linux-kernel - Re: [PATCH v3 1/2] media: v4l: Add definitions for the HEVC slice format and controls

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Date:   Fri, 19 Apr 2019 09:22:51 +0200
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     Hans Verkuil <hverkuil@...all.nl>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...glegroups.com
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Randy Li <ayaka@...lik.info>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>
Subject: Re: [PATCH v3 1/2] media: v4l: Add definitions for the HEVC slice
 format and controls

Hi,

On Tue, 2019-04-02 at 13:51 +0200, Hans Verkuil wrote:
> On 2/14/19 10:53 AM, Paul Kocialkowski wrote:

[...]

> > +.. c:type:: v4l2_hevc_dpb_entry
> > +
> > +.. cssclass:: longtable
> > +
> > +.. flat-table:: struct v4l2_hevc_dpb_entry
> > +    :header-rows:  0
> > +    :stub-columns: 0
> > +    :widths:       1 1 2
> > +
> > +    * - __u32
> > +      - ``buffer_tag``
> > +      - The V4L2 buffer tag that matches the associated reference picture.
> 
> This struct documentation is out of date with the actual struct!

Woops, sorry about that. I'll make another pass on the structures vs
docs before submitting the next version.

> > +#define V4L2_HEVC_DPB_ENTRY_RPS_ST_CURR_BEFORE	0x01
> > +#define V4L2_HEVC_DPB_ENTRY_RPS_ST_CURR_AFTER	0x02
> > +#define V4L2_HEVC_DPB_ENTRY_RPS_LT_CURR		0x03
> > +
> > +#define V4L2_HEVC_DPB_ENTRIES_NUM_MAX		16
> > +
> > +struct v4l2_hevc_dpb_entry {
> > +	__u64	timestamp;
> 
> I would call this 'reference_ts' rather than plain 'timestamp'.
> Or at least some other name that reflects what the timestamp
> refers to.

Well, I think using "ts" instead of "timestamp" is much less explicit
(I'm really not a big fan of abbreviations...).

Also, this field is in the dpb entry structure and since a dpb entry
describes a reference, I find it quite straightforward already without
any added prefix. The prefix feels redundant with the name of the
structure that holds the element.

I think the same should apply for mpeg-2 and h.264.

> > +	__u8	rps;
> > +	__u8	field_pic;
> > +	__u16	pic_order_cnt[2];
> > +	__u8	padding[2];
> 
> padding fields here or elsewhere are not documented. Do you really
> need them? What if you changed rps and field_pic to __u16 instead?
> 
> Padding fields are painful to maintain in my experience.

I think we'll need to rework the controls to include flags and such in
the not-too-distant future. So there will be an occasion to change it
then (the same work also needs to be done for MPEG-2).

If you prefer that we make the change now and later update it along
with the controls, I can definitely do that too.

Cheers,

Paul

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com