From e3e0a28b5b067d16b8e2e5ddaedecda5bd0c3ec2 Mon Sep 17 00:00:00 2001 From: "Hans J. Koch" Date: Mon, 11 Dec 2006 16:59:59 +0100 Subject: UIO: Documentation Documentation for the UIO interface From: Hans J. Koch Signed-off-by: Greg Kroah-Hartman --- Documentation/DocBook/uio-howto.tmpl | 611 +++++++++++++++++++++++++++++++++++ 1 file changed, 611 insertions(+) create mode 100644 Documentation/DocBook/uio-howto.tmpl (limited to 'Documentation/DocBook/uio-howto.tmpl') diff --git a/Documentation/DocBook/uio-howto.tmpl b/Documentation/DocBook/uio-howto.tmpl new file mode 100644 index 000000000000..e3bb29a8d8dd --- /dev/null +++ b/Documentation/DocBook/uio-howto.tmpl @@ -0,0 +1,611 @@ + + + + + +The Userspace I/O HOWTO + + + Hans-Jürgen + Koch + Linux developer, Linutronix + + + Linutronix + + +
+ hjk@linutronix.de +
+ + + +2006-12-11 + + + This HOWTO describes concept and usage of Linux kernel's + Userspace I/O system. + + + + + 0.3 + 2007-04-29 + hjk + Added section about userspace drivers. + + + 0.2 + 2007-02-13 + hjk + Update after multiple mappings were added. + + + 0.1 + 2006-12-11 + hjk + First draft. + + + + + + +About this document + + + +Copyright and License + + Copyright (c) 2006 by Hans-Jürgen Koch. + +This documentation is Free Software licensed under the terms of the +GPL version 2. + + + + + +Translations + +If you know of any translations for this document, or you are +interested in translating it, please email me +hjk@linutronix.de. + + + + +Preface + + For many types of devices, creating a Linux kernel driver is + overkill. All that is really needed is some way to handle an + interrupt and provide access to the memory space of the + device. The logic of controlling the device does not + necessarily have to be within the kernel, as the device does + not need to take advantage of any of other resources that the + kernel provides. One such common class of devices that are + like this are for industrial I/O cards. + + + To address this situation, the userspace I/O system (UIO) was + designed. For typical industrial I/O cards, only a very small + kernel module is needed. The main part of the driver will run in + user space. This simplifies development and reduces the risk of + serious bugs within a kernel module. + + + + +Acknowledgments + I'd like to thank Thomas Gleixner and Benedikt Spranger of + Linutronix, who have not only written most of the UIO code, but also + helped greatly writing this HOWTO by giving me all kinds of background + information. + + + +Feedback + Find something wrong with this document? (Or perhaps something + right?) I would love to hear from you. Please email me at + hjk@linutronix.de. + + + + + +About UIO + +If you use UIO for your card's driver, here's what you get: + + + + only one small kernel module to write and maintain. + + + develop the main part of your driver in user space, + with all the tools and libraries you're used to. + + + bugs in your driver won't crash the kernel. + + + updates of your driver can take place without recompiling + the kernel. + + + if you need to keep some parts of your driver closed source, + you can do so without violating the GPL license on the kernel. + + + + +How UIO works + + Each UIO device is accessed through a device file and several + sysfs attribute files. The device file will be called + /dev/uio0 for the first device, and + /dev/uio1, /dev/uio2 + and so on for subsequent devices. + + + /dev/uioX is used to access the + address space of the card. Just use + mmap() to access registers or RAM + locations of your card. + + + + Interrupts are handled by reading from + /dev/uioX. A blocking + read() from + /dev/uioX will return as soon as an + interrupt occurs. You can also use + select() on + /dev/uioX to wait for an interrupt. The + integer value read from /dev/uioX + represents the total interrupt count. You can use this number + to figure out if you missed some interrupts. + + + + To handle interrupts properly, your custom kernel module can + provide its own interrupt handler. It will automatically be + called by the built-in handler. + + + + For cards that don't generate interrupts but need to be + polled, there is the possibility to set up a timer that + triggers the interrupt handler at configurable time intervals. + See drivers/uio/uio_dummy.c for an + example of this technique. + + + + Each driver provides attributes that are used to read or write + variables. These attributes are accessible through sysfs + files. A custom kernel driver module can add its own + attributes to the device owned by the uio driver, but not added + to the UIO device itself at this time. This might change in the + future if it would be found to be useful. + + + + The following standard attributes are provided by the UIO + framework: + + + + + name: The name of your device. It is + recommended to use the name of your kernel module for this. + + + + + version: A version string defined by your + driver. This allows the user space part of your driver to deal + with different versions of the kernel module. + + + + + event: The total number of interrupts + handled by the driver since the last time the device node was + read. + + + + + These attributes appear under the + /sys/class/uio/uioX directory. Please + note that this directory might be a symlink, and not a real + directory. Any userspace code that accesses it must be able + to handle this. + + + Each UIO device can make one or more memory regions available for + memory mapping. This is necessary because some industrial I/O cards + require access to more than one PCI memory region in a driver. + + + Each mapping has its own directory in sysfs, the first mapping + appears as /sys/class/uio/uioX/maps/map0/. + Subsequent mappings create directories map1/, + map2/, and so on. These directories will only + appear if the size of the mapping is not 0. + + + Each mapX/ directory contains two read-only files + that show start address and size of the memory: + + + + + addr: The address of memory that can be mapped. + + + + + size: The size, in bytes, of the memory + pointed to by addr. + + + + + + From userspace, the different mappings are distinguished by adjusting + the offset parameter of the + mmap() call. To map the memory of mapping N, you + have to use N times the page size as your offset: + + +offset = N * getpagesize(); + + + + + + + +Using uio_dummy + + Well, there is no real use for uio_dummy. Its only purpose is + to test most parts of the UIO system (everything except + hardware interrupts), and to serve as an example for the + kernel module that you will have to write yourself. + + + +What uio_dummy does + + The kernel module uio_dummy.ko creates a + device that uses a timer to generate periodic interrupts. The + interrupt handler does nothing but increment a counter. The + driver adds two custom attributes, count + and freq, that appear under + /sys/devices/platform/uio_dummy/. + + + + The attribute count can be read and + written. The associated file + /sys/devices/platform/uio_dummy/count + appears as a normal text file and contains the total number of + timer interrupts. If you look at it (e.g. using + cat), you'll notice it is slowly counting + up. + + + + The attribute freq can be read and written. + The content of + /sys/devices/platform/uio_dummy/freq + represents the number of system timer ticks between two timer + interrupts. The default value of freq is + the value of the kernel variable HZ, which + gives you an interval of one second. Lower values will + increase the frequency. Try the following: + + +cd /sys/devices/platform/uio_dummy/ +echo 100 > freq + + + Use cat count to see how the interrupt + frequency changes. + + + + + + +Writing your own kernel module + + Please have a look at uio_dummy.c as an + example. The following paragraphs explain the different + sections of this file. + + + +struct uio_info + + This structure tells the framework the details of your driver, + Some of the members are required, others are optional. + + + + +char *name: Required. The name of your driver as +it will appear in sysfs. I recommend using the name of your module for this. + + + +char *version: Required. This string appears in +/sys/class/uio/uioX/version. + + + +struct uio_mem mem[ MAX_UIO_MAPS ]: Required if you +have memory that can be mapped with mmap(). For each +mapping you need to fill one of the uio_mem structures. +See the description below for details. + + + +long irq: Required. If your hardware generates an +interrupt, it's your modules task to determine the irq number during +initialization. If you don't have a hardware generated interrupt but +want to trigger the interrupt handler in some other way, set +irq to UIO_IRQ_CUSTOM. The +uio_dummy module does this as it triggers the event mechanism in a timer +routine. If you had no interrupt at all, you could set +irq to UIO_IRQ_NONE, though this +rarely makes sense. + + + +unsigned long irq_flags: Required if you've set +irq to a hardware interrupt number. The flags given +here will be used in the call to request_irq(). + + + +int (*mmap)(struct uio_info *info, struct vm_area_struct +*vma): Optional. If you need a special +mmap() function, you can set it here. If this +pointer is not NULL, your mmap() will be called +instead of the built-in one. + + + +int (*open)(struct uio_info *info, struct inode *inode) +: Optional. You might want to have your own +open(), e.g. to enable interrupts only when your +device is actually used. + + + +int (*release)(struct uio_info *info, struct inode *inode) +: Optional. If you define your own +open(), you will probably also want a custom +release() function. + + + + +Usually, your device will have one or more memory regions that can be mapped +to user space. For each region, you have to set up a +struct uio_mem in the mem[] array. +Here's a description of the fields of struct uio_mem: + + + + +int memtype: Required if the mapping is used. Set this to +UIO_MEM_PHYS if you you have physical memory on your +card to be mapped. Use UIO_MEM_LOGICAL for logical +memory (e.g. allocated with kmalloc()). There's also +UIO_MEM_VIRTUAL for virtual memory. + + + +unsigned long addr: Required if the mapping is used. +Fill in the address of your memory block. This address is the one that +appears in sysfs. + + + +unsigned long size: Fill in the size of the +memory block that addr points to. If size +is zero, the mapping is considered unused. Note that you +must initialize size with zero for +all unused mappings. + + + +void *internal_addr: If you have to access this memory +region from within your kernel module, you will want to map it internally by +using something like ioremap(). Addresses +returned by this function cannot be mapped to user space, so you must not +store it in addr. Use internal_addr +instead to remember such an address. + + + + +Please do not touch the kobj element of +struct uio_mem! It is used by the UIO framework +to set up sysfs files for this mapping. Simply leave it alone. + + + + +Adding an interrupt handler + + What you need to do in your interrupt handler depends on your + hardware and on how you want to handle it. You should try to + keep the amount of code in your kernel interrupt handler low. + If your hardware requires no action that you + have to perform after each interrupt, + then your handler can be empty. If, on the other + hand, your hardware needs some action to + be performed after each interrupt, then you + must do it in your kernel module. Note + that you cannot rely on the userspace part of your driver. Your + userspace program can terminate at any time, possibly leaving + your hardware in a state where proper interrupt handling is + still required. + + + + There might also be applications where you want to read data + from your hardware at each interrupt and buffer it in a piece + of kernel memory you've allocated for that purpose. With this + technique you could avoid loss of data if your userspace + program misses an interrupt. + + + + A note on shared interrupts: Your driver should support + interrupt sharing whenever this is possible. It is possible if + and only if your driver can detect whether your hardware has + triggered the interrupt or not. This is usually done by looking + at an interrupt status register. If your driver sees that the + IRQ bit is actually set, it will perform its actions, and the + handler returns IRQ_HANDLED. If the driver detects that it was + not your hardware that caused the interrupt, it will do nothing + and return IRQ_NONE, allowing the kernel to call the next + possible interrupt handler. + + + + If you decide not to support shared interrupts, your card + won't work in computers with no free interrupts. As this + frequently happens on the PC platform, you can save yourself a + lot of trouble by supporting interrupt sharing. + + + + + + + +Writing a driver in userspace + + Once you have a working kernel module for your hardware, you can + write the userspace part of your driver. You don't need any special + libraries, your driver can be written in any reasonable language, + you can use floating point numbers and so on. In short, you can + use all the tools and libraries you'd normally use for writing a + userspace application. + + + +Getting information about your UIO device + + Information about all UIO devices is available in sysfs. The + first thing you should do in your driver is check + name and version to + make sure your talking to the right device and that its kernel + driver has the version you expect. + + + You should also make sure that the memory mapping you need + exists and has the size you expect. + + + There is a tool called lsuio that lists + UIO devices and their attributes. It is available here: + + + + http://www.osadl.org/projects/downloads/UIO/user/ + + + With lsuio you can quickly check if your + kernel module is loaded and which attributes it exports. + Have a look at the manpage for details. + + + The source code of lsuio can serve as an + example for getting information about an UIO device. + The file uio_helper.c contains a lot of + functions you could use in your userspace driver code. + + + + +mmap() device memory + + After you made sure you've got the right device with the + memory mappings you need, all you have to do is to call + mmap() to map the device's memory + to userspace. + + + The parameter offset of the + mmap() call has a special meaning + for UIO devices: It is used to select which mapping of + your device you want to map. To map the memory of + mapping N, you have to use N times the page size as + your offset: + + + offset = N * getpagesize(); + + + N starts from zero, so if you've got only one memory + range to map, set offset = 0. + A drawback of this technique is that memory is always + mapped beginning with its start address. + + + + +Waiting for interrupts + + After you successfully mapped your devices memory, you + can access it like an ordinary array. Usually, you will + perform some initialization. After that, your hardware + starts working and will generate an interrupt as soon + as it's finished, has some data available, or needs your + attention because an error occured. + + + /dev/uioX is a read-only file. A + read() will always block until an + interrupt occurs. There is only one legal value for the + count parameter of + read(), and that is the size of a + signed 32 bit integer (4). Any other value for + count causes read() + to fail. The signed 32 bit integer read is the interrupt + count of your device. If the value is one more than the value + you read the last time, everything is OK. If the difference + is greater than one, you missed interrupts. + + + You can also use select() on + /dev/uioX. + + + + + + +Further information + + + + OSADL homepage. + + + + Linutronix homepage. + + + + + -- cgit v1.2.1