Return-Path: Date: Fri, 16 Mar 2012 20:38:57 -0300 From: Vinicius Costa Gomes To: Marcel Holtmann Cc: David Herrmann , Andre Guedes , linux-input@vger.kernel.org, jkosina@suse.cz, chen.ganir@ti.com, claudio.takahasi@openbossa.org, jprvita@openbossa.org, linux-bluetooth@vger.kernel.org, Vijaykumar.Dadmode@csr.com Subject: Re: [RFC 1/1] HID: User-space I/O driver support for HID subsystem Message-ID: <20120316233857.GA23369@samus> References: <1331909617-22106-1-git-send-email-dh.herrmann@googlemail.com> <1331909617-22106-2-git-send-email-dh.herrmann@googlemail.com> <1331918148.14217.173.camel@aeonflux> <1331930743.14217.187.camel@aeonflux> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1331930743.14217.187.camel@aeonflux> List-ID: 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 > > >> > --- > > >> > 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 > > >> > > >> 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. > > Regards > > Marcel > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-input" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html Cheers, -- Vinicius