[libcamera-devel] [PATCH v2] libcamera: v4l2_device: Workaround faulty control menus

[Jacopo Mondi]

Hi Marian

On Wed, Nov 02, 2022 at 09:44:41PM +0100, Marian Buschsieweke wrote:
> Hi,
>
> > If that's the case why only push the default to indices?
>
> see the note at
> https://www.kernel.org/doc/html/v6.0/userspace-api/media/v4l/vidioc-queryctrl.html#description
>
>     It is possible for VIDIOC_QUERYMENU to return an EINVAL error code for some
>     indices between minimum and maximum. In that case that particular menu item
>     is not supported by this driver. [...]
>
> I interpret this as "if VIDIOC_QUERYMENU fails, that menu item is not
> supported" and not as "if VIDIOC_QUERYMENU fails, that menu item is supported
> but has no label". This seems to also be the interpretation behind the
> implementation in libcamera that just removes menu items for which
> VIDIOC_QUERYMENU fail.
>
> I can test this tomorrow to confirm that exposure_auto cannot be set to any of
> [0..3], or whether indeed just the labels are missing.

Thanks this will help understanding what's going wrong here

>
> Assuming the interpretation in the code is correct and in fact the menu
> exposure_auto contains no supported item to pick, it is IMO most sensible to
> either not expose the control for it, or to provide a bogus single-item menu.
> The latter approach resulted in the cleaner and less complex patch (:

Again, not an UVC expert (we have a few here that might provide some
guidance) but I'm surprised the control gets even registered by the
driver...

Ofc, if not value can be set on the control without hitting an error,
I would rather not register the control to libcamera at all...

>
> Kind regards,
> Marian
>
>
> On Wed, 2 Nov 2022 17:22:37 +0100
> Jacopo Mondi <jacopo at jmondi.org> wrote:
>
> > Hi Kieran
> >
> > On Wed, Nov 02, 2022 at 10:38:04AM +0000, Kieran Bingham via libcamera-devel
> > wrote: [...]
> >
> > Not sure what this mean in practice... The bug report says:
> >
> > "Exposure Auto" menu has not a single supported item
> >
> > However it is reported as listing 3 menu entries by v4l2-ctl -l
> >
> >         exposure_auto 0x009a0901 (menu)   : min=0 max=3 default=0 value=0
> >
> > I guess the issue is then that QUERYMENU is not supported ? How does this
> > work in UVC, I guess the uvcdriver supports the ioctl, is it the
> > device fw which wrongly implements the uvcvideo specification ?
> >
> > If that's the case why only push the default to indices ? Aren't min
> > and max selectable too ? (yes, we're going to miss all other values in
> > between min/max which are != default, but if the device is buggy we
> > can't do more than this...)
> >
> > As this fix a user bug I don't want to get into the way of this fix,
> > but just to validate my understanding can you clarify this last part ?
> >
> >  [...]
>



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