[PATCH 06/15] drm/dp: Read AUX read interval from DPCD

[Lyude Paul]

On Tue, 2019-10-15 at 16:35 +0200, Thierry Reding wrote:
> From: Thierry Reding <treding at nvidia.com>
> 
> Store the AUX read interval from DPCD, so that it can be used to wait
> for the durations given in the specification during link training.
> 
> Signed-off-by: Thierry Reding <treding at nvidia.com>
> ---
>  include/drm/drm_dp_helper.h | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> index 7a537ffc2fb1..6c12de6f7e46 100644
> --- a/include/drm/drm_dp_helper.h
> +++ b/include/drm/drm_dp_helper.h
> @@ -25,8 +25,11 @@
>  
>  #include <linux/delay.h>
>  #include <linux/i2c.h>
> +#include <linux/time64.h>
>  #include <linux/types.h>
>  
> +#include <drm/drm_print.h>
> +
>  /*
>   * Unless otherwise noted, all values are from the DP 1.1a spec.  Note that
>   * DP and DPCD versions are independent.  Differences from 1.0 are not
> noted,
> @@ -1297,6 +1300,36 @@ drm_dp_alternate_scrambler_reset_cap(const u8
> dpcd[DP_RECEIVER_CAP_SIZE])
>  			DP_ALTERNATE_SCRAMBLER_RESET_CAP;
>  }
>  
> +/**
> + * drm_dp_read_aux_interval() - read the AUX read interval from the DPCD
> + * @dpcd: receiver capacity buffer
> + *
> + * Reads the AUX read interval (in microseconds) from the DPCD. Note that
> the
> + * TRAINING_AUX_RD_INTERVAL stores the value in units of 4 milliseconds. If
> no
> + * read interval is specified and for DPCD v1.4 and later, the read
> interval
> + * is always 100 microseconds.
> + *
> + * Returns:
> + * The read AUX interval in microseconds.
> + */
> +static inline unsigned int
> +drm_dp_aux_rd_interval(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
> +{
> +	unsigned int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> +					DP_TRAINING_AUX_RD_MASK;
> +
> +	if (rd_interval > 4)
> +		DRM_DEBUG_KMS("AUX interval %u, out of range (max: 4)\n",
> +			      rd_interval);
Do you think it might be worth clamping the value to 4 here?

> +
> +	if (rd_interval > 0 && dpcd[DP_DPCD_REV] < DP_DPCD_REV_14)

Also small  nit pick: you can just use rd_interval instead of rd_interval > 0
> +		rd_interval *= 4 * USEC_PER_MSEC;
> +	else
> +		rd_interval = 100;
> +
> +	return rd_interval;
> +}
> +
>  /*
>   * DisplayPort AUX channel
>   */
-- 
Cheers,
	Lyude Paul