Documentation: Point to canonical protocol documentation

Avoid conflicts in the protocol documentation by pointing back to the
reference implementation.

Change-Id: I380f32dab2e9b8bbeec25b9abffe9c5427b76bd3
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>

1 files changed, 2 insertions, 688 deletions

diff --git a/Documentation/mbox_protocol.md b/Documentation/mbox_protocol.md
index 45320dd..b6da187 100644
--- a/Documentation/mbox_protocol.md
+++ b/Documentation/mbox_protocol.md
@@ -1,689 +1,3 @@
-Copyright 2017 IBM
+Please reference the canonical mbox protocol documentation:
 
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-  http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-
-## Intro
-
-This document describes a protocol for host to BMC communication via the
-mailbox registers present on the Aspeed 2400 and 2500 chips.
-This protocol is specifically designed to allow a host to request and manage
-access to the flash with the specifics of how the host is required to control
-this described below.
-
-## Version
-
-Both version 1 and version 2 of the protocol are described below with version 2
-specificities represented with V2 in brackets - (V2).
-
-## Problem Overview
-
-"mbox" is the name we use to represent a protocol we have established between
-the host and the BMC via the Aspeed mailbox registers. This protocol is used
-for the host to control the flash.
-
-Prior to the mbox protocol, the host uses a backdoor into the BMC address space
-(the iLPC-to-AHB bridge) to directly manipulate the BMCs own flash controller.
-
-This is not sustainable for a number of reasons. The main ones are:
-
-1. Every piece of the host software stack that needs flash access (HostBoot,
-   OCC, OPAL, ...) has to have a complete driver for the flash controller,
-   update it on each BMC generation, have all the quirks for all the flash
-   chips supported etc... We have 3 copies on the host already in addition to
-   the one in the BMC itself.
-
-2. There are serious issues of access conflicts to that controller between the
-   host and the BMC.
-
-3. It's very hard to support "BMC reboots" when doing that
-
-4. It's slow
-
-5. Last but probably most important, having that backdoor open is a security
-   risk. It means the host can access any address on the BMC internal bus and
-   implant malware in the BMC itself. So if the host is a "bare metal" shared
-   system in some kind of data center, not only the host flash needs to be
-   reflashed when switching from one customer to another, but the entire BMC
-   flash too as nothing can be trusted. So we want to disable it.
-
-To address all these, we have implemented a new mechanism that we call mbox.
-
-When using this mechanism, the BMC is solely responsible for directly accessing
-the flash controller. All flash erase and write operations are performed by the
-BMC and the BMC only. (We can allow direct reads from flash under some
-circumstances but we tend to prefer going via memory).
-
-The host uses the mailbox registers to send "commands" to the BMC, which
-responds via the same mechanism. Those commands allow the host to control a
-"window" (which is the LPC -> AHB FW space mapping) that is either a read
-window or a write window onto the flash.
-
-When set for writing, the BMC makes the window point to a chunk of RAM instead.
-When the host "commits" a change (via MBOX), then the BMC can perform the
-actual flashing from the data in the RAM window.
-
-The idea is to have the LPC FW space be routed to an active "window".  That
-window can be a read or a write window. The commands allow to control which
-window and which offset into the flash it maps.
-
-* A read window can be a direct window to the flash controller space (ie.
-  0x3000\_0000) or it can be a window to a RAM image of a flash. It doesn't have
-  to be the full size of the flash per protocol (commands can be use to "slide"
-  it to various parts of the flash) but if its set to map the actual flash
-  controller space at 0x3000\_0000, it's probably simpler to make it the full
-  flash. The host makes no assumption, it's your choice what to provide. The
-  simplest implementation is to just route to the flash read/only.
-
-* A write window has to be a chunk of BMC memory. The minimum size is not
-  defined in the spec, but it should be at least one block (4k for now but it
-  should support larger block sizes in the future). When the BMC receive the
-  command to map the write window at a given offset of the flash, the BMC should
-  copy that portion of the flash into a reserved memory buffer, and modify the
-  LPC mapping to point to that buffer.
-
-The host can then write to that window directly (updating the BMC memory) and
-send a command to "commit" those updates to flash.
-
-Finally there is a `RESET_STATE`. It's the state in which the bootloader in the
-SEEPROM of the POWER9 chip will find what it needs to load HostBoot. The
-details are still being ironed out: either mapping the full flash read only or
-reset to a "window" that is either at the bottom or top of the flash. The
-current implementation resets to point to the full flash.
-
-## Where is the code?
-
-The mbox userspace is available [on GitHub](https://github.com/openbmc/mboxbridge)
-This is Apache licensed but we are keen to see any enhancements you may have.
-
-The kernel driver is still in the process of being upstreamed but can be found
-in the OpenBMC Linux kernel staging tree:
-
-https://github.com/openbmc/linux/commit/85770a7d1caa6a1fa1a291c33dfe46e05755a2ef
-
-## Building
-
-The autotools of this requires the autoconf-archive package for your
-system
-
-## The Hardware
-
-The Aspeed mailbox consists of 16 (8 bit) data registers see Layout for their
-use. Mailbox interrupt enabling, masking and triggering is done using a pair
-of control registers, one accessible by the host the other by the BMC.
-Interrupts can also be raised per write to each data register, for BMC and
-host. Write tiggered interrupts are configured using two 8 bit registers where
-each bit represents a data register and if an interrupt should fire on write.
-Two 8 bit registers are present to act as a mask for write triggered
-interrupts.
-
-### Layout
-
-```
-Byte 0: COMMAND
-Byte 1: Sequence
-Byte 2-12: Arguments
-Byte 13: Response code
-Byte 14: Host controlled status reg
-Byte 15: BMC controlled status reg
-```
-
-Note: when the BMC is writing a response to the mbox registers (as described
-above), the "Response Code" (Register 13) must be the last register written to.
-Writing register 13 will trigger an interrupt to the host indicating a complete
-response has been written. Triggering the interrupt by writing register 13
-prior to completing the response may lead to a data race, and must therefore
-be avoided.
-
-## Low Level Protocol Flow
-
-What we essentially have is a set of registers which either the host or BMC can
-write to in order to communicate to the other which will respond in some way.
-There are 3 basic types of communication.
-
-1. Commands sent from the Host to the BMC
-2. Responses sent from the BMC to the Host in response to commands
-3. Asyncronous events raised by the BMC
-
-### General Use
-
-Messages usually originate from the host to the BMC. There are special
-cases for a back channel for the BMC to pass new information to the
-host which will be discussed later.
-
-To initiate a request the host must set a command code (see Commands) into
-mailbox data register 0, and generate a sequence number (see Sequence Numbers)
-to write to mailbox register data 1. After these two values, any
-command-specific data should be written (see Layout). The host must then
-generate an interrupt to the BMC by using bit 0 of its control register and
-wait for an interrupt on the response register.  Generating an interrupt
-automatically sets bit 7 of the corresponding control register. This bit can be
-used to poll for messages.
-
-On receiving an interrupt (or polling on bit 7 of its Control
-Register) the BMC should read the message from the general registers
-of the mailbox and perform the necessary action before responding. On
-responding the BMC must ensure that the sequence number is the same as
-the one in the request from the host. The BMC must also ensure that
-mailbox data register 13 is a valid response code (see Responses). The
-BMC should then use its control register to generate an interrupt for
-the host to notify it of a response.
-
-### Asynchronous BMC to Host Events
-
-BMC to host communication is also possible for notification of events
-from the BMC. This requires that the host have interrupts enabled on
-mailbox data register 15 (or otherwise poll on bit 7 of mailbox status
-register 1). On receiving such a notification the host should read
-mailbox data register 15 to determine the event code which was set by the
-BMC (see BMC Event notifications in Commands for detail). Events which are
-defined as being able to be acknowledged by the host must be with a
-BMC_EVENT_ACK command.
-
-## High Level Protocol Flow
-
-When a host wants to communicate with the BMC via the mbox protocol the first
-thing it should do it call MBOX_GET_INFO in order to establish the protocol
-version which each understands. Before this the only other commands which are
-allowed are RESET_STATE and BMC_EVENT_ACK.
-
-After this the host can open and close windows with the CREATE_READ_WINDOW,
-CREATE_WRITE_WINDOW and CLOSE_WINDOW commands. Creating a window is how the
-host requests access to a section of flash. It is worth noting that the host
-can only ever have one window that it is accessing at a time - hence forth
-referred to as the active window.
-
-When the active window is a write window the host can perform MARK_WRITE_DIRTY,
-MARK_WRITE_ERASED and WRITE_FLUSH commands to identify changed blocks and
-control when the changed blocks are written to flash.
-
-Independently, and at any point not during an existing mbox command
-transaction, the BMC may raise raise asynchronous events with the host to
-communicate a change in state.
-
-### Version Negotiation
-
-Given that a majority of command and response arguments are specified as a
-multiple of block size it is necessary for the host and BMC to agree on a
-protocol version as this determines the block size. In V1 it is hard coded at
-4K and in V2 the BMC chooses and specifies this to the host as a response
-argument to `MBOX_GET_INFO`. Thus the host must always call `MBOX_GET_INFO`
-before any other command which specifies an argument in block size.
-
-When invoking `MBOX_GET_INFO` the host must provide the BMC its highest
-supported version of the protocol. The BMC must respond with a protocol version
-less than or equal to that requested by the host, or in the event that there is
-no such value, an error code. In the event that an error is returned the host
-must not continue to communicate with the BMC. Otherwise, the protocol version
-returned by the BMC is the agreed protocol version for all further
-communication. The host may at a future point request a change in protocol
-version by issuing a subsequent `MBOX_GET_INFO` command.
-
-### Window Management
-
-In order to access flash contents the host must request a window be opened at
-the flash offset it would like to access. The host may give a hint as to how
-much data it would like to access or otherwise set this argument to zero. The
-BMC must respond with the lpc bus address to access this window and the
-window size. The host must not access past the end of the active window.
-
-There is only ever one active window which is the window created by the most
-recent CREATE_READ_WINDOW or CREATE_WRITE_WINDOW call which succeeded. Even
-though there are two types of windows there can still only be one active window
-irrespective of type. A host must not write to a read window. A host may read
-from a write window and the BMC must guarantee that the window reflects what
-the host has written there.
-
-A window can be closed by calling CLOSE_WINDOW in which case there is no active
-window and the host must not access the LPC window after it has been closed.
-If the host closes an active write window then the BMC must perform an
-implicit flush. If the host tries to open a new window with an already active
-window then the active window is closed (and implicitly flushed if it was a
-write window). If the new window is successfully opened then it is the new
-active window, if the command fails then there is no active window and the
-previous active window must no longer be accessed.
-
-The host must not access an lpc address other than that which is contained by
-the active window. The host must not use write management functions (see below)
-if the active window is a read window or if there is no active window.
-
-### Write Management
-
-The BMC has no method for intercepting writes that occur over the LPC bus. Thus
-the host must explicitly notify the BMC of where and when a write has
-occured. The host must use the MARK_WRITE_DIRTY command to tell the BMC where
-within the write window it has modified. The host may also use the
-MARK_WRITE_ERASED command to erase large parts of the active window without the
-need to write 0xFF. The BMC must ensure that if the host
-reads from an area it has erased that the read values are 0xFF. Any part of the
-active window marked dirty/erased is only marked for the lifetime of the current
-active write window and does not persist if the active window is closed either
-implicitly or explicitly by the host or the BMC. The BMC may at any time
-or must on a call to WRITE_FLUSH flush the changes which it has been notified
-of back to the flash, at which point the dirty or erased marking is cleared
-for the active window. The host must not assume that any changes have been
-written to flash unless an explicit flush call was successful, a close of an
-active write window was successful or a create window command with an active
-write window was successful - otherwise consistency between the flash and memory
-contents cannot be guaranteed.
-
-The host is not required to perform an erase before a write command and the
-BMC must ensure that a write performs as expected - that is if an erase is
-required before a write then the BMC must perform this itself.
-
-### BMC Events
-
-The BMC can raise events with the host asynchronously to communicate to the
-host a change in state which it should take notice of. The host must (if
-possible for the given event) acknowledge it to inform the BMC it has been
-received.
-
-If the BMC raises a BMC Reboot event then the host must renegotiate the
-protocol version so that both the BMC and the host agree on the block size.
-A BMC Reboot event implies a BMC Windows Reset event.
-If the BMC raises a BMC Windows Reset event then the host must
-assume that there is no longer an active window - that is if there was an
-active window it has been closed by the BMC and if it was a write window
-then the host must not assume that it was flushed unless a previous explicit
-flush call was successful.
-
-The BMC may at some points require access to the flash and the BMC daemon must
-set the BMC Flash Control Lost event when the BMC is accessing the flash behind
-the BMC daemons back. When this event is set the host must assume that the
-contents of the active window could be inconsistent with the contents of flash.
-
-## Protocol Definition
-
-### Commands
-
-```
-RESET_STATE          0x01
-GET_MBOX_INFO        0x02
-GET_FLASH_INFO       0x03
-CREATE_READ_WINDOW   0x04
-CLOSE_WINDOW         0x05
-CREATE_WRITE_WINDOW  0x06
-MARK_WRITE_DIRTY     0x07
-WRITE_FLUSH          0x08
-BMC_EVENT_ACK        0x09
-MARK_WRITE_ERASED    0x0a	(V2)
-```
-
-### Responses
-
-```
-SUCCESS		1
-PARAM_ERROR	2
-WRITE_ERROR	3
-SYSTEM_ERROR	4
-TIMEOUT		5
-BUSY		6	(V2)
-WINDOW_ERROR	7	(V2)
-SEQ_ERROR	8	(V2)
-```
-
-### Sequence Numbers
-
-Sequence numbers are included in messages for correlation of commands and
-responses. V1 and V2 of the protocol permit either zero or one commands to be
-in progress (yet to receive a response).
-
-For generality, the host must generate a sequence number that is unique with
-respect to the previous command (one that has received a response) and any
-in-progress commands. Sequence numbers meeting this requirement are considered
-valid. The BMC's response to a command must contain the same sequence number
-issued by the host as found in the relevant command.
-
-Sequence numbers may be reused in accordance with the constraints outlined
-above, however it is not an error if the BMC receives a `GET_MBOX_INFO` with an
-invalid sequence number. For all other cases, the BMC must respond with
-`SEQ_ERROR` if the constraints are violated. If the host receives a `SEQ_ERROR`
-response it must consider any in-progress commands to have failed. The host may
-retry the affected command(s) after generating a suitable sequence number.
-
-#### Description:
-
-SUCCESS		- Command completed successfully
-
-PARAM_ERROR	- Error with parameters supplied or command invalid
-
-WRITE_ERROR	- Error writing to the backing file system
-
-SYSTEM_ERROR	- Error in BMC performing system action
-
-TIMEOUT		- Timeout in performing action
-
-BUSY		- Daemon in suspended state (currently unable to access flash)
-		- Retry again later
-
-WINDOW_ERROR	- Command not valid for active window or no active window
-		- Try opening an appropriate window and retrying the command
-
-### Information
-- All multibyte messages are LSB first (little endian)
-- All responses must have a valid return code in byte 13
-
-
-### Commands in detail
-
-Block size refers to an agreed value which is used as a unit for the
-arguments of various commands and responses. Having a block size multiplier
-allows us to specify larger values with fewer command and response fields.
-
-In V1 block size is hard coded to 4K.
-In V2 it is variable and must be queried with the GET_MBOX_INFO command.
-Note that for simplicity block size must always be a power-of-2.
-Block size must also be greater than or equal to 4K. This is due to the
-fact that we have a 28-bit LPC address space and commands which return an
-LPC address do so in 16 bits, thus we need at least a 12-bit unit to ensure
-that we can specify the entire address space. This additionally allows us
-to specify flash addresses of at least 256MB.
-
-Sizes and addresses are specified in either bytes - (bytes)
-					 or blocks - (blocks)
-Sizes and addresses specified in blocks must be converted to bytes by
-multiplying by the block size.
-```
-Command:
-	RESET_STATE
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		-
-	Response:
-		-
-	Notes:
-		This command is designed to inform the BMC that it should put
-		host LPC mapping back in a state where the SBE will be able to
-		use it. Currently this means pointing back to BMC flash
-		pre mailbox protocol. Final behavour is still TBD.
-
-Command:
-	GET_MBOX_INFO
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		V1:
-		Args 0: API version
-
-		V2:
-		Args 0: API version
-
-	Response:
-		V1:
-		Args 0: API version
-		Args 1-2: default read window size (blocks)
-		Args 3-4: default write window size (blocks)
-
-		V2:
-		Args 0: API version
-		Args 1-2: reserved
-		Args 3-4: reserved
-		Args 5: Block size as power of two (encoded as a shift)
-		Args 6-7: Suggested Timeout (seconds)
-	Notes:
-		The suggested timeout is a hint to the host as to how long
-		it should wait after issuing a command to the BMC before it
-		times out waiting for a response. This is the maximum time
-		which the BMC thinks it could take to service any command which
-		the host could issue. This may be set to zero to indicate that
-		the BMC	does not wish to provide a hint in which case the host
-		must choose some reasonable value.
-
-Command:
-	GET_FLASH_INFO
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		-
-	Response:
-		V1:
-		Args 0-3: Flash size (bytes)
-		Args 4-7: Erase granule (bytes)
-
-		V2:
-		Args 0-1: Flash size (blocks)
-		Args 2-3: Erase granule (blocks)
-
-Command:
-	CREATE_{READ/WRITE}_WINDOW
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		V1:
-		Args 0-1: Requested flash offset (blocks)
-
-		V2:
-		Args 0-1: Requested flash offset (blocks)
-		Args 2-3: Requested flash size to access (blocks)
-
-	Response:
-		V1:
-		Args 0-1: LPC bus address of window (blocks)
-
-		V2:
-		Args 0-1: LPC bus address of window (blocks)
-		Args 2-3: Window size (blocks)
-		Args 4-5: Flash offset mapped by window (blocks)
-	Notes:
-		The flash offset which the host requests access to is always
-		taken from the start of flash - that is it is an absolute
-		offset into flash.
-
-		LPC bus address is always given from the start of the LPC
-		address space - that is it is an absolute address.
-
-		The requested access size is only a hint. The response
-		indicates the actual size of the window. The BMC may
-		want to use the requested size to pre-load the remainder
-		of the request. The host must not access past the end of the
-		active window.
-
-		The flash offset mapped by the window is an absolute flash
-		offset and must be less than or equal to the flash offset
-		requested by the host. It is the responsibility of the host
-		to use this information to access any offset which is required.
-
-		The requested window size may be zero. In this case the
-		BMC is free to create any sized window but it must contain
-		atleast the first block of data requested by the host. A large
-		window is of course preferred and should correspond to
-		the default size returned in the GET_MBOX_INFO command.
-
-		If this command returns successfully then the created window
-		is the active window. If it fails then there is no active
-		window.
-
-Command:
-	CLOSE_WINDOW
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		V1:
-		-
-
-		V2:
-		Args 0: Flags
-	Response:
-		-
-	Notes:
-		Closes the active window. Any further access to the LPC bus
-		address specified to address the previously active window will
-		have undefined effects. If the active window is a
-		write window then the BMC must perform an implicit flush.
-
-		The Flags argument allows the host to provide some
-		hints to the BMC. Defined Values:
-			0x01 - Short Lifetime:
-				The window is unlikely to be accessed
-				anytime again in the near future. The effect of
-				this will depend on BMC implementation. In
-				the event that the BMC performs some caching
-				the BMC daemon could mark data contained in a
-				window closed with this flag as first to be
-				evicted from the cache.
-
-Command:
-	MARK_WRITE_DIRTY
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		V1:
-		Args 0-1: Flash offset to mark from base of flash (blocks)
-		Args 2-5: Number to mark dirty at offset (bytes)
-
-		V2:
-		Args 0-1: Window offset to mark (blocks)
-		Args 2-3: Number to mark dirty at offset (blocks)
-
-	Response:
-		-
-	Notes:
-		The BMC has no method for intercepting writes that
-		occur over the LPC bus. The host must explicitly notify
-		the daemon of where and when a write has occured so it
-		can be flushed to backing storage.
-
-		Offsets are given as an absolute (either into flash (V1) or the
-		active window (V2)) and a zero offset refers to the first
-		block. If the offset + number exceeds the size of the active
-		window then the command must not succeed.
-
-Command
-	WRITE_FLUSH
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		V1:
-		Args 0-1: Flash offset to mark from base of flash (blocks)
-		Args 2-5: Number to mark dirty at offset (bytes)
-
-		V2:
-		-
-
-	Response:
-		-
-	Notes:
-		Flushes any dirty/erased blocks in the active window to
-		the backing storage.
-
-		In V1 this can also be used to mark parts of the flash
-		dirty and flush in a single command. In V2 the explicit
-		mark dirty command must be used before a call to flush
-		since there are no longer any arguments. If the offset + number
-		exceeds the size of the active window then the command must not
-		succeed.
-
-
-Command:
-	BMC_EVENT_ACK
-	Implemented in Versions:
-		V1, V2
-	Arguments:
-		Args 0:	Bits in the BMC status byte (mailbox data
-			register 15) to ack
-	Response:
-		*clears the bits in mailbox data register 15*
-	Notes:
-		The host should use this command to acknowledge BMC events
-		supplied in mailbox register 15.
-
-Command:
-	MARK_WRITE_ERASED
-	Implemented in Versions:
-		V2
-	Arguments:
-		V2:
-		Args 0-1: Window offset to erase (blocks)
-		Args 2-3: Number to erase at offset (blocks)
-	Response:
-		-
-	Notes:
-		This command allows the host to erase a large area
-		without the need to individually write 0xFF
-		repetitively.
-
-		Offset is the offset within the active window to start erasing
-		from (zero refers to the first block of the active window) and
-		number is the number of blocks of the active window to erase
-		starting at offset. If the offset + number exceeds the size of
-		the active window then the command must not succeed.
-```
-
-### BMC Events in Detail:
-
-If the BMC needs to tell the host something then it simply
-writes to Byte 15. The host should have interrupts enabled
-on that register, or otherwise be polling it.
-
-#### Bit Definitions:
-
-Events which should be ACKed:
-```
-0x01: BMC Reboot
-0x02: BMC Windows Reset (V2)
-```
-
-Events which cannot be ACKed (BMC will clear when no longer
-applicable):
-```
-0x40: BMC Flash Control Lost (V2)
-0x80: BMC MBOX Daemon Ready (V2)
-```
-
-#### Event Description:
-
-Events which must be ACKed:
-The host should acknowledge these events with BMC_EVENT_ACK to
-let the BMC know that they have been received and understood.
-```
-0x01 - BMC Reboot:
-	Used to inform the host that a BMC reboot has occured.
-	The host must perform protocol verison negotiation again and
-	must assume it has no active window. The host must not assume
-	that any commands which didn't respond as such succeeded.
-0x02 - BMC Windows Reset: (V2)
-	The host must assume that its active window has been closed and
-	that it no longer has an active window. The host is not
-	required to perform protocol version negotiation again. The
-	host must not assume that any commands which didn't respond as such
-	succeeded.
-```
-
-Events which cannot be ACKed:
-These events cannot be acknowledged by the host and a call to
-BMC_EVENT_ACK with these bits set will have no effect. The BMC
-will clear these bits when they are no longer applicable.
-```
-0x40 - BMC Flash Control Lost: (V2)
-	The BMC daemon has been suspended and thus no longer
-	controls access to the flash (most likely because some
-	other process on the BMC required direct access to the
-	flash and has suspended the BMC daemon to preclude
-	concurrent access).
-	The BMC daemon must clear this bit itself when it regains
-	control of the flash (the host isn't able to clear it
-	through an acknowledge command).
-	The host must not assume that the contents of the active window
-	correctly reflect the contents of flash while this bit is set.
-0x80 - BMC MBOX Daemon Ready: (V2)
-	Used to inform the host that the BMC daemon is ready to
-	accept command requests. The host isn't able to clear
-	this bit through an acknowledge command, the BMC daemon must
-	clear it before it terminates (assuming it didn't
-	terminate unexpectedly).
-	The host should not expect a response while this bit is
-	not set.
-	Note that this bit being set is not a guarantee that the BMC daemon
-	will respond as it or the BMC may have crashed without clearing
-	it.
-```
+    https://github.com/openbmc/mboxbridge/blob/master/Documentation/mbox_protocol.md