From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Prabhakar Lad <prabhakar.csengg@gmail.com>,
        linux-media <linux-media@vger.kernel.org>,
        Hans Verkuil <hans.verkuil@cisco.com>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Sakari Ailus <sakari.ailus@iki.fi>
Subject: Re: [PATCH] media: v4l2-subdev.h: drop the guard CONFIG_VIDEO_V4L2_SUBDEV_API for v4l2_subdev_get_try_*()
Date: Tue, 02 Dec 2014 22:56:18 +0200
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Hi Hans,

On Tuesday 02 December 2014 20:34:22 Hans Verkuil wrote:
> On 12/02/2014 01:59 PM, Laurent Pinchart wrote:
> >>>> Basically,
> >>>> 
> >>>> 1. Create a subdev pad configuration store structure to store the
> >>>> formats and selection rectangles for each pad.
> >>> 
> >>> I wouldn't call it a 'store'. Just call it fmt_config or pad_config
> >>> something like that.
> > 
> > Sure, the name doesn't matter too much.
> > 
> >>>> 2. Embed an instance of that structure in v4l2_subdev_fh.
> >>>> 
> >>>> 3. Modify the subdev pad ops to take a configuration store pointer
> >>>> instead of a file handle pointer.
> >>>> 
> >>>> The userspace API implementation (v4l2-subdev.c) would then pass
> >>>> &fh->store to the pad operations instead of fh.
> >>>> 
> >>>> Bridge drivers that need to implement TRY_FMT on top of pad ops would
> >>>> create a temporary store (or temporary stores when multiple subsdevs
> >>>> are involved), call the pad ops with a pointer to the temporary store
> >>>> to propagate TRY formats, destroy the store(s) and return the resulting
> >>>> format.
> >>> 
> >>> That will work. I think this is a good approach and it shouldn't be too
> >>> difficult.
> >> 
> >> Laurent, just so I understand this correctly: does this mean that all
> >> occurrences of 'struct v4l2_subdev_fh *fh' will be replaced by 'struct
> >> v4l2_subdev_pad_config *cfg'?
> > 
> > That's the plan, yes.
> > 
> >> Is there any reason why the 'fh' should still be passed on?
> > 
> > We might find out reasons to still pass the fh, but in that case I think
> > they should be addressed and the fh just dropped from the pad ops
> > arguments.
> >
> >> Personally I am in favor of this since the 'fh' always made it hard for
> >> bridge drivers to use these pad ops. So if we can replace it by something
> >> that can be used by bridge drivers as well, then that will make it easier
> >> to move all drivers over to the pad ops.
> > 
> > Good, looks like we have a plan for world domination :-)
> 
> OK, so I couldn't help myself and I did this conversion. The code is here:
> 
> http://git.linuxtv.org/cgit.cgi/hverkuil/media_tree.git/log/?h=remcrop
> 
> And it introduces a new struct:
> 
> struct v4l2_subdev_pad_config {
>         struct v4l2_mbus_framefmt try_fmt;
>         struct v4l2_rect try_crop;
>         struct v4l2_rect try_compose;
> };
> 
> And subdev_fh now looks like this:
> 
> struct v4l2_subdev_fh {
>         struct v4l2_fh vfh;
>         struct v4l2_subdev_pad_config *pad;
> };
> 
> But I realized that it could be simplified: right now we pass a pointer to
> the pad config array to the pad ops and the v4l2_subdev_get_try_* functions
> pick the right entry based on the pad index.
> 
> Wouldn't it be easier to pass the right pad config directly?

You can't do that, because format and selection rectangles on a pad can depend 
on the formats and selection rectangles on other pads of the same subdev. For 
instance setting the format on the source pad of a scaler will ignore the 
media bus code and return the code configured on the sink pad. A full 
configuration must thus be passed to the pad operations.

> So instead of passing fh->pad in v4l2-subdev.c you pass e.g. 'fh->pad +
> sel->pad'. That way the pad_config pointer points straight to the
> configuration of the requested pad.
> 
> This also simplifies things in a bridge driver: it doesn't need to allocate
> a pad config array, it only has to create pad config structs for those pads
> that it actually uses.
> 
> And the v4l2_subdev_get_try_* functions can effectively be removed, since
> you would just access cfg->try_fmt directly. I never liked those helper
> functions, so I'm not sorry to see them go.
> 
> The only reason why this would fail is if drivers need to access the
> pad_config of other pads than the one currently requested.

Exactly :-)

> I have not seen any code that does that, though. And I don't think that is
> something we want anyway since that would make things extremely complex.

That's done all over the place, look at the OMAP3 ISP subdev for instance.

> Let me know what you think,

-- 
Regards,

Laurent Pinchart

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