Re: [PATCH v7 01/10] media: v4l2-subdev: Introduce [get|set]_mbus_config pad ops

[Janusz Krzysztofik <jmkrzyszt@gmail.com>]

Hi Jacopo,

On Thursday, July 16, 2020 4:27:04 P.M. CEST Jacopo Mondi wrote:
> Introduce two new pad operations to allow retrieving and configuring the
> media bus parameters on a subdevice pad.
> 
> The newly introduced operations aims to replace the s/g_mbus_config video
> operations, which have been on their way for deprecation since a long
> time.
> 
> Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> ---
>  include/media/v4l2-subdev.h | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> index f7fe78a6f65a..d8b9d5735307 100644
> --- a/include/media/v4l2-subdev.h
> +++ b/include/media/v4l2-subdev.h
> @@ -670,6 +670,29 @@ struct v4l2_subdev_pad_config {
>   *
>   * @set_frame_desc: set the low level media bus frame parameters, @fd array
>   *                  may be adjusted by the subdev driver to device capabilities.
> + *
> + * @get_mbus_config: get the media bus configuration of a remote sub-device.
> + *		     The media bus configuration is usually retrieved from the
> + *		     firmware interface at sub-device probe time, immediately
> + *		     applied to the hardware and eventually adjusted by the
> + *		     driver. Remote sub-devices (usually video receivers) shall
> + *		     use this operation to query the transmitting end bus
> + *		     configuration in order to adjust their own one accordingly.
> + *		     Callers should make sure they get the most up-to-date as
> + *		     possible configuration from the remote end, likely calling
> + *		     this operation as close as possible to stream on time. The
> + *		     operation shall fail if the pad index it has been called on
> + *		     is not valid.
> + *
> + * @set_mbus_config: set the media bus configuration of a remote sub-device.
> + *		     This operations is intended to allow, in combination with
> + *		     the get_mbus_config operation, the negotiation of media bus
> + *		     configuration parameters between media sub-devices. The
> + *		     operation shall not fail if the requested configuration is
> + *		     not supported, but the driver shall update the content of
> + *		     the %config argument to reflect what has been actually
> + *		     applied to the hardware. The operation shall fail if the
> + *		     pad index it has been called on is not valid.

Could this description also clarify what results are expected in case of 
hardware errors?  The ov6650 implementation you propose may suggest such
errors may be expected to be ignored silently as long as current configuration 
can be successfully obtained from hardware and passed back to the caller.

Moreover, since validity of the pad argument is expected to be verified, I 
think this should be handled by the media infrastructure layer with the 
drivers/media/v4l2-core/v4l2-subdev.c:check_pad() helper called from a 
.set_mbus_config() wrapper added to v4l2_subdev_call_pad_wrappers, freeing 
drivers from reimplementing it.

Thanks,
Janusz

>   */
>  struct v4l2_subdev_pad_ops {
>  	int (*init_cfg)(struct v4l2_subdev *sd,
> @@ -710,6 +733,10 @@ struct v4l2_subdev_pad_ops {
>  			      struct v4l2_mbus_frame_desc *fd);
>  	int (*set_frame_desc)(struct v4l2_subdev *sd, unsigned int pad,
>  			      struct v4l2_mbus_frame_desc *fd);
> +	int (*get_mbus_config)(struct v4l2_subdev *sd, unsigned int pad,
> +			       struct v4l2_mbus_config *config);
> +	int (*set_mbus_config)(struct v4l2_subdev *sd, unsigned int pad,
> +			       struct v4l2_mbus_config *config);
>  };
>  
>  /**
>