Message-ID: <CAB0kiBLTmybSJH-KDMaYc=DQBatQBf=UeZkE1PGGGuEEgTkObg@mail.gmail.com>
Date: Tue, 13 Aug 2024 08:47:21 -0400
From: Chris Wulff <crwulff@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-usb@...r.kernel.org, Konstantin Aladyshev <aladyshev22@...il.com>, 
	David Sands <david.sands@...mp.com>, Jeff Johnson <quic_jjohnson@...cinc.com>, 
	Christophe JAILLET <christophe.jaillet@...adoo.fr>, linux-kernel@...r.kernel.org, 
	Chris Wulff <Chris.Wulff@...mp.com>
Subject: Re: [PATCH v3] USB: gadget: f_hid: Add GET_REPORT via userspace IOCTL

On Tue, Aug 13, 2024 at 4:38 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Sat, Aug 10, 2024 at 10:18:35AM -0400, crwulff@...il.com wrote:
> > From: Chris Wulff <Chris.Wulff@...mp.com>
> >
> > While supporting GET_REPORT is a mandatory request per the HID
> > specification the current implementation of the GET_REPORT request responds
> > to the USB Host with an empty reply of the request length. However, some
> > USB Hosts will request the contents of feature reports via the GET_REPORT
> > request. In addition, some proprietary HID 'protocols' will expect
> > different data, for the same report ID, to be to become available in the
> > feature report by sending a preceding SET_REPORT to the USB Device that
> > defines what data is to be presented when that feature report is
> > subsequently retrieved via GET_REPORT (with a very fast < 5ms turn around
> > between the SET_REPORT and the GET_REPORT).
> >
> > There are two other patch sets already submitted for adding GET_REPORT
> > support. The first [1] allows for pre-priming a list of reports via IOCTLs
> > which then allows the USB Host to perform the request, with no further
> > userspace interaction possible during the GET_REPORT request. And another
> > [2] which allows for a single report to be setup by userspace via IOCTL,
> > which will be fetched and returned by the kernel for subsequent GET_REPORT
> > requests by the USB Host, also with no further userspace interaction
> > possible.
> >
> > This patch, while loosely based on both the patch sets, differs by allowing
> > the option for userspace to respond to each GET_REPORT request by setting
> > up a poll to notify userspace that a new GET_REPORT request has arrived. To
> > support this, two extra IOCTLs are supplied. The first of which is used to
> > retrieve the report ID of the GET_REPORT request (in the case of having
> > non-zero report IDs in the HID descriptor). The second IOCTL allows for
> > storing report responses in a list for responding to requests.
> >
> > The report responses are stored in a list (it will be either added if it
> > does not exist or updated if it exists already). A flag (userspace_req) can
> > be set to whether subsequent requests notify userspace or not.
> >
> > Basic operation when a GET_REPORT request arrives from USB Host:
> >
> > - If the report ID exists in the list and it is set for immediate return
> >   (i.e. userspace_req == false) then response is sent immediately,
> > userspace is not notified
> >
> > - The report ID does not exist, or exists but is set to notify userspace
> >   (i.e. userspace_req == true) then notify userspace via poll:
> >
> >       - If userspace responds, and either adds or update the response in
> >         the list and respond to the host with the contents
> >
> >       - If userspace does not respond within the fixed timeout (2500ms)
> >         but the report has been set prevously, then send 'old' report
> >         contents
> >
> >       - If userspace does not respond within the fixed timeout (2500ms)
> >         and the report does not exist in the list then send an empty
> >         report
> >
> > Note that userspace could 'prime' the report list at any other time.
> >
> > While this patch allows for flexibility in how the system responds to
> > requests, and therefore the HID 'protocols' that could be supported, a
> > drawback is the time it takes to service the requests and therefore the
> > maximum throughput that would be achievable. The USB HID Specification
> > v1.11 itself states that GET_REPORT is not intended for periodic data
> > polling, so this limitation is not severe.
> >
> > Testing on an iMX8M Nano Ultra Lite with a heavy multi-core CPU loading
> > showed that userspace can typically respond to the GET_REPORT request
> > within 1200ms - which is well within the 5000ms most operating systems seem
> > to allow, and within the 2500ms set by this patch.
> >
> > [1] https://lore.kernel.org/all/20220805070507.123151-2-sunil@amarulasolutions.com/
> > [2] https://lore.kernel.org/all/20220726005824.2817646-1-vi@endrift.com/
> >
> > Signed-off-by: David Sands <david.sands@...mp.com>
> > Signed-off-by: Chris Wulff <chris.wulff@...mp.com>
> > ---
> > v3: rebased to usb-next, checkpatch cleanup (formatting, lore.kernel.org links)
> > v2: https://lore.kernel.org/all/CO1PR17MB541952864266039BAA7BBBD3E10F2@CO1PR17MB5419.namprd17.prod.outlook.com/
> > v1: https://lore.kernel.org/all/20230215231529.2513236-1-david.sands@biamp.com/
> > ---
> >  drivers/usb/gadget/function/f_hid.c | 272 +++++++++++++++++++++++++++-
> >  include/uapi/linux/usb/g_hid.h      |  40 ++++
> >  include/uapi/linux/usb/gadgetfs.h   |   2 +-
> >  3 files changed, 306 insertions(+), 8 deletions(-)
> >  create mode 100644 include/uapi/linux/usb/g_hid.h
>
> Breaks the build:
>
> drivers/usb/gadget/function/f_hid.c:567:6: error: no previous prototype for ‘get_report_workqueue_handler’ [-Werror=missing-prototypes]
>   567 | void get_report_workqueue_handler(struct work_struct *work)
>       |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors
>

I must not have warnings-as-errors enabled. I didn't notice the warning
when I last built it. I will clean that up and make a new patch.

  -- Chris Wulff

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