Re: [PATCH v2 5/7] media: i2c: max9286: Configure reverse channel amplitude

[Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>]

Hi Jacopo,

On 15/10/2020 19:27, 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.
> 
> Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> ---
>  drivers/media/i2c/max9286.c | 74 +++++++++++++++++++++++--------------
>  1 file changed, 47 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/media/i2c/max9286.c b/drivers/media/i2c/max9286.c
> index 163e102199e3..4b59a9e0228a 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;
>  
> @@ -436,6 +438,32 @@ static int max9286_check_config_link(struct max9286_priv *priv,
>  	return 0;
>  }
>  
> +static void max9286_reverse_channel_setup(struct max9286_priv *priv,
> +					  unsigned int chan_amplitude)

This looks like you're adding a new function - how about we add this
function here, in the first place when you add it in 3/7

> +{
> +	/* Reverse channel transmission time: default to 1. */
> +	u8 chan_config = MAX9286_REV_TRF(1);
> +
> +	/*
> +	 * Reverse channel setup.
> +	 *
> +	 * - Enable custom reverse channel configuration (through register 0x3f)
> +	 *   and set the first pulse length to 35 clock cycles.
> +	 * - Adjust reverse channel amplitude: values > 130 are programmed
> +	 *   using the additional +100mV REV_AMP_X boost flag
> +	 */
> +	max9286_write(priv, 0x3f, MAX9286_EN_REV_CFG | MAX9286_REV_FLEN(35));
> +
> +	if (chan_amplitude > 100) {
> +		/* It is not possible express values (100 < x < 130) */
> +		chan_amplitude = chan_amplitude < 130
> +			       ? 30 : chan_amplitude - 100;
> +		chan_config |= MAX9286_REV_AMP_X;
> +	}
> +	max9286_write(priv, 0x3b, chan_config | MAX9286_REV_AMP(chan_amplitude));
> +	usleep_range(2000, 2500);
> +}
> +
>  /* -----------------------------------------------------------------------------
>   * V4L2 Subdev
>   */
> @@ -531,10 +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);

is it troublesome to re-set it if it's already set? I guess it's just
unnecessary. so that's fine.

>  	max9286_check_config_link(priv, priv->source_mask);
>  
>  	/*
> @@ -906,32 +938,6 @@ static void max9286_v4l2_unregister(struct max9286_priv *priv)
>   * Probe/Remove
>   */
>  
> -static void max9286_reverse_channel_setup(struct max9286_priv *priv,
> -					  unsigned int chan_amplitude)
> -{
> -	/* Reverse channel transmission time: default to 1. */
> -	u8 chan_config = MAX9286_REV_TRF(1);
> -
> -	/*
> -	 * Reverse channel setup.
> -	 *
> -	 * - Enable custom reverse channel configuration (through register 0x3f)
> -	 *   and set the first pulse length to 35 clock cycles.
> -	 * - Adjust reverse channel amplitude: values > 130 are programmed
> -	 *   using the additional +100mV REV_AMP_X boost flag
> -	 */
> -	max9286_write(priv, 0x3f, MAX9286_EN_REV_CFG | MAX9286_REV_FLEN(35));
> -
> -	if (chan_amplitude > 100) {
> -		/* It is not possible express values (100 < x < 130) */
> -		chan_amplitude = chan_amplitude < 130
> -			       ? 30 : chan_amplitude - 100;
> -		chan_config |= MAX9286_REV_AMP_X;
> -	}
> -	max9286_write(priv, 0x3b, chan_config | MAX9286_REV_AMP(chan_amplitude));
> -	usleep_range(2000, 2500);
> -}
> -
>  static int max9286_setup(struct max9286_priv *priv)
>  {
>  	/*
> @@ -967,7 +973,15 @@ 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.
> +	 */
> +	if (priv->high_threshold)
> +		max9286_reverse_channel_setup(priv, 170);
> +	else
> +		max9286_reverse_channel_setup(priv, 100);

Hrm... ternery is more concise here, but is it helpful?

  max9286_reverse_channel_setup(priv, priv->high_threshold ? 170 : 100);

The high-threshold could also be parsed in
max9286_reverse_channel_setup(), but I like it being passed in.

>  
>  	/*
>  	 * Enable GMSL links, mask unused ones and autodetect link
> @@ -1235,6 +1249,12 @@ 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");
> +

Oh, I think I like this, it's a neat way to express what it needs to do
from the DT depending on the attached cameras.

It's sort of dependant upon the cameras though, I guess making this
something that we query from the remote endpoint isn't so easy ...?



>  	priv->route_mask = priv->source_mask;
>  
>  	return 0;
>