|author||Andrew Jeffery <email@example.com>||2018-03-29 14:52:09 +1030|
|committer||Andrew Jeffery <firstname.lastname@example.org>||2018-04-04 17:27:31 +0930|
Change-Id: I68d3f453325c15ef524d0fdce5228d65148427be Signed-off-by: Andrew Jeffery <email@example.com>
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+Virtual PNOR Functionality
+In the abstract, the virtual PNOR function shifts mboxd away from accessing raw
+flash to dynamically presenting a raw flash image to the host from a set of
+Enabling the feature virtualises both the host's access to the flash (the mbox
+protocol), and the BMC's access to the flash (via some filesystem on top of the
+Do I want to use this feature in my platform?
+Maybe. It depends on how the image construction is managed, particularly the
+behaviour around writes from the host. It is likely the scheme will prevent
+firmware updates from being correctly applied when using flash tools on the
+Currently, the virtual PNOR implementation requires that:
+* The host expect an FFS layout (OpenPOWER systems)
+* The BMC provide a directory tree presenting the backing files in a hierarchy
+ that reflects the partition properties in the FFS table of contents.
+Implementation Behavioural Properties
+1. The FFS ToC defines the set of valid access ranges in terms of partitions
+2. The read-only property of partitions is enforced
+3. The ToC is considered read-only
+4. Read access to valid ranges must be granted
+5. Write access to valid ranges may be granted
+6. Access ranges that are valid may map into a backing file associated with
+ the partition
+7. A read of a valid access range that maps into the backing file will render
+ the data held in the backing file at the appropriate offset
+8. A read of a valid access range that does not map into the backing file will
+ appear erased
+9. A read of an invalid access range will appear erased
+10. A write to a valid access range that maps into the backing file will update
+ the data in the file at the appropriate offset
+11. A write to a valid access range that does not map into the backing file
+ will expand the backing file to accommodate the write.
+12. A write to a valid access range may fail if the range is not marked as
+ writeable. The error should be returned in response to the request to open
+ the write window intersecting the read-only range.
+13. A write of an invalid access range will return an error. The error should
+ be returned in response to the request to open the write window covering
+ the invalid range.
+The clarification on when the failure occurs in points 11 and 12 is useful for
+host-side error handling. Opening a write window gives the indication that
+future writes are expected to succeed, but in both cases we define them as
+always failing. Therefore we should not give the impression to the host that
+what it is asking for can be satisfied.