Hi Jacopo,
Thank you for the patch.
On Mon, Mar 15, 2021 at 02:15:10PM +0100, Jacopo Mondi wrote:
> The init() subdev core operation is deemed to be deprecated for new
> subdevice drivers. However it could prove useful for complex
> architectures to defer operation that require access to the
> communication bus if said bus is not available (or fully configured)
> at the time when the subdevice probe() function is run.
>
> As an example, the GMSL architecture requires the GMSL configuration
> link to be configured on the host side after the remote subdevice
> has completed its probe function. After the configuration on the host
> side has been performed, the subdevice registers can be accessed through
> the communication bus.
>
> In particular:
>
> HOST REMOTE
>
> probe()
> |
> ---------------------> |
> probe() {
> bus config()
> }
> |<--------------------|
> v4l2 async bound {
> bus config()
> call subdev init()
> |-------------------->|
> init() {
> access register on the bus()
> }
> |<-------------------
> }
>
> In the GMSL use case the bus configuration requires the enablement of the
> noise immunity threshold on the remote side which ensures reliability
> of communications in electrically noisy environments. After the subdevice
> has enabled the threshold at the end of its probe() sequence the host
> side shall compensate it with an higher signal amplitude. Once this
> sequence has completed the bus can be accessed with noise protection
> enabled and all the operations that require a considerable number of
> transactions on the bus (such as the image sensor configuration
> sequence) are run in the subdevice init() operation implementation.
I think this can be considered as a reasonable use of .init(). I'd like
to get feedback from Hans and Sakari (CC'ed) though.
> Signed-off-by: Jacopo Mondi <jacopo+rene...@jmondi.org>
> ---
> include/media/v4l2-subdev.h | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> index d0e9a5bdb08b..3068d9940669 100644
> --- a/include/media/v4l2-subdev.h
> +++ b/include/media/v4l2-subdev.h
> @@ -148,9 +148,18 @@ struct v4l2_subdev_io_pin_config {
> * each pin being configured. This function could be called at times
> * other than just subdevice initialization.
> *
> - * @init: initialize the sensor registers to some sort of reasonable default
> - * values. Do not use for new drivers and should be removed in existing
> - * drivers.
> + * @init: initialize the subdevice registers to some sort of reasonable
> default
> + * values. Do not use for new drivers (and should be removed in existing
> + * ones) for regular architectures where the image sensor is connected to
> + * the host receiver. For more complex architectures where the subdevice
> + * initialization should be deferred to the completion of the probe
> + * sequence of some intermediate component, or the communication bus
> + * requires configurations on the host side that depend on the completion
> + * of the probe sequence of the remote subdevices, the usage of this
> + * operation could be considered to allow the devices along the pipeline to
> + * probe and register in the media graph and to defer any operation that
> + * require actual access to the communication bus to their init() function
> + * implementation.
> *
> * @load_fw: load firmware.
> *
--
Regards,
Laurent Pinchart
