linux-kernel - Re: [PATCH v7 02/11] media: uapi: h264: Rename pixel format

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Date:   Mon, 19 Aug 2019 11:38:51 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     linux-media@...r.kernel.org, kernel@...labora.com,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Tomasz Figa <tfiga@...omium.org>,
        linux-rockchip@...ts.infradead.org,
        Heiko Stuebner <heiko@...ech.de>,
        Jonas Karlman <jonas@...boo.se>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Alexandre Courbot <acourbot@...omium.org>,
        fbuergisser@...omium.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 02/11] media: uapi: h264: Rename pixel format

On Mon, 2019-08-19 at 14:41 +0200, Paul Kocialkowski wrote:
> Hi,
> 
> On Fri 16 Aug 19, 13:01, Ezequiel Garcia wrote:
> > The V4L2_PIX_FMT_H264_SLICE_RAW name was originally suggested
> > because the pixel format would represent H264 slices without any
> > start code.
> > 
> > However, as we will now introduce a start code menu control,
> > give the pixel format a more meaningful name, while it's
> > still early enough to do so.
> 
> I definitely agree that SLICE_RAW is not the suffix we are looking for, but I'm
> not sure that _SLICE is self-describing given that we can operate either
> per-frame or per-slice, and _SLICE sort of implies the latter.

This is not entirely so, per-frame or per-slice mode refer to the granularity
of the stateless API, the pixel format is still composed of H264 NALU slices.

As per the documentation, the decode_mode and the start_code are modifiers
of the pixel format. As long as this is properly speced, the
V4L2_PIX_FMT_H264_SLICE looks pretty OK to me.

>  Also, VP8 uses
> _FRAME to clearly indicate that it operates per-frame.
> 

This is because VP8 doesn't have any concept of slices, the encoded unit
is a video frame.

> In addition, the _SLICE suffix is used by MPEG-2 in the stable API. Since we
> certainly want MPEG-2 to allow per-slice and per-frame decoding as well as
> H.264 and that the _SLICE format is specified to be the broken "concatenated
> slices" that cedrus expects, we probably want to use another suffix. This way,
> we could deprecated MPEG2_SLICE and introduce a new format for MPEG-2 that would
> have consistent naming with the other mpeg formats.
> 
> One suggestion I had was to call it H264_PARSED (and apply this to MPEG-2 and
> HEVC when similar controls to H.264 are set in place for them). I think Hans had
> another suggestion for the name but I don't recall what it was at this point.
> 
> Either way, if this has to be some debate, we could perhaps take it off your
> series and stay with SLICE_RAW for now, as long as we do rename it before making
> the API stable.
> 
> What do you think?
> 

With the new pixel format modifiers (decode_mode and start_code), the _RAW suffix
now has no meaning. Now, we could name it _PARSED or _SLICE. As long as this is
properly documented (as it is now), that'd be fine.

Now, to be completely honest, this discussion sounds like
bikeshedding to me.

Thanks,
Eze