Re: [PATCH v2 4/7] media: i2c: max9286: Make channel amplitude programmable

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

Hi Jacopo,

On 15/10/2020 19:27, Jacopo Mondi wrote:
> Instrument the function that configures the reverse channel with a
> programmable amplitude value.
> 
> This change serves to prepare to adjust the reverse channel amplitude
> depending on the remote end high-threshold configuration.
> 
> Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> ---
>  drivers/media/i2c/max9286.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/media/i2c/max9286.c b/drivers/media/i2c/max9286.c
> index 89a7248f5c25..163e102199e3 100644
> --- a/drivers/media/i2c/max9286.c
> +++ b/drivers/media/i2c/max9286.c
> @@ -906,19 +906,29 @@ static void max9286_v4l2_unregister(struct max9286_priv *priv)
>   * Probe/Remove
>   */
>  
> -static void max9286_reverse_channel_setup(struct max9286_priv *priv)
> +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.
> -	 * - Increase the reverse channel amplitude to 170mV to accommodate the
> -	 *   high threshold enabled by the serializer driver.
> +	 * - 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));
> -	max9286_write(priv, 0x3b, MAX9286_REV_TRF(1) | MAX9286_REV_AMP(70) |
> -		      MAX9286_REV_AMP_X);
> +

Should we also clamp to min/max values at all?
Probably not needed, as it's only an internal helper.


> +	if (chan_amplitude > 100) {
> +		/* It is not possible express values (100 < x < 130) */

'possible to express'

> +		chan_amplitude = chan_amplitude < 130
> +			       ? 30 : chan_amplitude - 100;

We could round < 115 to 100, and >= 115 to 130, but that's probably more
effort that it's worth, so I think this is fine.

I think it's really helpful to codify this parameter though:

Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

> +		chan_config |= MAX9286_REV_AMP_X;
> +	}
> +	max9286_write(priv, 0x3b, chan_config | MAX9286_REV_AMP(chan_amplitude));
>  	usleep_range(2000, 2500);
>  }
>  
> @@ -957,7 +967,7 @@ 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);
> +	max9286_reverse_channel_setup(priv, 170);
>  
>  	/*
>  	 * Enable GMSL links, mask unused ones and autodetect link
>