linux-kernel - Re: [PATCH v3 5/7] media: i2c: max9286: Configure reverse channel amplitude

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Message-ID: <b27d0984-a1c5-d8a5-6e5a-1fefc3b87fd5@ideasonboard.com>
Date:   Fri, 16 Oct 2020 12:04:29 +0100
From:   Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
To:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        laurent.pinchart+renesas@...asonboard.com,
        niklas.soderlund+renesas@...natech.se
Cc:     linux-media@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Hyun Kwon <hyunk@...inx.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Subject: Re: [PATCH v3 5/7] media: i2c: max9286: Configure reverse channel
 amplitude

Hi Jacopo,

On 16/10/2020 13:06, Jacopo Mondi wrote:
> Adjust reverse channel amplitude according to the presence of
> the 'high-threshold" DTS property.
> 
> If no high threshold compensation is required, start with a low
> amplitude (100mV) and increase it after the remote serializers
> have probed and have enabled noise immunity on their reverse
> channels.
> 
> If high threshold compensation is required, configure the reverse
> channel with a 170mV amplitude before the remote serializers have
> probed.
> 
> This change is required for both rdacm20 and rdacm21 camera modules
> to be correctly probed when used in combination with the max9286
> deserializer.

My only fear here would be that perhaps on other cameras we need a more
fine-grained control of the amplitudes?

But I'll leave that discussion to the binding itself,

For this patch ...

Reviewed-by: Kieran Bingham <kieran.bingham@...asonboard.com>


> 
> Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> ---
>  drivers/media/i2c/max9286.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/i2c/max9286.c b/drivers/media/i2c/max9286.c
> index 31e27d0f34f1..4c72e1e6b27b 100644
> --- a/drivers/media/i2c/max9286.c
> +++ b/drivers/media/i2c/max9286.c
> @@ -163,6 +163,8 @@ struct max9286_priv {
>  	unsigned int mux_channel;
>  	bool mux_open;
>  
> +	bool high_threshold;
> +
>  	struct v4l2_ctrl_handler ctrls;
>  	struct v4l2_ctrl *pixelrate;
>  
> @@ -557,10 +559,14 @@ static int max9286_notify_bound(struct v4l2_async_notifier *notifier,
>  	 * All enabled sources have probed and enabled their reverse control
>  	 * channels:
>  	 *
> +	 * - Increase the reverse channel amplitude to compensate for the
> +	 *   remote ends high threshold, if not done already
>  	 * - Verify all configuration links are properly detected
>  	 * - Disable auto-ack as communication on the control channel are now
>  	 *   stable.
>  	 */
> +	if (!priv->high_threshold)
> +		max9286_reverse_channel_setup(priv, 170);
>  	max9286_check_config_link(priv, priv->source_mask);
>  
>  	/*
> @@ -967,7 +973,12 @@ static int max9286_setup(struct max9286_priv *priv)
>  	 * only. This should be disabled after the mux is initialised.
>  	 */
>  	max9286_configure_i2c(priv, true);
> -	max9286_reverse_channel_setup(priv, 170);
> +
> +	/*
> +	 * Compensate the remote end high threshold with a larger channel
> +	 * amplitude if necessary.
> +	 */
> +	max9286_reverse_channel_setup(priv, priv->high_threshold ? 170 : 100);
>  
>  	/*
>  	 * Enable GMSL links, mask unused ones and autodetect link
> @@ -1235,6 +1246,12 @@ static int max9286_parse_dt(struct max9286_priv *priv)
>  	}
>  	of_node_put(node);
>  
> +	/*
> +	 * Parse 'high_threshold' property to configure the reverse channel
> +	 * amplitude.
> +	 */
> +	priv->high_threshold = device_property_present(dev, "high_threshold");
> +
>  	priv->route_mask = priv->source_mask;
>  
>  	return 0;
>