linux-kernel - Re: [PATCH] v4l: doc: clarify v4l2_mbus

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Message-ID: <20180108161149.GB23075@bigcity.dyn.berto.se>
Date:   Mon, 8 Jan 2018 17:11:49 +0100
From:   Niklas Söderlund 
        <niklas.soderlund@...natech.se>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Sakari Ailus <sakari.ailus@....fi>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        linux-media@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Archit Taneja <architt@...eaurora.org>,
        Sean Paul <seanpaul@...omium.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] v4l: doc: clarify v4l2_mbus_fmt height definition

On 2018-01-08 16:48:35 +0100, Hans Verkuil wrote:
> On 01/08/2018 04:32 PM, Niklas Söderlund wrote:
> > Hi,
> > 
> > Thanks for your patch.
> > 
> > On 2018-01-08 17:13:53 +0200, Sakari Ailus wrote:
> >> Hi Kieran,
> >>
> >> On Mon, Jan 08, 2018 at 02:45:49PM +0000, Kieran Bingham wrote:
> >>> The v4l2_mbus_fmt width and height corresponds directly with the
> >>> v4l2_pix_format definitions, yet the differences in documentation make
> >>> it ambiguous what to do in the event of field heights.
> >>>
> >>> Clarify this by referencing the v4l2_pix_format which is explicit on the
> >>> matter, and by matching the terminology of 'image height' rather than
> >>> the misleading 'frame height'.
> > 
> > Nice that this relationship is documented as it have contributed to some 
> > confusion on my side in the past!
> > 
> >>>
> >>> Signed-off-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> >>> ---
> >>>  Documentation/media/uapi/v4l/subdev-formats.rst | 6 ++++--
> >>>  include/uapi/linux/v4l2-mediabus.h              | 4 ++--
> >>>  2 files changed, 6 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/Documentation/media/uapi/v4l/subdev-formats.rst b/Documentation/media/uapi/v4l/subdev-formats.rst
> >>> index b1eea44550e1..a2a00202b430 100644
> >>> --- a/Documentation/media/uapi/v4l/subdev-formats.rst
> >>> +++ b/Documentation/media/uapi/v4l/subdev-formats.rst
> >>> @@ -16,10 +16,12 @@ Media Bus Formats
> >>>  
> >>>      * - __u32
> >>>        - ``width``
> >>> -      - Image width, in pixels.
> >>> +      - Image width in pixels. See struct
> >>> +	:c:type:`v4l2_pix_format`.
> >>>      * - __u32
> >>>        - ``height``
> >>> -      - Image height, in pixels.
> >>> +      - Image height in pixels. See struct
> >>> +	:c:type:`v4l2_pix_format`.
> >>>      * - __u32
> >>>        - ``code``
> >>>        - Format code, from enum
> >>> diff --git a/include/uapi/linux/v4l2-mediabus.h b/include/uapi/linux/v4l2-mediabus.h
> >>> index 6e20de63ec59..6b34108d0338 100644
> >>> --- a/include/uapi/linux/v4l2-mediabus.h
> >>> +++ b/include/uapi/linux/v4l2-mediabus.h
> >>> @@ -18,8 +18,8 @@
> >>>  
> >>>  /**
> >>>   * struct v4l2_mbus_framefmt - frame format on the media bus
> >>> - * @width:	frame width
> >>> - * @height:	frame height
> >>> + * @width:	image width
> >>> + * @height:	image height (see struct v4l2_pix_format)
> >>
> >> Hmm. This is the media bus format and it has no direct relation to
> >> v4l2_pix_format. So no, I can't see what would be the point in making such
> >> a reference.
> > 
> > Well we have functions like v4l2_fill_pix_format() that do
> > 
> >     pix_fmt->width = mbus_fmt->width;
> >     pix_fmt->height = mbus_fmt->height;
> > 
> > So I think there at least is an implicit relation between the two 
> > structs. The issue I think Kieran is trying to address is in the case of 
> > TOP, BOTTOM and ALTERNATE field formats. From the v4l2_pix_format 
> > documentation on the height field:
> > 
> >    "Image height in pixels. If field is one of V4L2_FIELD_TOP, 
> >    V4L2_FIELD_BOTTOM or V4L2_FIELD_ALTERNATE then height refers to the 
> >    number of lines in the field, otherwise it refers to the number of 
> >    lines in the frame (which is twice the field height for interlaced 
> >    formats)."
> 
> Right, and I'd just copy this text to subdev-formats.rst rather than referring
> to it.
> 
> > 
> > But there are no such clear definition of the height field for 
> > v4l2_mbus_framefmt. This have cased some confusion for us which would be 
> > nice to clarify. I think it would be a good thing to add to the 
> > documentation if the height in v4l2_mbus_framefmt should describe the 
> > height of a frame or field. And if it should represent the frame height 
> > then v4l2_fill_pix_format() and v4l2_fill_mbus_format() should be 
> > updated to support converting from the two different formats for height.
> 
> And that makes no sense as it would make things even more difficult.
> 
> So I believe it's best to be clear about it and talk about image height
> instead of frame height.

And to be clear, image height for ALTERNATE should be half the frame 
height?

> 
> Just double check first in the code if there are any subdevs that support
> TOP/BOTTOM/ALTERNATE and how they fill in the height, I don't think we
> have any at the moment, but I'm not 100% certain.

A quick look and I find no subdevs which supports TOP or BOTTOM but two 
who supports ALTERNATE, adv748x and tvp5150. And of course they handle 
this differently.

* adv748x - adv748x_afe_fill_format()

  fmt->field = V4L2_FIELD_ALTERNATE;
  fmt->height = afe->curr_norm & V4L2_STD_525_60 ? 480 : 576;
  fmt->height /= 2;

* tvp5150 - tvp5150_fill_fmt()

  f->height = decoder->rect.height;
  f->field = V4L2_FIELD_ALTERNATE;

  Where rect is from tvp5150_probe() and tvp5150_s_std()

  #define TVP5150_V_MAX_525_60    480U
  #define TVP5150_V_MAX_OTHERS    576U

  if (tvp5150_read_std(sd) & V4L2_STD_525_60)
          core->rect.height = TVP5150_V_MAX_525_60;
  else
          core->rect.height = TVP5150_V_MAX_OTHERS

  But it can be changed to any value < TVP5150_V_MAX_OTHERS in 
  tvp5150_set_selection(). And interestingly enough it is reported as 
  half size in .enum_frame_size() - tvp5150_enum_frame_size():

  fse->min_height = decoder->rect.height / 2;
  fse->max_height = decoder->rect.height / 2;

Not sure how this two different solutions best should be handled and 
which one is correct.

> 
> Regards,
> 
> 	Hans
> 
> > 
> >>
> >>>   * @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)
> >>
> >> -- 
> >> Regards,
> >>
> >> Sakari Ailus
> >> e-mail: sakari.ailus@....fi
> > 
> 

-- 
Regards,
Niklas Söderlund