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Date:   Thu, 12 Jul 2018 18:38:21 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     hans.verkuil@...co.com, acourbot@...omium.org,
        sakari.ailus@...ux.intel.com,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        tfiga@...omium.org, posciak@...omium.org,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        nicolas.dufresne@...labora.com, jenskuske@...il.com,
        linux-sunxi@...glegroups.com,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Guenter Roeck <groeck@...omium.org>
Subject: Re: [PATCH 1/9] CHROMIUM: v4l: Add H264 low-level decoder API
 compound controls.

Hi Hans,

Thanks for your feedback, I have a few things I'm not really sure
about though.

On Fri, Jun 15, 2018 at 01:59:58PM +0200, Hans Verkuil wrote:
> > +struct v4l2_ctrl_h264_sps {
> > +	__u8 profile_idc;
> > +	__u8 constraint_set_flags;
> > +	__u8 level_idc;
> > +	__u8 seq_parameter_set_id;
> > +	__u8 chroma_format_idc;
> > +	__u8 bit_depth_luma_minus8;
> > +	__u8 bit_depth_chroma_minus8;
> > +	__u8 log2_max_frame_num_minus4;
> > +	__u8 pic_order_cnt_type;
> > +	__u8 log2_max_pic_order_cnt_lsb_minus4;
> 
> There is a hole in the struct here. Is that OK? Are there alignment
> requirements?

This structure represents an equivalent structure in the H264
bitstream, but it's not a 1:1 mapping, so I don't think there's any
alignment issues.

As of the padding, is it an issue? Isn't it defined by the ABI, and
therefore the userspace will have the same padding rules?

> 
> > +	__s32 offset_for_non_ref_pic;
> > +	__s32 offset_for_top_to_bottom_field;
> > +	__u8 num_ref_frames_in_pic_order_cnt_cycle;
> > +	__s32 offset_for_ref_frame[255];
> 
> Perhaps use a define instead of hardcoding 255? Not sure if that makes sense.
> Same for other arrays below.
> 
> > +	__u8 max_num_ref_frames;
> > +	__u16 pic_width_in_mbs_minus1;
> > +	__u16 pic_height_in_map_units_minus1;
> > +	__u8 flags;
> > +};
> 
> You have to test the struct layout for 32 bit and 64 bit systems
> (the latter for both 64 bit arm and Intel). The layout should be the
> same for all of them since the control framework does not support
> compat32 conversions for compound controls.

I'm not really sure how to test that though? Should I write a program
doing a bunch of offset_of calls to make sure it matches by hand, or
is there anything smarter?

Thanks!
Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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