Re: [PATCH 2/3] media: Documentation: v4l: LINK_FREQ can be an INTEGER64 control

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Hi Hans,

On Fri, Apr 26, 2024 at 09:22:40AM +0200, Hans Verkuil wrote:
> On 20/02/2024 14:03, Sakari Ailus wrote:
> > When a device passes through the video data while still having its own
> > receiver and transmitter, it can use the same frequency as the upstream
> > link does. The Intel MEI CSI device is an example of this. An integer menu
> > control isn't useful in conveying the actual frequency to the receiver in
> > this case.
> > 
> > Document that the LINK_FREQ control may also be a 64-bit integer control.
> > 
> > Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> > ---
> >  .../userspace-api/media/v4l/ext-ctrls-image-process.rst         | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-image-process.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-image-process.rst
> > index b1c2ab2854af..7a3ccb100e1d 100644
> > --- a/Documentation/userspace-api/media/v4l/ext-ctrls-image-process.rst
> > +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-image-process.rst
> > @@ -22,7 +22,7 @@ Image Process Control IDs
> >  
> >  .. _v4l2-cid-link-freq:
> >  
> > -``V4L2_CID_LINK_FREQ (integer menu)``
> > +``V4L2_CID_LINK_FREQ (integer menu or 64-bit integer)``
> >      The frequency of the data bus (e.g. parallel or CSI-2).
> 
> I really think a new control should be created for this.
> 
> As I understand it, V4L2_CID_LINK_FREQ gives a set of supported frequencies,
> and the application has to pick one (I think?). Is it supposed to be a
> read-only control? Some driver seem to set the READ_ONLY flag, and some do not.
> The documentation isn't helpful in that respect.

This is read-only effectively in current IVSC implementation.

> 
> In the case of the Intel MEI CSI and similar devices a new control would be
> better, I think. Do I understand it correctly that for these devices it would
> always be a read-only control? I.e. it just reports the frequency, but applications
> cannot change it?

How would you call the new control?

V4L2_CID_LINK_FREQ_READ_ONLY?

Originally the reason for changing LINK_FREQ for sensors was be part of
changing sensor's configuration to achieve a given frame interval.

-- 
Regards,

Sakari Ailus




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