linux-kernel - Re: [PATCH v2 16/18] media: v4l2-subdev: De-deprecate init() subdev op

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Message-ID: <YE/X9LLrv1Hq/BL6@pendragon.ideasonboard.com>
Date:   Mon, 15 Mar 2021 23:56:04 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Jacopo Mondi <jacopo+renesas@...ndi.org>
Cc:     kieran.bingham+renesas@...asonboard.com,
        niklas.soderlund+renesas@...natech.se, geert@...ux-m68k.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Sakari Ailus <sakari.ailus@....fi>
Subject: Re: [PATCH v2 16/18] media: v4l2-subdev: De-deprecate init() subdev
 op

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+renesas@...ndi.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