[RFC] Reserved fields in v4l2_mbus_framefmt, v4l2_subdev_format alignment

Hi Sakari,

On 09/05/2011 05:55 PM, Sakari Ailus wrote:
> Hi all,
> 
> I recently came across a few issues in the definitions of v4l2_subdev_format
> and v4l2_mbus_framefmt when I was working on sensor control that I wanted to
> bring up here. The appropriate structure right now look like this:
> 
> include/linux/v4l2-subdev.h:
> ---8<---
> /**
>   * struct v4l2_subdev_format - Pad-level media bus format
>   * @which: format type (from enum v4l2_subdev_format_whence)
>   * @pad: pad number, as reported by the media API
>   * @format: media bus format (format code and frame size)
>   */
> struct v4l2_subdev_format {
>          __u32 which;
>          __u32 pad;
>          struct v4l2_mbus_framefmt format;
>          __u32 reserved[8];
> };
> ---8<---
> 
> include/linux/v4l2-mediabus.h:
> ---8<---
> /**
>   * struct v4l2_mbus_framefmt - frame format on the media bus
>   * @width:      frame width
>   * @height:     frame height
>   * @code:       data format code (from enum v4l2_mbus_pixelcode)
>   * @field:      used interlacing type (from enum v4l2_field)
>   * @colorspace: colorspace of the data (from enum v4l2_colorspace)
>   */
> struct v4l2_mbus_framefmt {
>          __u32                   width;
>          __u32                   height;
>          __u32                   code;
>          __u32                   field;
>          __u32                   colorspace;
>          __u32                   reserved[7];
> };
> ---8<---
> 
> Offering a lower level interface for sensors which allows better control of
> them from the user space involves providing the link frequency to the user
> space. While the link frequency will be a control, together with the bus
> type and number of lanes (on serial links), this will define the pixel
> clock.
> 
> <URL:http://www.spinics.net/lists/linux-media/msg36492.html>
> 
> After adding pixel clock to v4l2_mbus_framefmt there will be six reserved
> fields left, one of which will be further possibly consumed by maximum image
> size:
> 
> <URL:http://www.spinics.net/lists/linux-media/msg35949.html>

Yes, thanks for remembering about it. I have done some experiments with a sensor
producing JPEG data and I'd like to add '__u32 framesamples' field to struct
v4l2_mbus_framefmt, even though it solves only part of the problem.
I'm not sure when I'll be able to get this finished though. I've just attached
the initial patch now.

> 
> Frame blanking (horizontal and vertical) and number of lanes might be needed
> in the struct as well in the future, bringing the reserved count down to
> two. I find this alarmingly low for a relatively new structure definition
> which will potentially have a few different uses in the future.

Sorry, could you explain why we need to put the blanking information in struct
v4l2_mbus_framefmt ? I thought it had been initially agreed that the control
framework will be used for this.

> 
> The another issue is that the size of the v4l2_subdev_format struct is not
> aligned to a power of two. Instead of the intended 32 u32's, the size is
> actually 22 u32's.

hmm, is this really an issue ? What is advantage of having the structure size
being the power of 2 ? Isn't multiple of 4 just enough ?

> 
> The interface is present in the 3.0 and marked experimental. My proposal is
> to add reserved fields to v4l2_mbus_framefmt to extend its size up to 32
> u32's. I understand there are already few which use the interface right now
> and thus this change must be done now or left as-is forever.

hmm, I feel a bit uncomfortable with increasing size of data structure which
is quite widely used, not only in sensors, also in TV capture cards, tuners, etc.
So far struct v4l2_mbus_framefmt was quite generic. IMHO it might be good to try
to avoid extending it widely with properties specific to single subsystem.

--
Regards,
Sylwester

[RFC] Reserved fields in v4l2_mbus_framefmt, v4l2_subdev_format alignment

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