Re: [PATCH] HID: Support Microsoft Surface Duo SPI-based touch controller driver as a module.

Hi,

Benjamin Tissoires [off-list ref] writes:

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Benjamin Tissoires [off-list ref] writes:

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On Thu, Aug 12, 2021 at 2:13 AM Dmitry Antipov [off-list ref] wrote:

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Surface Duo uses a touch digitizer that communicates to the main SoC via SPI
and presents itself as a HID device. This patch contains the changes needed
for the driver to work as a module: HID Core affordances for SPI devices,
addition of the new Device IDs, and a new quirk in hid-microsoft. The driver
itself is being prepared for a submission in the near future.

Signed-off-by: Dmitry Antipov dmanti@microsoft.com

Though I really appreciate seeing a microsoft.com submission, the
commit description makes me wonder if we should not postpone the
inclusion of this patch with the "submission in the near future".

AFAIK, HID is not SPI aware. So basically, we are introducing dead
code in the upstream kernel if we take this patch.

Right, unfortunately spec isn't public yet (albeit having products
shipped using it and a driver available in a public tree somewhere I
couldn't find).

I _do_ have one question though:

Is there a way to tell hid core that $this device wants hidraw? If we

Depending on what you want and can do I can think of several solutions:
- add a quirk for this device (either at boot time, either in
hid-quirks.c) There must be one that tells to only bind to hidraw
- provide an out of the tree driver that declares the BUS:VID:PID, and
start the HID device with HIDRAW only.

sounds good

I did some more digging this morning.

The quirk option is not especially good, there is no "hidraw only" quirk.
However, there is a "HID_QUIRK_HAVE_SPECIAL_DRIVER" that prevents
hid-generic from binding to your device. This has the same effect as
adding the device in the hid-quirks.c in this submission, so for
development purposes, if the device is messed up by hid-generic, I
would advise to add this quirk (maybe from the development spi
transport driver as a temporary work around).

To have the device bound to hidraw only, then yes, if you provide a
simple out of the tree module that binds to this module, hid-generic
will release the device and let this out of the tree module deal with
it.

AFAICT, the hid-core changes you are asking here should not block the
development process if they are not merged. You'll get an "UNKOWN" bus
in the logs, and the SPI_HID_DEVICE macro can be defined in the out of
the tree driver.

that's correct, indeed.

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remove the hid-microsoft changes, hid-generic will pick the device as
expected, but this really needs hidraw. Any hints?

I am fine with the hid-microsoft changes, I just want them in a
separate commit. But I don't see why we would take only the first one
without the SPI transport and the hid-microsoft code...

Basically: not sure why you need hidraw for it.

Yeah, the touch controller is "peculiar". It sends to the host raw
frames which needs to be processed by a userspace application. We don't
get coordinates, pressure, etc. We get raw values from the touch
digitizer; these are passed to a daemon which runs your usual filters
(palm rejection, denoising, yada yada yada) and produces the actual
touch inputs. Those are, in turn, given to uinput.

In that case, maybe hidraw is not the best way to forward the events.

V4L has a capability to export raw touch events. You can have a look
at drivers/input/rmi4/rmi_f54.c, drivers/input/touchscreen/sur40.c or
drivers/input/touchscreen/atmel_mxt_ts.c for some examples.
The nice thing is you'll get parallel processing and DMA between the
kernel and userspace. Also, there must be userspace tools around that
are already capable of dealing with that kind of input. Though I also
understand the need to have your own sauce.

Right, I saw those but I'm not sure it applies to spi-hid. OTOH, maybe
spi-hid is just a dumb transport and the V4L2 interface would be
implemented in a hid-microsoft-surface.c driver, or something.

Just to make things clear, the way spi-hid works is like shown
below. First the enumeration phase:

1. power on & reset deassert
2. device asserts interrupt
3. Linux sends ack
4. device responds with a header
5. Linux sends ack (containing body size)
6. device responds with Reset Response
7. Linux requests device descriptor
8. device asserts interrupts
9. Linux sends ack
11. device responds with header
12. Linux sends ack (with body size)
13. device responds with device descriptor

Then an actual transfer.

1. User touches the digitizer
2. device asserts interrupt
3. Linux sends ack
4. device responds with a header
5. Linux sends ack (containing body size)
6. device responds with Data packet (raw digitizer matrix read)

As you can see, there's no polling at a constant rate. Everything is
driven by the device's interrrupts. Also, this is a transport layer
meaning we can have a touch controller, or a keyboard, or a joystick, or
whatever HID device on top of this.

There are also some "hazards" allowed by the spec. For example, the
interrupt is edge triggered and the interrupt can be reasserted after
completion of the ACK prior to the body data portion. I.e., the
following is perfectly valid behavior (assuming rising edge IRQs):

        __                __     ____
IRQ  __/  \______________/  \___/    \_______

         ACK              ACK
MOSI __/XXXXX\__________/XXXXX\______________

                 header             body
MISO ___________/XXXXXX\_________/XXXXXXXX\__


There's also the possibility that the device can reset at any time. The
spec calls these "Device Initiated Reset". The idea for this is to cope
with e.g. such as static discharge causing the touch controller to
reset and so on.

These two little features make it rather hard to make sure that Linux
won't loose any interrupt events and still deal with the fact that the
device can reset at unexpected times. The way we've been dealing with
this in the past was to teardown and recreate the HID device from
scratch. This can result in race conditions between the IRQ handler and
a workqueue which offloads the work of recreating the device. There's a
possibility of race between kernel and userspace here.

Remember that we don't know, ahead of time, which device is attached to
the bus, we have to fetch the device descriptor to get VID/PID pair in
order to start the entire machinery.

I'm saying this because it's very likely that when we submit this driver
publicly (after cleaning it up a bit) we end up creating a bigger
discussion of how to support this new transport layer. We'll work
through the comments, of course, but at the same time we want to reduce
the amount of out-of-tree code we're keeping (for reasons which should
be obvious :-)

That being said, we'll have a look at the V4L2 interface for touch
devices and check how well it would fit for our touch controller :-)

cheers

-- 
balbi