linux-kernel - Re: [PATCH v2 12/12] media: uvcvideo: Do not create MC entities for virtual entities

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Message-ID: <20250714143617.GK8243@pendragon.ideasonboard.com>
Date: Mon, 14 Jul 2025 17:36:17 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Hans Verkuil <hverkuil@...all.nl>, Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Bartosz Golaszewski <brgl@...ev.pl>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Len Brown <lenb@...nel.org>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	devicetree@...r.kernel.org, linux-gpio@...r.kernel.org,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2 12/12] media: uvcvideo: Do not create MC entities for
 virtual entities

On Tue, Jul 08, 2025 at 08:28:21AM +0200, Ricardo Ribalda wrote:
> On Tue, 1 Jul 2025 at 13:20, Ricardo Ribalda wrote:
> > On Sun, 29 Jun 2025 at 20:06, Laurent Pinchart wrote:
> > > Hi Ricardo,
> > >
> > > Thank you for the patch.
> > >
> > > I would use "software entities" and not "virtual entities" in the
> > > subject line and everywhere else, as those entities are not virtual.
> > >
> > > On Thu, Jun 05, 2025 at 05:53:05PM +0000, Ricardo Ribalda wrote:
> > > > Neither the GPIO nor the SWENTITY entities form part of the device
> > > > pipeline. We just create them to hold emulated uvc controls.
> > > >
> > > > When the device initializes, a warning is thrown by the v4l2 core:
> > > > uvcvideo 1-1:1.0: Entity type for entity SWENTITY was not initialized!
> > > >
> > > > There are no entity function that matches what we are doing here, and
> > > > it does not make to much sense to create a function for entities that
> > > > do not really exist.
> > >
> > > I don't agree with this. The purpose of reporting entities to userspace
> > > through the MC API is to let application enumerate what entities a
> > > device contains. Being able to enumerate software entities seems as
> > > useful as being able to enumerate hardware entities.
> >
> > What function shall we use in this case? Nothing here seems to match a
> > software entity
> > https://www.kernel.org/doc/html/latest/userspace-api/media/mediactl/media-types.html
> >
> > Any suggestion for name?
> > Shall we just live with the warning in dmesg?
> 
>  I just realised that if/when we move to the control framework, the
> software entity will be gone.... So to avoid introducing a uAPI change
> that will be reverted later I think that we should keep this patch.

You know my opinion about moving to the control framework, so that's not
a very compelling argument :-)

We could use MEDIA_ENT_F_PROC_VIDEO_PIXEL_FORMATTER, as that's the
function already used by XUs, and the SWENTITY fulfills the same role as
XUs in some devices.

> > > > Do not create MC entities for them and pretend nothing happened here.
> > > >
> > > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > > ---
> > > >  drivers/media/usb/uvc/uvc_entity.c | 10 ++++++++++
> > > >  1 file changed, 10 insertions(+)
> > > >
> > > > diff --git a/drivers/media/usb/uvc/uvc_entity.c b/drivers/media/usb/uvc/uvc_entity.c
> > > > index d1a652ef35ec34801bd39a5124b834edf838a79e..2dbeb4ab0c4c8cc763ff2dcd2d836a50f3c6a040 100644
> > > > --- a/drivers/media/usb/uvc/uvc_entity.c
> > > > +++ b/drivers/media/usb/uvc/uvc_entity.c
> > > > @@ -72,6 +72,16 @@ static int uvc_mc_init_entity(struct uvc_video_chain *chain,
> > > >  {
> > > >       int ret;
> > > >
> > > > +     /*
> > > > +      * Do not initialize virtual entities, they do not really exist
> > > > +      * and are not connected to any other entities.
> > > > +      */
> > > > +     switch (UVC_ENTITY_TYPE(entity)) {
> > > > +     case UVC_EXT_GPIO_UNIT:
> > > > +     case UVC_SWENTITY_UNIT:
> > > > +             return 0;
> > > > +     }
> > > > +
> > > >       if (UVC_ENTITY_TYPE(entity) != UVC_TT_STREAMING) {
> > > >               u32 function;
> > > >

-- 
Regards,

Laurent Pinchart