The Userspace I/O HOWTO
Hans-Jürgen
Koch
Linux developer, Linutronix
Linutronix
hjk@hansjkoch.de
2006-2008
Hans-Jürgen Koch.
2009
Red Hat Inc, Michael S. Tsirkin (mst@redhat.com)
This documentation is Free Software licensed under the terms of the
GPL version 2.
2006-12-11
This HOWTO describes concept and usage of Linux kernel's
Userspace I/O system.
0.9
2009-07-16
mst
Added generic pci driver
0.8
2008-12-24
hjk
Added name attributes in mem and portio sysfs directories.
0.7
2008-12-23
hjk
Added generic platform drivers and offset attribute.
0.6
2008-12-05
hjk
Added description of portio sysfs attributes.
0.5
2008-05-22
hjk
Added description of write() function.
0.4
2007-11-26
hjk
Removed section about uio_dummy.
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
Translations
If you know of any translations for this document, or you are
interested in translating it, please email me
hjk@hansjkoch.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.
Please note that UIO is not an universal driver interface. Devices
that are already handled well by other kernel subsystems (like
networking or serial or USB) are no candidates for an UIO driver.
Hardware that is ideally suited for an UIO driver fulfills all of
the following:
The device has memory that can be mapped. The device can be
controlled completely by writing to this memory.
The device usually generates interrupts.
The device does not fit into one of the standard kernel
subsystems.
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@hansjkoch.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.
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.
For some hardware that has more than one interrupt source internally,
but not separate IRQ mask and status registers, there might be
situations where userspace cannot determine what the interrupt source
was if the kernel handler disables them by writing to the chip's IRQ
register. In such a case, the kernel has to disable the IRQ completely
to leave the chip's register untouched. Now the userspace part can
determine the cause of the interrupt, but it cannot re-enable
interrupts. Another cornercase is chips where re-enabling interrupts
is a read-modify-write operation to a combined IRQ status/acknowledge
register. This would be racy if a new interrupt occurred
simultaneously.
To address these problems, UIO also implements a write() function. It
is normally not used and can be ignored for hardware that has only a
single interrupt source or has separate IRQ mask and status registers.
If you need it, however, a write to /dev/uioX
will call the irqcontrol() function implemented
by the driver. You have to write a 32-bit value that is usually either
0 or 1 to disable or enable interrupts. If a driver does not implement
irqcontrol(), write() will
return with -ENOSYS.
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.
This interrupt simulation is done by calling
uio_event_notify()
from the timer's event handler.
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 four read-only files
that show attributes of the memory:
name: A string identifier for this mapping. This
is optional, the string can be empty. Drivers can set this to make it
easier for userspace to find the correct mapping.
addr: The address of memory that can be mapped.
size: The size, in bytes, of the memory
pointed to by addr.
offset: The offset, in bytes, that has to be
added to the pointer returned by mmap() to get
to the actual device memory. This is important if the device's memory
is not page aligned. Remember that pointers returned by
mmap() are always page aligned, so it is good
style to always add this offset.
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();
Sometimes there is hardware with memory-like regions that can not be
mapped with the technique described here, but there are still ways to
access them from userspace. The most common example are x86 ioports.
On x86 systems, userspace can access these ioports using
ioperm(), iopl(),
inb(), outb(), and similar
functions.
Since these ioport regions can not be mapped, they will not appear under
/sys/class/uio/uioX/maps/ like the normal memory
described above. Without information about the port regions a hardware
has to offer, it becomes difficult for the userspace part of the
driver to find out which ports belong to which UIO device.
To address this situation, the new directory
/sys/class/uio/uioX/portio/ was added. It only
exists if the driver wants to pass information about one or more port
regions to userspace. If that is the case, subdirectories named
port0, port1, and so on,
will appear underneath
/sys/class/uio/uioX/portio/.
Each portX/ directory contains four read-only
files that show name, start, size, and type of the port region:
name: A string identifier for this port region.
The string is optional and can be empty. Drivers can set it to make it
easier for userspace to find a certain port region.
start: The first port of this region.
size: The number of ports in this region.
porttype: A string describing the type of port.
Writing your own kernel module
Please have a look at uio_cif.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.
const char *name: Required. The name of your driver as
it will appear in sysfs. I recommend using the name of your module for this.
const 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.
struct uio_port port[ MAX_UIO_PORTS_REGIONS ]: Required
if you want to pass information about ioports to userspace. For each port
region you need to fill one of the uio_port 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.
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.
int (*irqcontrol)(struct uio_info *info, s32 irq_on)
: Optional. If you need to be able to enable or disable
interrupts from userspace by writing to /dev/uioX,
you can implement this function. The parameter irq_on
will be 0 to disable interrupts and 1 to enable them.
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.
Sometimes, your device can have one or more port regions which can not be
mapped to userspace. But if there are other possibilities for userspace to
access these ports, it makes sense to make information about the ports
available in sysfs. For each region, you have to set up a
struct uio_port in the port[] array.
Here's a description of the fields of struct uio_port:
char *porttype: Required. Set this to one of the predefined
constants. Use UIO_PORT_X86 for the ioports found in x86
architectures.
unsigned long start: Required if the port region is used.
Fill in the number of the first port of this region.
unsigned long size: Fill in the number of ports in this
region. If size is zero, the region is considered unused.
Note that you must initialize size
with zero for all unused regions.
Please do not touch the portio element of
struct uio_port! It is used internally by the UIO
framework to set up sysfs files for this region. 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.
Using uio_pdrv for platform devices
In many cases, UIO drivers for platform devices can be handled in a
generic way. In the same place where you define your
struct platform_device, you simply also implement
your interrupt handler and fill your
struct uio_info. A pointer to this
struct uio_info is then used as
platform_data for your platform device.
You also need to set up an array of struct resource
containing addresses and sizes of your memory mappings. This
information is passed to the driver using the
.resource and .num_resources
elements of struct platform_device.
You now have to set the .name element of
struct platform_device to
"uio_pdrv" to use the generic UIO platform device
driver. This driver will fill the mem[] array
according to the resources given, and register the device.
The advantage of this approach is that you only have to edit a file
you need to edit anyway. You do not have to create an extra driver.
Using uio_pdrv_genirq for platform devices
Especially in embedded devices, you frequently find chips where the
irq pin is tied to its own dedicated interrupt line. In such cases,
where you can be really sure the interrupt is not shared, we can take
the concept of uio_pdrv one step further and use a
generic interrupt handler. That's what
uio_pdrv_genirq does.
The setup for this driver is the same as described above for
uio_pdrv, except that you do not implement an
interrupt handler. The .handler element of
struct uio_info must remain
NULL. The .irq_flags element
must not contain IRQF_SHARED.
You will set the .name element of
struct platform_device to
"uio_pdrv_genirq" to use this driver.
The generic interrupt handler of uio_pdrv_genirq
will simply disable the interrupt line using
disable_irq_nosync(). After doing its work,
userspace can reenable the interrupt by writing 0x00000001 to the UIO
device file. The driver already implements an
irq_control() to make this possible, you must not
implement your own.
Using uio_pdrv_genirq not only saves a few lines of
interrupt handler code. You also do not need to know anything about
the chip's internal registers to create the kernel part of the driver.
All you need to know is the irq number of the pin the chip is
connected to.
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 occurred.
/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.
Generic PCI UIO driver
The generic driver is a kernel module named uio_pci_generic.
It can work with any device compliant to PCI 2.3 (circa 2002) and
any compliant PCI Express device. Using this, you only need to
write the userspace driver, removing the need to write
a hardware-specific kernel module.
Making the driver recognize the device
Since the driver does not declare any device ids, it will not get loaded
automatically and will not automatically bind to any devices, you must load it
and allocate id to the driver yourself. For example:
modprobe uio_pci_generic
echo "8086 10f5" > /sys/bus/pci/drivers/uio_pci_generic/new_id
If there already is a hardware specific kernel driver for your device, the
generic driver still won't bind to it, in this case if you want to use the
generic driver (why would you?) you'll have to manually unbind the hardware
specific driver and bind the generic driver, like this:
echo -n 0000:00:19.0 > /sys/bus/pci/drivers/e1000e/unbind
echo -n 0000:00:19.0 > /sys/bus/pci/drivers/uio_pci_generic/bind
You can verify that the device has been bound to the driver
by looking for it in sysfs, for example like the following:
ls -l /sys/bus/pci/devices/0000:00:19.0/driver
Which if successful should print
.../0000:00:19.0/driver -> ../../../bus/pci/drivers/uio_pci_generic
Note that the generic driver will not bind to old PCI 2.2 devices.
If binding the device failed, run the following command:
dmesg
and look in the output for failure reasons
Things to know about uio_pci_generic
Interrupts are handled using the Interrupt Disable bit in the PCI command
register and Interrupt Status bit in the PCI status register. All devices
compliant to PCI 2.3 (circa 2002) and all compliant PCI Express devices should
support these bits. uio_pci_generic detects this support, and won't bind to
devices which do not support the Interrupt Disable Bit in the command register.
On each interrupt, uio_pci_generic sets the Interrupt Disable bit.
This prevents the device from generating further interrupts
until the bit is cleared. The userspace driver should clear this
bit before blocking and waiting for more interrupts.
Writing userspace driver using uio_pci_generic
Userspace driver can use pci sysfs interface, or the
libpci libray that wraps it, to talk to the device and to
re-enable interrupts by writing to the command register.
Example code using uio_pci_generic
Here is some sample userspace driver code using uio_pci_generic:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
int main()
{
int uiofd;
int configfd;
int err;
int i;
unsigned icount;
unsigned char command_high;
uiofd = open("/dev/uio0", O_RDONLY);
if (uiofd < 0) {
perror("uio open:");
return errno;
}
configfd = open("/sys/class/uio/uio0/device/config", O_RDWR);
if (uiofd < 0) {
perror("config open:");
return errno;
}
/* Read and cache command value */
err = pread(configfd, &command_high, 1, 5);
if (err != 1) {
perror("command config read:");
return errno;
}
command_high &= ~0x4;
for(i = 0;; ++i) {
/* Print out a message, for debugging. */
if (i == 0)
fprintf(stderr, "Started uio test driver.\n");
else
fprintf(stderr, "Interrupts: %d\n", icount);
/****************************************/
/* Here we got an interrupt from the
device. Do something to it. */
/****************************************/
/* Re-enable interrupts. */
err = pwrite(configfd, &command_high, 1, 5);
if (err != 1) {
perror("config write:");
break;
}
/* Wait for next interrupt. */
err = read(uiofd, &icount, 4);
if (err != 4) {
perror("uio read:");
break;
}
}
return errno;
}
Further information
OSADL homepage.
Linutronix homepage.