Message-ID: <20241114230630.GE31681@pendragon.ideasonboard.com>
Date: Fri, 15 Nov 2024 01:06:30 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Armin Wolf <W_Armin@....de>, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, Yunke Cao <yunkec@...omium.org>,
	Hans Verkuil <hverkuil@...all.nl>, stable@...r.kernel.org,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v3 0/8] media: uvcvideo: Implement the Privacy GPIO as a
 evdev

Hello,

On Thu, Nov 14, 2024 at 08:21:26PM +0100, Ricardo Ribalda wrote:
> On Wed, 13 Nov 2024 at 18:57, Hans de Goede wrote:
> > On 12-Nov-24 6:30 PM, Ricardo Ribalda wrote:
> > > Some notebooks have a button to disable the camera (not to be mistaken
> > > with the mechanical cover). This is a standard GPIO linked to the
> > > camera via the ACPI table.
> > >
> > > 4 years ago we added support for this button in UVC via the Privacy control.
> > > This has three issues:
> > > - If the camera has its own privacy control, it will be masked.
> > > - We need to power-up the camera to read the privacy control gpio.
> > > - Other drivers have not followed this approach and have used evdev.
> > >
> > > We tried to fix the power-up issues implementing "granular power
> > > saving" but it has been more complicated than anticipated...
> > >
> > > This patchset implements the Privacy GPIO as a evdev.
> > >
> > > The first patch of this set is already in Laurent's tree... but I
> > > include it to get some CI coverage.
> > >
> > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > ---
> > > Changes in v3:
> > > - CodeStyle (Thanks Sakari)
> > > - Re-implement as input device
> >
> > Thank you for your enthusiasm for my suggestion to implement this
> > as an input device.
> 
> I wanted to give it a try... and it turned out to be quite simple to
> implement. I thought it could be a good idea to share it, so we can
> have something tangible to talk about ;).
> 
> > As I mentioned in my reply in the v2 thread, the goal of my
> > enumeration of various way camera privacy-controls are exposed to
> > userspace today is to try and get everyone to agree on a single
> > userspace API for this.
> >
> > Except for this v3 patch-set, which I take as an implied vote
> > from you (Ricardo) for the evdev SW_CAMERA_LENS_COVER approach,
> > we have not heard anything on this subject from Sakari or Laurent
> > yet. So for now I would like to first focus on / circle back to
> > the userspace API discussion and then once we have a plan for
> > the userspace API we can implement that for uvcvideo.
> >
> > First lets look at the API question top down, iow what use-cases
> > do we expect there to be for information about the camera-privacy
> > switch state:
> >
> > a) Having an app which is using (trying to use) the camera show
> > a notification to the user that the camera is turned-off by
> > a privacy switch .
> >
> > Ricardo, AFAICT this is the main use-case for chrome-os, do I have
> > this right ?
> 
> b) is as important as a) for us.  If you do not give instant feedback
> to the user when they change the status of the camera they might not
> be able to find the button later on :)

How do you handle cameras that suffer from
UVC_QUIRK_PRIVACY_DURING_STREAM ?

> > b) Showing on on-screen-display (OSD) with a camera /
> > crossed-out-camera icon when the switch is toggled, similar to how
> > muting speakers/mic show an OSD . Laptop vendor Windows add-on
> > software does this and I know that some users have been asking
> > for this.
> >
> > Then lets look at the question bottom-up which hardware interfaces
> > do we have exposing this information:
> >
> > 1. Internal UVC camera with an input privacy GPIO resource in
> > the ACPI fwnode for the UVC camera, with the GPIO reporting
> > the privacy-switch state. Found on some chrome-books

Ricardo, is this found only in ACPI-based (x86) chromebooks, or also in
DT-based chromebooks ?

Can we assume that the UVC module will not be powered off (and therefore
disappear from USB) when the privacy switch is toggled to disable the
camera ?

> > 2. Laptop firmware (EC/ACPI/WMI) which reports privacy-switch
> > state, without a clear 1:1 relation between the reported state and
> > which camera it applies to. In this case sometimes the whole UVC
> > camera module (if it is UVC) is simply dropped of the bus when
> > the camera is disabled through the privacy switch, removing
> > the entire /dev/video# node for the camera. Found on many windows
> > laptops.
> >
> > 3. UVC cameras which report privacy-switch status through
> > a UVC_CT_PRIVACY_CONTROL. Found on ... ?
> 
> Some logitech cameras and also internal ones.
> 
> > Note this will only work while the camera is streaming and
> > even then may require polling of the ctrl because not all
> > cameras reliably send UVC status messages when it changes.
> > This renders this hardware interface as not usable

In general I agree, but maybe the situation is better with the UVC
cameras that implement UVC_CT_PRIVACY_CONTROL ?

Note that, in theory, and as far as I understand, it should be possible
to get the UVC_CT_PRIVACY_CONTROL events when the camera is not
streaming, if the device implement remote wakeup. In practice that's
hardly ever the case, among the ~450 sets of USB descriptors I've
collected over time, only 8 report support for remote wakeup in the
configuration descriptor's bmAttributes field, and I'm not even sure we
could trust those devices to implement this feature correctly.

Ricardo, do you know if the internal UVC cameras used in chromebooks
that implement UVC_CT_PRIVACY_CONTROL support remote wakeup to notify
changes in the privacy control when the camera is suspended ?

> > Currently there are 2 ways this info is being communicated
> > to userspace, hw-interfaces 1. + 3. are exposed as a v4l2
> > privacy-ctrl where as hw-if 2. uses and input evdev device.
> >
> > The advantage of the v4l2 privacy-ctrl is that it makes it
> > very clear which camera is controlled by the camera
> > privacy-switch.
> >
> > The disadvantage is that it will not work for hw-if 2,
> > because the ACPI / WMI drivers have no v4l2 device to report
> > the control on. We could try to add some magic glue code,
> > but even then with e.g. IPU6 cameras it would still be
> > unclear which v4l2(sub)device we should put the control on
> > and if a UVC camera is just dropped from the bus there is
> > no /dev/video# device at all.

Is there any ACPI- or WMI-provided information that could assist with
associating a privacy GPIO with a camera ?

> > Using an input device does not has this disadvantage and
> > it has the advantage of not requiring to power-up the camera
> > as currently happens with a v4l2 ctrl on a UVC camera.

API-wise, and with the current uvcvideo implementation, I agree. We
could of course also try to improve the uvcvideo driver to not power the
device unless it is streaming (depending on whether or not the known
drawbacks are considered acceptable).

Devices in the 3rd category will still need to be powered up to report
the status of the privacy control, as well as some devices in the 1st
category (see patch 8/8 in this series that introduces
UVC_QUIRK_PRIVACY_DURING_STREAM).

> > But using an input device makes it harder to determine
> > which camera the privacy-switch applies to.

We could include the evdev in the MC graph. That will of course only be
possible if the kernel knows about that association in the first place.
At least the 1st category of devices would benefit from this.

> > We can specify
> > that SW_CAMERA_LENS_COVER only applies to device internal
> > cameras, but then it is up to userspace to determine which
> > cameras that are.
> 
> I am working on wiring up this to userspace right now.. I will report
> back if it cannot do it.
> 
> > Another problem with using an input device is that it will
> > not work for "UVC cameras which report privacy-switch status
> > through a UVC_CT_PRIVACY_CONTROL." since those need the camera
> > on and even then need to be polled to get a reliable reading.
> >
> > Taking this all into account my proposal would be to go
> > with an input device and document that SW_CAMERA_LENS_COVER
> > only applies to device internal cameras.
> >
> > This should work well for both use-cases a) and b) described
> > above and also be easy to support for both hw interfaces
> > 1. and 2.
> >
> > My proposal for hw-if 3. (UVC_CT_PRIVACY_CONTROL) would be
> > to keep reporting this as V4L2_CID_PRIVACY. This means it
> > will not work out of the box for userspace which expects
> > the input device method, but giving the limitations of
> > this hw interface I think that requiring userspace to have
> > to explicitly support this use-case (and e.g. poll the
> > control) is a good thing rather then a bad thing.
> >
> > Still before moving forward with switching the hw-if 1.
> > case to an input device as this patch-series does I would
> > like to hear input from others.
> >
> > Sakari, Laurent, any comments ?

Assuming the kernel could report the association between an evdev and
camera, we would need to report which evdev SW_CAMERA_LENS_COVER
originates from all the way from the evdev to the consumer of the event.
How well is that supported in standard Linux system architectures ? If
I'm not mistaken libinput will report the originating device, but how
far up the stack is it propagated ? And which component would we expect
to consume those events, should the camera evdev be managed by e.g.
libcamera ?

> > > - Make the code depend on UVC_INPUT_EVDEV
> > > - Link to v2: https://lore.kernel.org/r/20241108-uvc-subdev-v2-0-85d8a051a3d3@chromium.org
> > >
> > > Changes in v2:
> > > - Rebase on top of https://patchwork.linuxtv.org/project/linux-media/patch/20241106-uvc-crashrmmod-v6-1-fbf9781c6e83@chromium.org/
> > > - Create uvc_gpio_cleanup and uvc_gpio_deinit
> > > - Refactor quirk: do not disable irq
> > > - Change define number for MEDIA_ENT_F_GPIO
> > > - Link to v1: https://lore.kernel.org/r/20241031-uvc-subdev-v1-0-a68331cedd72@chromium.org
> > >
> > > ---
> > > Ricardo Ribalda (8):
> > >       media: uvcvideo: Fix crash during unbind if gpio unit is in use
> > >       media: uvcvideo: Factor out gpio functions to its own file
> > >       media: uvcvideo: Re-implement privacy GPIO as an input device
> > >       Revert "media: uvcvideo: Allow entity-defined get_info and get_cur"
> > >       media: uvcvideo: Create ancillary link for GPIO subdevice
> > >       media: v4l2-core: Add new MEDIA_ENT_F_GPIO
> > >       media: uvcvideo: Use MEDIA_ENT_F_GPIO for the GPIO entity
> > >       media: uvcvideo: Introduce UVC_QUIRK_PRIVACY_DURING_STREAM
> > >
> > >  .../userspace-api/media/mediactl/media-types.rst   |   4 +
> > >  drivers/media/usb/uvc/Kconfig                      |   2 +-
> > >  drivers/media/usb/uvc/Makefile                     |   3 +
> > >  drivers/media/usb/uvc/uvc_ctrl.c                   |  40 +-----
> > >  drivers/media/usb/uvc/uvc_driver.c                 | 112 +---------------
> > >  drivers/media/usb/uvc/uvc_entity.c                 |  21 ++-
> > >  drivers/media/usb/uvc/uvc_gpio.c                   | 144 +++++++++++++++++++++
> > >  drivers/media/usb/uvc/uvc_status.c                 |  13 +-
> > >  drivers/media/usb/uvc/uvc_video.c                  |   4 +
> > >  drivers/media/usb/uvc/uvcvideo.h                   |  31 +++--
> > >  drivers/media/v4l2-core/v4l2-async.c               |   3 +-
> > >  include/uapi/linux/media.h                         |   1 +
> > >  12 files changed, 223 insertions(+), 155 deletions(-)
> > > ---
> > > base-commit: 1b3bb4d69f20be5931abc18a6dbc24ff687fa780
> > > change-id: 20241030-uvc-subdev-89f4467a00b5

-- 
Regards,

Laurent Pinchart

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