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:

quoted

+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?

quoted

+	__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.

quoted

+	__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