Re: [RFC 1/3] v4l: Add pixel clock to struct v4l2_mbus_framefmt

[Sakari Ailus <sakari.ailus@iki.fi>]

Hi Sylwester,

Thanks for the comments!

On Thu, Dec 15, 2011 at 10:46:22PM +0100, Sylwester Nawrocki wrote:
> Hi Sakari,
> 
> thanks for the patch.
> 
> On 12/14/2011 04:22 PM, Sakari Ailus wrote:
> > Pixel clock is an essential part of the image data parameters. Add this.
> > Together, the current parameters also define the frame rate.
> > 
> > Sensors do not have a concept of frame rate; pixel clock is much more
> > meaningful in this context. Also, it is best to combine the pixel clock with
> > the other format parameters since there are dependencies between them.
> > 
> > Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
> > ---
> >  Documentation/DocBook/media/v4l/subdev-formats.xml |    9 ++++++++-
> >  include/linux/v4l2-mediabus.h                      |    4 +++-
> >  2 files changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/DocBook/media/v4l/subdev-formats.xml b/Documentation/DocBook/media/v4l/subdev-formats.xml
> > index 49c532e..b4591ef 100644
> > --- a/Documentation/DocBook/media/v4l/subdev-formats.xml
> > +++ b/Documentation/DocBook/media/v4l/subdev-formats.xml
> > @@ -35,7 +35,14 @@
> >  	</row>
> >  	<row>
> >  	  <entry>__u32</entry>
> > -	  <entry><structfield>reserved</structfield>[7]</entry>
> > +	  <entry><structfield>pixel_clock</structfield></entry>
> > +	  <entry>Pixel clock in kHz. This clock is the maximum rate at
> > +	  which pixels are transferred on the bus. The pixel_clock
> > +	  field is read-only.</entry>
> 
> I searched a couple of datasheets to find out where I could use this pixel_clock
> field but didn't find any so far. I haven't tried too hard though ;)
> There seems to be more benefits from having the link frequency control.

There are a few reasons to have the pixel clock available to the user space.

The previously existing reason is that the user may get information on the
pixel rates, including cases where the pixel rate of a subdev isn't enough
for the streaming to be possible. Earlier on it just failed. Such cases are
common on the OMAP 3 ISP, for example.

The second reason is to provide that for timing calculations in the user
space.

> It might be easy to confuse pixel_clock with the bus clock. The bus clock is
> often referred in datasheets as Pixel Clock (PCLK, AFAIU it's described with
> link frequency in your RFC). IMHO your original proposal was better, i.e.
> using more explicit pixel_rate. Also why it is in kHz ? Doesn't it make more
> sense to use bits or pixels  per second ?

Oh, yes, now that you mention it I did call it pixel rate. I'm fine
withrenaming it back to e.g. "pixelrate".

I picked kHz since the 32-bit field would allow rates up to 4 GiP/s. Not
sure if that's overkill though. Could be. But in practice it should give
good enough precision this way, too.

> > +	</row>
> > +	<row>
> > +	  <entry>__u32</entry>
> > +	  <entry><structfield>reserved</structfield>[6]</entry>
> >  	  <entry>Reserved for future extensions. Applications and drivers must
> >  	  set the array to zero.</entry>
> >  	</row>
> > diff --git a/include/linux/v4l2-mediabus.h b/include/linux/v4l2-mediabus.h
> > index 5ea7f75..76a0df2 100644
> > --- a/include/linux/v4l2-mediabus.h
> > +++ b/include/linux/v4l2-mediabus.h
> > @@ -101,6 +101,7 @@ enum v4l2_mbus_pixelcode {
> >   * @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)
> > + * @pixel_clock: pixel clock, in kHz
> >   */
> >  struct v4l2_mbus_framefmt {
> >  	__u32			width;
> > @@ -108,7 +109,8 @@ struct v4l2_mbus_framefmt {
> >  	__u32			code;
> >  	__u32			field;
> >  	__u32			colorspace;
> > -	__u32			reserved[7];
> > +	__u32			pixel_clock;
> 
> I'm wondering, whether it is worth to make it 'pixelclock' for consistency
> with other fields? Perhaps it would make more sense to have color_space and
> pixel_clock.

"pixelrate" is fine for me.

-- 
Sakari Ailus
e-mail: sakari.ailus@iki.fi	jabber/XMPP/Gmail: sailus@retiisi.org.uk