Message-ID: <CANiDSCvaL-fqyBKWbsD_rMGWmzzqLhYhfioDO-MapO4VOpjXCg@mail.gmail.com>
Date: Tue, 26 Nov 2024 18:29:41 +0100
From: Ricardo Ribalda <ribalda@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Hans de Goede <hdegoede@...hat.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, 
	Sakari Ailus <sakari.ailus@...ux.intel.com>, linux-kernel@...r.kernel.org, 
	linux-media@...r.kernel.org, Yunke Cao <yunkec@...omium.org>, 
	Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [PATCH v2 0/6] media: uvcvideo: Implement the Privacy GPIO as a subdevice

On Tue, 26 Nov 2024 at 18:18, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> On Tue, Nov 26, 2024 at 05:22:20PM +0100, Ricardo Ribalda wrote:
> > On Mon, 25 Nov 2024 at 15:02, Hans de Goede wrote:
> > > On 25-Nov-24 2:39 PM, Ricardo Ribalda wrote:
> > > > On Mon, 25 Nov 2024 at 13:25, Hans de Goede wrote:
> > > >> On 9-Nov-24 5:29 PM, Ricardo Ribalda wrote:
> > > >>
> > > >> <snip>
> > > >>
> > > >>>> I have been discussing UVC power-management with Laurent, also
> > > >>>> related to power-consumption issues caused by libcamera's pipeline
> > > >>>> handler holding open the /dev/video# node as long as the camera
> > > >>>> manager object exists.
> > > >>
> > > >> <snip>
> > > >>
> > > >>>> Here is what I have in mind for this:
> > > >>>>
> > > >>>> 1. Assume that the results of trying a specific fmt do not change over time.
> > > >>>>
> > > >>>> 2. Only allow userspace to request fmts which match one of the enum-fmts ->
> > > >>>>    enum-frame-sizes -> enum-frame-rates tripplet results
> > > >>>>    (constrain what userspace requests to these)
> > > >>>>
> > > >>>> 3. Run the equivalent of tryfmt on all possible combinations (so the usaul
> > > >>>>    3 levels nested loop for this) on probe() and cache the results
> > > >>>>
> > > >>>> 4. Make try_fmt / set_fmt not poweron the device but instead constrain
> > > >>>>    the requested fmt to one from our cached fmts
> > > >>>>
> > > >>>> 5. On stream-on do the actual power-on + set-fmt + verify that we get
> > > >>>>    what we expect based on the cache, and otherwise return -EIO.
> > > >>>
> > > >>> Can we start powering up the device during try/set fmt and then
> > > >>> implement the format caching as an improvement?
> > > >>
> > > >> Yes, actually looking at how complex this is when e.g. also taking
> > > >> controls into account I think that taking small steps is a good idea.
> > > >>
> > > >> I have lately mostly been working on sensor drivers where delaying
> > > >> applying format settings + all controls to stream-on is normal.
> > > >>
> > > >> So that is the mental model I'm applying to uvc here, but that might
> > > >> not be entirely applicable.
> > > >>
> > > >>> Laurent mentioned that some cameras missbehave if a lot of controls
> > > >>> are set during probing. I hope that this approach does not trigger
> > > >>> those, and if it does it would be easier to revert if we do the work
> > > >>> in two steps.
> > > >>
> > > >> Ack, taking small steps sounds like a good plan.
> > > >>
> > > >> <snip>
> > > >>
> > > >>>> This should also make camera enumeration faster for apps, since
> > > >>>> most apps / frameworks do the whole 3 levels nested loop for this
> > > >>>> on startup, for which atm we go out to the hw, which now instead
> > > >>>> will come from the fmts cache and thus will be much much faster,
> > > >>>> so this should lead to a noticeable speedup for apps accessing UVC
> > > >>>> cameras which would be another nice win.
> > > >>>>
> > > >>>> Downside is that the initial probe will take longer see we do
> > > >>>> all the tryfmt-s there now. But I think that taking a bit longer
> > > >>>> to probe while the machine is booting should not be an issue.
> > > >>>
> > > >>> How do you pretend to handle the controls? Do you plan to power-up the
> > > >>> device during s_ctrl() or set them only during streamon()?
> > > >>> If we power-up the device during s_ctrl we need to take care of the
> > > >>> asynchronous controls (typically pan/tilt/zoom), The device must be
> > > >>> powered until the control finishes, and the device might never reply
> > > >>> control_done if the firmware is not properly implemented.
> > > >>> If we set the controls only during streamon, we will break some
> > > >>> usecases. There are some video conferencing equipment that do homing
> > > >>> during streamoff. That will be a serious API breakage.
> > > >>
> > > >> How to handle controls is a good idea.
> > > >>
> > > >> Based on my sensor experience my initial idea was to just cache them
> > > >> all. Basically make set_ctrl succeed but do not actually do anyhing
> > > >> when the camera is not already powered on and then on stream-on call
> > > >> __v4l2_ctrl_handler_setup() to get all current values applied.
> > > >>
> > > >> But as you indicate that will likely not work well with async controls,
> > > >> although we already have this issue when using v4l2-ctl from the cmdline
> > > >> on such a control and that seems to work fine.
> > > >
> > > > -----
> > > >> Just because we allow
> > > >> the USB connection to sleep, does not mean that the camera cannot finish
> > > >> doing applying the async control.
> > > >>
> > > > Not sure what you mean with this sentence. Could you explain it
> > > > differently? Sorry
> > > >
> > > >> But I can see how some cameras might not like this and having 2 different
> > > >> paths for different controls also is undesirable.
> > > >>
> > > >> Combine that with what Laurent said about devices not liking it when
> > > >> you set too much controls in a short time and I do think we need to
> > > >> immediately apply ctrls.
> > > >>
> > > >> I see 2 ways of doing that:
> > > >>
> > > >> 1. Use pm_runtime_set_autosuspend_delay() with a delay of say 1 second
> > > >> and then on set_ctrl do a pm_runtime_get_sync() +
> > > >> pm_runtime_put_autosuspend() giving the camera 1 second to finish
> > > >> applying the async ctrl (which might not be enough for e.g homing) +
> > > >> also avoid doing suspend + resume all the time if multiple ctrls are send
> > > >
> > > > What about 1.5:
> > > >
> > > > during s_ctrl():
> > > > usb_autopm_get_interface()
> > > > if the control is UVC_CTRL_FLAG_ASYNCHRONOUS.
> > > >        usb_autopm_get_interface()
> > > > set the actual control in the hardware
> > > > usb_autopm_put_interface()
> > > >
> > > > during uvc_ctrl_status_event():
> > > >    usb_autopm_put_interface()
> > >
> > > How do we match this to the usb_autopm_get_interface()
> > > call ? At a minimum we would need some counter to
> > > track pending (not acked through status interrupt urb)
> > > async control requests and only do the put() if that
> > > counter >= 1 (and then decrease the counter).
> > >
> > > We don't want to do unbalanced puts here in case of
> > > buggy cameras sending unexpected / too many
> > > ctrl status events.
>
> We would need a counter indeed, which is a big red flag of bad
> engineering. It will be fragile at best.
>
> > > > during close():
> > > >    send all the missing usb_autopm_put_interface()
> > >
> > > Except for my one remark this is an interesting
> > > proposal.
> >
> > I have just upload a patchset implementing this. I tried
> > v4l2-compliance and using the camera app.
> >
> > I think it looks promissing
> >
> > Shall we move the discussion there?
> >
> > https://lore.kernel.org/linux-media/20241126-uvc-granpower-ng-v1-0-6312bf26549c@chromium.org/T/#t
>
> You're sending too many patch series too quickly, even before we can
> come to an agreement on any item being discussed. Experimenting is
> helpful, but if we keep moving the discussion from one series to the
> next, that won't work. Let's keep it here, and focus on one problem at a
> time, or the end result will be slower merging of the patches.

I'd argue that it is better to discuss power management in a series called
"uvcvideo: Implement Granular Power Saving"
than in another called
"media: uvcvideo: Implement the Privacy GPIO as a subdevice"

Those two problems are orthogonal.

I also believe that Hans agreed that that approach was worth exploring....

>
> > > Maybe also do a dev_warn() if there are missing
> > > usb_autopm_put_interface() calls pending on close() ?
> > >
> > > > This way:
> > > > - we do not have an artificial delay that might not work for all the use cases
> > > > - cameras with noncompliant async controls will have the same PM
> > > > behaviour as now  (will be powered on until close() )
> > > >
> > > > We do the same with the rest of the actions that require hardware access, like:
> > > > https://lore.kernel.org/linux-media/20220920-resend-powersave-v5-2-692e6df6c1e2@chromium.org/
> > > >
> > > > This way:
> > > > - Apps that do not need to access the hardware, do not wake it up, and
> > > > we do not break usecases.
> > > >
> > > > Next steps will be:
> > > >  - cache the formats
> > > >  - move the actual set_ctrl to streamon... but if we can do that I
> > > > would argue than we can move completely to the control framework.
> > >
> > > Right I had forgotten that the UVC driver does not use the control
> > > framework. I think moving to that would be a prerequisite for moving
> > > the set_ctrl to stream_on.
>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda

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