Re: [PATCH v5 1/3] drm/msm/dp: cleared DP_DOWNSPREAD_CTRL register before start link training

[Dmitry Baryshkov]

On 12/09/2022 19:23, Kuogee Hsieh wrote:
> DOWNSPREAD_CTRL (0x107) shall be cleared to 0 upon power-on reset or an
> upstream device disconnect. This patch will enforce this rule by always
> cleared DOWNSPREAD_CTRL register to 0 before start link training. At rare
> case that DP MSA timing parameters may be mis-interpreted by the sink
> which causes audio sampling rate be calculated wrongly and cause audio
> did not work at sink if DOWNSPREAD_CTRL register is not cleared to 0.
>
> Changes in v2:
> 1) fix spelling at commit text
> 2) merge ssc variable into encoding[0]
>
> Changes in v3:
> -- correct spelling of DOWNSPREAD_CTRL
> -- replace err with len of ssize_t
>
> Changes in v4:
> -- split into 2 patches
>
> Fixes: 154b5a7da0fd ("drm/msm/dp: add displayPort driver support")
> Signed-off-by: Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx>
> ---
>   drivers/gpu/drm/msm/dp/dp_ctrl.c | 13 +++++--------
>   1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> index ab6aa13..2c74c59 100644
> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> @@ -1245,8 +1245,7 @@ static int dp_ctrl_link_train(struct dp_ctrl_private *ctrl,
>   {
>   	int ret = 0;
>   	const u8 *dpcd = ctrl->panel->dpcd;
> -	u8 encoding = DP_SET_ANSI_8B10B;
> -	u8 ssc;
> +	u8 encoding[] = { 0, DP_SET_ANSI_8B10B };
>   	u8 assr;
>   	struct dp_link_info link_info = {0};
>   
> @@ -1258,13 +1257,11 @@ static int dp_ctrl_link_train(struct dp_ctrl_private *ctrl,
>   
>   	dp_aux_link_configure(ctrl->aux, &link_info);
>   
> -	if (drm_dp_max_downspread(dpcd)) {
> -		ssc = DP_SPREAD_AMP_0_5;
> -		drm_dp_dpcd_write(ctrl->aux, DP_DOWNSPREAD_CTRL, &ssc, 1);
> -	}
> +	if (drm_dp_max_downspread(dpcd))
> +		encoding[0] |= DP_SPREAD_AMP_0_5;

It would be simpler to call drm_dp_dpcd_write(ssc, DP_DOWNSPREAD_CTRL, 
1) unconditionally here. You won't have to change the 
encoding/DP_MAIN_LINK_CHANNEL_CODING_SET/etc.

>   
> -	drm_dp_dpcd_write(ctrl->aux, DP_MAIN_LINK_CHANNEL_CODING_SET,
> -				&encoding, 1);
> +	/* config DOWNSPREAD_CTRL and MAIN_LINK_CHANNEL_CODING_SET */
> +	drm_dp_dpcd_write(ctrl->aux, DP_DOWNSPREAD_CTRL, encoding, 2);
>   
>   	if (drm_dp_alternate_scrambler_reset_cap(dpcd)) {
>   		assr = DP_ALTERNATE_SCRAMBLER_RESET_ENABLE;

--
With best wishes
Dmitry