Re: [PATCH 1/1] platform/x86/tuxedo: Add virtual LampArray for TUXEDO NB04 devices

[Benjamin Tissoires]

On Oct 08 2024, Werner Sembach wrote:
> 
> Am 08.10.24 um 11:53 schrieb Benjamin Tissoires:
> > On Oct 07 2024, Werner Sembach wrote:
> > > Hi,
> > > 
> > > Am 02.10.24 um 10:31 schrieb Benjamin Tissoires:
> > > > On Oct 01 2024, Werner Sembach wrote:
> > > > > Hi Benjamin,
> > > > > 
> > > > > Am 01.10.24 um 15:41 schrieb Benjamin Tissoires:
> > > > > > [...]
> > > > > > PPS: sorry for pushing that hard on HID-BPF, but I can see that it fits
> > > > > > all of the requirements here:
> > > > > > - need to be dynamic
> > > > > > - still unsure of the userspace implementation, meaning that userspace
> > > > > >      might do something wrong, which might require kernel changes
> > > > > Well the reference implementetion for the arduiono macropad from microsoft
> > > > > ignores the intensity (brightness) channel on rgb leds contrary to the HID
> > > > > spec, soo yeah you have a point here ...
> > > > Heh :)
> > > > 
> > > > > > - possibility to extend later the kernel API
> > > > > > - lots of fun :)
> > > > > You advertise it good ;). More work for me now but maybe less work for me
> > > > > later, I will look into it.
> > > > Again, I'm pushing this because I see the benefits and because I can
> > > > probably reuse the same code on my Corsair and Logitech keyboards. But
> > > > also, keep in mind that it's not mandatory because you can actually
> > > > attach the BPF code on top of your existing driver to change the way it
> > > > behaves. It'll be slightly more complex if you don't let a couple of
> > > > vendor passthrough reports that we can use to directly talk to the
> > > > device without any tampering, but that's doable. But if you want to keep
> > > > the current implementation and have a different layout, this can easily
> > > > be done in BPF on top.
> > > > 
> > > > Cheers,
> > > > Benjamin
> > > > 
> > > > 
> > > > [0] https://lore.kernel.org/linux-input/20241001-hid-bpf-hid-generic-v3-0-2ef1019468df@xxxxxxxxxx/T/#t
> > > Thinking about the minimal WMI to HID today, but found a problem: a HID
> > > feature report is either strictly input or output afaik, but the WMI
> > > interface has both in some functions.
> > Not sure you are talking about feature reports, because they are
> > read/write. It's just that they are synchronous over the USB control
> > endpoint (on USB).
> 
> I'm confused about the split between get and send feature reports
> https://www.kernel.org/doc/html/latest/hid/hidraw.html
> 
> I guess then a get feature report can also carry input data and the
> difference is that a send feature report doesn't wait for a reply? but then
> what is it's reason of existence in contrast to an output report?

I'm under the impression you are mixing the 3 types of reports (just
re-stating that here in case I wasn't clear).

- Input reports: 
  `Input()` in the report descriptor
  -> data emitted by the device to the host, and notified through an IRQ
  mechanism
  -> obtained in hidraw through a blocking read() operation
- Output reports:
  `Output()` in the report descriptor
  -> data sent asynchronously by the host to the device.
  -> sent from hidraw by calling write() on the dev node (no feedback
  except how many bytes were sent)
- Feature reports:
  `Feature()` in the report descriptor
  -> way to synchronously configure the device. Think of it like a
  register on the device: you can read it, write it, but you never get
  an interrupt when there is a change
  -> read/written by using an ioctl on the hidraw node

And BTW, it's perfectly fine to have a dedicated report ID which has
Input, Output and Feature attached to it :)

> 
> > 
> > An input report is strictly directed from the device, and an output
> > report is from the host to the device.
> > 
> > But a feature report is bidirectional.
> > 
> > > How would I map that?
> > Depending on the WMI interface, if you want this to be synchronous,
> > defining a Feature report is correct, otherwise (if you don't need
> > feedback from WMI), you can declare the commands to WMI as Output
> > reports.
> Thanks for reminding me that output reports exist xD.

hehe

> > 
> > > If I split everything in input and output the new interface wouldn't
> > > actually be much smaller.
> > The HID report descriptor doesn't need to be smaller. The fact that by
> > default it exposes only one or two LEDs so we don't have the micrometers
> > arrays is the only purpose.
> > 
> > But if we also implement a not-full HID implementation of LampArray, we
> > should be able to strip out the parts that we don't care in the LED
> > class implementation, like the exact positioning, or the multiupdate.
> > 
> > > Also what would I write for the usage for the reserved padding in the report
> > > descriptor. Usage: 0x00?
> > padding are ignored by HID. So whatever current usage you have is fine.
> > 
> > However, if you are talking about the custom WMI vendor access, I'd go
> > with a vendor collection (usage page 0xff00, usage 0x08 for the 8 bytes
> > long WMI command for instance, 0x10 for the 16 bytes long one).
> > 
> > Side note: in drivers/hid/bpf/progs/hid_report_helpers.h we have some
> > autogenerated macros to help writing report descriptors (see
> > drivers/hid/bpf/progs/Huion__Dial-2.bpf.c for an example of usage). It's
> > in the hid-bpf tree but I think we might be able to include this in
> > other drivers (or do a minimal rewrite/move into include).
> > I'm not asking you to use it on your code right now, but this has the
> > advantage of becoming less "binary blob" in your code, and prevent
> > mistakes where you edit the comments but not the values.
> 
> I will look into it.
> 
> Since the interface is fixed I don't need to flesh out the whole descriptor
> (which i thought i must do) and usage page (0xff42, because NB04 and the wmi
> has 2 other ec controlling wmi interfaces besides the AB one), report usage
> (matching the wmi comand id's) and report size should be enough.

I'm a little confused by that last sentence. But yeah, I would expect
some minimal sanity check before handing over the HID report to the WMI
interface :)

Cheers,
Benjamin