The Userspace I/O HOWTO¶
Author: | Hans-Jürgen Koch Linux developer, Linutronix |
---|---|
Date: | 2006-12-11 |
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 bymmap()
to get to the actual device memory. This is important if the device’s memory is not page aligned. Remember that pointers returned bymmap()
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 withmmap()
. For each mapping you need to fill one of theuio_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 theuio_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, setirq
toUIO_IRQ_CUSTOM
. If you had no interrupt at all, you could setirq
toUIO_IRQ_NONE
, though this rarely makes sense.unsigned long irq_flags
: Required if you’ve setirq
to a hardware interrupt number. The flags given here will be used in the call torequest_irq()
.int (*mmap)(struct uio_info *info, struct vm_area_struct *vma)
: Optional. If you need a specialmmap()
function, you can set it here. If this pointer is not NULL, yourmmap()
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 ownopen()
, 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 ownopen()
, you will probably also want a customrelease()
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 parameterirq_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
:
const char *name
: Optional. Set this to help identify the memory region, it will show up in the corresponding sysfs node.int memtype
: Required if the mapping is used. Set this toUIO_MEM_PHYS
if you you have physical memory on your card to be mapped. UseUIO_MEM_LOGICAL
for logical memory (e.g. allocated withkmalloc()
). There’s alsoUIO_MEM_VIRTUAL
for virtual memory.phys_addr_t addr
: Required if the mapping is used. Fill in the address of your memory block. This address is the one that appears in sysfs.resource_size_t size
: Fill in the size of the memory block thataddr
points to. Ifsize
is zero, the mapping is considered unused. Note that you must initializesize
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 likeioremap()
. Addresses returned by this function cannot be mapped to user space, so you must not store it inaddr
. Useinternal_addr
instead to remember such an address.
Please do not touch the map
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. UseUIO_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. Ifsize
is zero, the region is considered unused. Note that you must initializesize
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.
Using uio_dmem_genirq for platform devices¶
In addition to statically allocated memory ranges, they may also be a
desire to use dynamically allocated regions in a user space driver. In
particular, being able to access memory made available through the
dma-mapping API, may be particularly useful. The uio_dmem_genirq
driver provides a way to accomplish this.
This driver is used in a similar manner to the "uio_pdrv_genirq"
driver with respect to interrupt configuration and handling.
Set the .name
element of struct platform_device
to
"uio_dmem_genirq"
to use this driver.
When using this driver, fill in the .platform_data
element of
struct platform_device
, which is of type
struct uio_dmem_genirq_pdata
and which contains the following
elements:
struct uio_info uioinfo
: The same structure used as theuio_pdrv_genirq
platform dataunsigned int *dynamic_region_sizes
: Pointer to list of sizes of dynamic memory regions to be mapped into user space.unsigned int num_dynamic_regions
: Number of elements indynamic_region_sizes
array.
The dynamic regions defined in the platform data will be appended to the
`` mem[] `` array after the platform device resources, which implies
that the total number of static and dynamic memory regions cannot exceed
MAX_UIO_MAPS
.
The dynamic memory regions will be allocated when the UIO device file,
/dev/uioX
is opened. Similar to static memory resources, the memory
region information for dynamic regions is then visible via sysfs at
/sys/class/uio/uioX/maps/mapY/*
. The dynamic memory regions will be
freed when the UIO device file is closed. When no processes are holding
the device file open, the address returned to userspace is ~0.
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 you’re 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 library 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 (configfd < 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;
}
Generic Hyper-V UIO driver¶
The generic driver is a kernel module named uio_hv_generic. It supports devices on the Hyper-V VMBus similar to uio_pci_generic on PCI bus.
Making the driver recognize the device¶
Since the driver does not declare any device GUID’s, 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, to use the network device class GUID:
modprobe uio_hv_generic
echo "f8615163-df3e-46c5-913f-f2d2f965ed0e" > /sys/bus/vmbus/drivers/uio_hv_generic/new_id
If there already is a hardware specific kernel driver for the device, the generic driver still won’t bind to it, in this case if you want to use the generic driver for a userspace library you’ll have to manually unbind the hardware specific driver and bind the generic driver, using the device specific GUID like this:
echo -n ed963694-e847-4b2a-85af-bc9cfc11d6f3 > /sys/bus/vmbus/drivers/hv_netvsc/unbind
echo -n ed963694-e847-4b2a-85af-bc9cfc11d6f3 > /sys/bus/vmbus/drivers/uio_hv_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/vmbus/devices/ed963694-e847-4b2a-85af-bc9cfc11d6f3/driver
Which if successful should print:
.../ed963694-e847-4b2a-85af-bc9cfc11d6f3/driver -> ../../../bus/vmbus/drivers/uio_hv_generic
Things to know about uio_hv_generic¶
On each interrupt, uio_hv_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.
When host rescinds a device, the interrupt file descriptor is marked down and any reads of the interrupt file descriptor will return -EIO. Similar to a closed socket or disconnected serial device.
- The vmbus device regions are mapped into uio device resources:
- Channel ring buffers: guest to host and host to guest
- Guest to host interrupt signalling pages
- Guest to host monitor page
- Network receive buffer region
- Network send buffer region
If a subchannel is created by a request to host, then the uio_hv_generic device driver will create a sysfs binary file for the per-channel ring buffer. For example:
/sys/bus/vmbus/devices/3811fe4d-0fa0-4b62-981a-74fc1084c757/channels/21/ring