Re: [RFC 2/7] media: v4l: subdev: Support INTERNAL_SOURCE pads in routing IOCTLs

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Mon, Jun 05, 2023 at 08:06:58AM +0000, Sakari Ailus wrote:
> On Sun, Jun 04, 2023 at 05:26:26PM +0300, Laurent Pinchart wrote:
> > On Fri, Jun 02, 2023 at 01:10:40PM +0000, Sakari Ailus wrote:
> > > On Fri, Jun 02, 2023 at 12:46:50PM +0300, Laurent Pinchart wrote:
> > > > On Fri, Jun 02, 2023 at 12:44:12PM +0300, Laurent Pinchart wrote:
> > > > > On Mon, May 08, 2023 at 03:24:10PM +0300, Sakari Ailus wrote:
> > > > > > On Mon, May 08, 2023 at 01:14:07PM +0300, Tomi Valkeinen wrote:
> > > > > > > On 06/05/2023 00:52, Sakari Ailus wrote:
> > > > > > > > Take the new INTERNAL_SOURCE pad flag into account in validating routing
> > > > > > > > IOCTL argument. Effectively this is a SINK pad in this respect. Due to the
> > > > > > > > union there's no need to check for the particular name.
> > > > > > > 
> > > > > > > What union? The one you add in the next patch?
> > > > > > 
> > > > > > Oops. I think we can reorder the patches.
> > > > > > 
> > > > > > > As a concept the internal source pads sound good, and they work in practice
> > > > > > > in my tests. But this union is what grates me a bit. We have a flag,
> > > > > > > MEDIA_PAD_FL_INTERNAL_SOURCE, which tells which field of the union to use,
> > > > > > > and then we go and do not use the new union field. Well, and also the
> > > > > > > naming, as we normally have source and sink pads, but now we also have
> > > > > > > internal source pads, which are actually identical to sink pads...
> > > > > > 
> > > > > > The union still should be used by the user space. We're testing flags here
> > > > > > and those flags are the same independently of the INTERNAL_SOURCE flag.
> > > > > > 
> > > > > > I'm fine by not adding that union though, but for the user space I think
> > > > > > it's better we have it: explaining that the sink_pad has a different
> > > > > > meaning if that flag is set is harder than making it a union member.
> > > > > > 
> > > > > > > I understand the idea and reasoning, but the two points above do confuse me
> > > > > > > and I'm sure would confuse others also.
> > > > > > > 
> > > > > > > I wonder if it would be less or more confusing to simplify this by just
> > > > > > > adding a MEDIA_PAD_FL_INTERNAL, which could be applied to either a source or
> > > > > > > a sink pad, and would prevent linking to it. The flag would indicate that
> > > > > > > the stream from/to that pad is generated/consumed internally. (I don't know
> > > > > > > when you would need an internal pad to consume data, but... who knows, the
> > > > > > > need might pop up =).
> > > > > > 
> > > > > > This is another option. But I envision there will be more compatibility
> > > > > > issues. Although... generally using such hardware requires knowledge of the
> > > > > > device, and we haven't obviously had any support for devices needing this
> > > > > > functionality in the tree. So in the end it might not matter much.
> > > > > >
> > > > > > > That would mean that an "internal sink pad" would generate a data stream,
> > > > > > > i.e. it's a "source", but I think a normal sink pad is almost the same
> > > > > > > anyway: when considering entity's internal routing, the normal sink pad is a
> > > > > > > "source", and the same logic would apply with the internal pads too.
> > > > > > > 
> > > > > > > An internal sink pad that generates a stream is a bit more confusing than an
> > > > > > > internal source pad, but I think that confusion is less than the rest of the
> > > > > > > confusion in the code-side that comes with the internal source pads.
> > > > > > 
> > > > > > I prefer having the possible sources of the confusion in the framework than
> > > > > > in the drivers. Therefore I think INTERNAL_SOURCE flag is a (slightly)
> > > > > > better option.
> > > > > > 
> > > > > > I wonder what Llaurent thinks.
> > > > > 
> > > > > I like the idea of a MEDIA_PAD_FL_INTERNAL flag. That's actually how I
> > > > > read patch 1/7, I didn't notice it used MEDIA_PAD_FL_INTERNAL*_SOURCE*
> > > > > :-)
> > > > > 
> > > > > We could define MEDIA_PAD_FL_INTERNAL_SOURCE
> > > > > 
> > > > > #define MEDIA_PAD_FL_INTERNAL_SOURCE 	(MEDIA_PAD_FL_SINK | MEDIA_PAD_FL_INTERNAL)
> > > > 
> > > > One option would be to find terms different from sink and source in this
> > > > case. It would generate less confusion to map (e.g., not a really good
> > > > name) MEDIA_PAD_FL_INTERNAL_PRODUCER to MEDIA_PAD_FL_SINK and to the
> > > > sink_pad field than using MEDIA_PAD_FL_INTERNAL_SOURCE.
> > > 
> > > I don't think this helps as you'd still be accessing the sink pad related
> > > fields that are there for another purpose.
> > > 
> > > Alternatively I'd have the (plain) INTERNAL flag and drop the union,
> > > without masking or adding compound flags.
> > > 
> > > I can switch to that if you promise not to change your mind again. ;-)
> > 
> > I'll do my best :-) Could you show the impact (if any) it would have on
> > drivers when accessing the routing fields ?
> 
> I don't think there's much of an impact for the drivers. It's mainly
> affecting setting up pads for the entities. Tomi?

I think it would be useful to discuss it with an actual sensor driver
using the API.

-- 
Regards,

Laurent Pinchart