Hi
On Sat, Mar 17, 2012 at 12:38 AM, Vinicius Costa Gomes
<vinicius.gomes@xxxxxxxxxxxxx> wrote:
> Hi Marcel,
>
> On 13:45 Fri 16 Mar, Marcel Holtmann wrote:
>> Hi David,
>>
>> > >> > This driver allows to write I/O drivers in user-space and feed the input
>> > >> > into the HID subsystem. It operates on the same level as USB-HID and
>> > >> > Bluetooth-HID (HIDP). It does not provide support to write special HID
>> > >> > device drivers but rather provides support for user-space I/O devices to
>> > >> > feed their data into the kernel HID subsystem. The HID subsystem then
>> > >> > loads the HID device drivers for the device and provides input-devices
>> > >> > based on the user-space HID I/O device.
>> > >> >
>> > >> > This driver register a new char-device (/dev/uhid). A user-space process
>> > >> > has to open this file for each device that it wants to provide to the
>> > >> > kernel. It can then use write/read to communicate with the UHID driver.
>> > >> > Both input and output data is sent with a uhid_event structure. The "type"
>> > >> > field of the structure specifies what kind of event is sent. There is a
>> > >> > file in Documentation/ explaining the ABI.
>> > >> >
>> > >> > Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxxxxxxx>
>> > >> > ---
>> > >> > Documentation/hid/uhid.txt | 95 +++++++++
>> > >> > drivers/hid/Kconfig | 21 ++
>> > >> > drivers/hid/Makefile | 2 +-
>> > >> > drivers/hid/uhid.c | 502 ++++++++++++++++++++++++++++++++++++++++++++
>> > >> > include/linux/uhid.h | 71 +++++++
>> > >> > 5 files changed, 690 insertions(+), 1 deletion(-)
>> > >> > create mode 100644 Documentation/hid/uhid.txt
>> > >> > create mode 100644 drivers/hid/uhid.c
>> > >> > create mode 100644 include/linux/uhid.h
>> > >> >
>> > >> > diff --git a/Documentation/hid/uhid.txt b/Documentation/hid/uhid.txt
>> > >> > new file mode 100644
>> > >> > index 0000000..67b138d
>> > >> > --- /dev/null
>> > >> > +++ b/Documentation/hid/uhid.txt
>> > >> > @@ -0,0 +1,95 @@
>> > >> > + UHID - User-space I/O driver support for HID subsystem
>> > >> > + ========================================================
>> > >> > +
>> > >> > +The UHID driver provides an interface for user-space I/O drivers to feed their
>> > >> > +data into the HID subsystem. The HID subsystem then parses the HID reports and
>> > >> > +loads the corresponding HID device driver which then provides the parsed data
>> > >> > +via input-devices to user-space.
>> > >> > +
>> > >> > +This allows user-space to operate on the same level as USB-HID, Bluetooth-HID
>> > >> > +and similar. It does not provide a way to write HID device drivers, though! Use
>> > >> > +HIDRAW for this purpose.
>> > >> > +
>> > >> > +UHID dynamically allocates the minor/major number, meaning that you should rely
>> > >> > +on udev to create the UHID device node. Typically this is created as /dev/uhid.
>> > >> > +
>> > >> > +The UHID API
>> > >> > +------------
>> > >> > +
>> > >> > +For each device that you want to register with the HID core, you need to open a
>> > >> > +separate file-descriptor on /dev/uhid. All communication is done by read()'ing
>> > >> > +or write()'ing "struct uhid_event" objects to the file. Non-blocking operations
>> > >> > +via O_NONBLOCK are supported.
>> > >> > +
>> > >> > +struct uhid_event {
>> > >> > + __u32 type;
>> > >> > + ... payload ...
>> > >> > +};
>> > >> > +
>> > >> > +write()
>> > >> > +-------
>> > >> > +write() allows you to modify the state of the device and feed input data into
>> > >> > +the kernel. The following types are supported: UHID_CREATE, UHID_DESTROY and
>> > >> > +UHID_INPUT.
>> > >> > +
>> > >> > + UHID_CREATE:
>> > >> > + This creates the internal HID device. No I/O is possible until you send this
>> > >> > + event to the kernel. The payload is of type struct uhid_create_req and
>> > >> > + contains information about your device.
>> > >> > +
>> > >> > + UHID_DESTROY:
>> > >> > + This destroys the internal HID device. No further I/O will be accepted. There
>> > >> > + may still be pending messages that you can receive with read() but no further
>> > >> > + UHID_INPUT events can be sent to the kernel.
>> > >> > + You can create a new device by sending UHID_CREATE again. There is no need to
>> > >> > + reopen the character device.
>> > >> > +
>> > >> > + UHID_INPUT:
>> > >> > + You must send UHID_CREATE before sending input to the kernel! This event
>> > >> > + contains a data-payload. This is the raw data that you read from your device.
>> > >> > + The kernel will parse the HID reports and react on it.
>> > >> > +
>> > >> > +read()
>> > >> > +------
>> > >> > +read() will return a queued ouput report. These output reports can be of type
>> > >> > +UHID_START, UHID_STOP, UHID_OPEN, UHID_CLOSE, UHID_OUTPUT or UHID_OUTPUT_EV. No
>> > >> > +reaction is required to any of them but you should handle them according to your
>> > >> > +needs. Only UHID_OUTPUT and UHID_OUTPUT_EV have payloads.
>> > >> > +
>> > >> > + UHID_START:
>> > >> > + This is sent when the HID device is started. Consider this as an answer to
>> > >> > + UHID_CREATE. This is always the first event that is sent. No I/O is possible
>> > >> > + before you read this.
>> > >> > +
>> > >> > + UHID_STOP:
>> > >> > + This is sent when the HID device is stopped. Consider this as an answer to
>> > >> > + UHID_DESTROY. No further I/O will be possible after receiving this.
>> > >> > + If the kernel HID device driver closes the device manually (that is, you
>> > >> > + didn't send UHID_DESTROY) then you should consider this device closed and send
>> > >> > + an UHID_DESTROY event. You may want to reregister your device, though.
>> > >> > +
>> > >> > + UHID_OPEN:
>> > >> > + This is sent when the HID device is opened. That is, the data that the HID
>> > >> > + device provides is read by some other process. You may ignore this event but
>> > >> > + it is useful for power-management. As long as you haven't received this event
>> > >> > + there is actually no other process that reads your data so there is no need to
>> > >> > + send UHID_INPUT events to the kernel.
>> > >> > +
>> > >> > + UHID_CLOSE:
>> > >> > + This is sent when there are no more processes which read the HID data. It is
>> > >> > + the counterpart of UHID_OPEN and you may as well ignore this event.
>> > >> > +
>> > >> > + UHID_OUTPUT:
>> > >> > + This is sent if the HID device driver wants to send raw data to the I/O
>> > >> > + device. You should read the payload and forward it to the device. The payload
>> > >> > + is of type "struct uhid_data_req".
>> > >> > + This may be received even though you haven't received UHID_OPEN, yet.
>> > >> > +
>> > >> > + UHID_OUTPUT_EV:
>> > >> > + Same as UHID_OUTPUT but this contains a "struct input_event" as payload. This
>> > >> > + is called for force-feedback, LED or similar events which are received through
>> > >> > + an input device by the HID subsystem. You should convert this into raw reports
>> > >> > + and send them to your device similar to events of type UHID_OUTPUT.
>> > >> > +
>> > >> > +Document by:
>> > >> > + David Herrmann <dh.herrmann@xxxxxxxxxxxxxx>
>> > >>
>> > >> What do you think about using ioctl() to handle creating, destroying
>> > >> and configuring internal hid devices and leave read() and write() to
>> > >> handle HID reports?
>> >
>> > I need to notify user-space about START/STOP/OPEN/CLOSE/... signals so read()
>> > will always need some event-structure. Therefore, I thought it would be more
>> > consistent if write() would use the same structure. We can also avoid using
>> > ioctl()'s entirely, they're ugly anyway.
>>
>> I like it this way. Just make sure the header is fixed length all the
>> time and we only give one header + data per read() / write().
>>
>> That way we can nicely use scatter gather and avoid complex data copy in
>> userspace for report data.
>>
>> We still have to do some measurement with the latency, but we might just
>> even run HIDP in userspace in the end.
>>
>> > >> This way, at user-space, we wouldn't need to build uhid_event messages
>> > >> for every HID report we get. We would just write() the HID report
>> > >> right away.
>> > >
>> > > we could also just use scatter gather writes and reads. So that is not
>> > > really a problem as long as the "header" has a fixed length.
>> >
>> > What is the problem with building uhid_event structures? Is it performance?
>> > Do you really think one single memcpy() is that important?
>>
>> We have a latency here already and I rather avoid any memcpy if we do
>> not have to. And scatter gather is a nice way to avoid this here.
>>
>> > If the size of the uhid_event structure is a problem, you should have a look at
>> > the internal handling. You can crop the structure if you want. For events like
>> > UHID_CREATE you can simply send a single struct { __u16 type; }; object. You
>> > just need to make sure that if the event-type requires a payload, then
>> > the payload
>> > must be included.
>> > However, I also considered using a pointer instead of the 4096-bytes array so we
>> > would avoid that heavy payload for lazy programmers. I am open for suggestions.
>>
>> I am not following this one. Maybe I confused with word scatter gather,
>> I wanna use recvmsg and sendmsg with iovec. But coming to think about it
>> now, I am not sure that it is actually supported by character devices.
>> Maybe I am just spoiled by sockets.
>
> From the readv/writev manpage:
> "The readv() system call works just like read(2) except that multiple
> buffers are filled.
>
> The writev() system call works just like write(2) except that
> multiple buffers are written out."
>
> From this one may expect that it would work on anything that plain
> read() and write() work. But in the end, I guess that we have to try.
readv() and writev() are handled internally exactly the same as if user-space
would call read() and write() for every single iovec entry. Drivers may provide
aio_read() and aio_write() callbacks, though, which allows them to retrieve the
exact copy of the whole iovec at once. See kernel VFS layer for more
information.
Hence, everything works as expected.
Regards
David
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