linux-kernel - Re: [PATCH] drm/dp: Avoid Reading DPCD

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Date:   Fri, 02 Sep 2022 17:35:00 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     Fangzhi Zuo <Jerry.Zuo@....com>, Daniel Vetter <daniel@...ll.ch>,
        Ville Syrjala <ville.syrjala@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Alex Deucher <alexander.deucher@....com>,
        Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org
Subject: Re: [PATCH] drm/dp: Avoid Reading DPCD_REV Before Native Aux Read

Reviewed-by: Lyude Paul <lyude@...hat.com>

On Wed, 2022-08-31 at 18:13 -0400, Fangzhi Zuo wrote:
> The attempt to read DPCD_REV before any native aux read breaks
> majority of DP2 compliance.
> 
> The spec. requires DP_SINK_STATUS to be polled for the reset status
> DP_INTRA_HOP_AUX_REPLY_INDICATION during the clear training stage.
> 
> Polling DP_SINK_STATUS each time gets DPCD_REV read first
> that makes non link training regsiter DPCD_REV get read
> during UHBR link training. It violates DP2 compliance.
> 
> Cc: Ville Syrjala <ville.syrjala@...ux.intel.com>
> Cc: Lyude Paul <lyude@...hat.com>
> Signed-off-by: Fangzhi Zuo <Jerry.Zuo@....com>
> ---
>  drivers/gpu/drm/display/drm_dp_helper.c | 18 ------------------
>  1 file changed, 18 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index e7c22c2ca90c..c7aa5bafa667 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -571,24 +571,6 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
>  {
>  	int ret;
>  
> -	/*
> -	 * HP ZR24w corrupts the first DPCD access after entering power save
> -	 * mode. Eg. on a read, the entire buffer will be filled with the same
> -	 * byte. Do a throw away read to avoid corrupting anything we care
> -	 * about. Afterwards things will work correctly until the monitor
> -	 * gets woken up and subsequently re-enters power save mode.
> -	 *
> -	 * The user pressing any button on the monitor is enough to wake it
> -	 * up, so there is no particularly good place to do the workaround.
> -	 * We just have to do it before any DPCD access and hope that the
> -	 * monitor doesn't power down exactly after the throw away read.
> -	 */
> -	if (!aux->is_remote) {
> -		ret = drm_dp_dpcd_probe(aux, DP_DPCD_REV);
> -		if (ret < 0)
> -			return ret;
> -	}
> -
>  	if (aux->is_remote)
>  		ret = drm_dp_mst_dpcd_read(aux, offset, buffer, size);
>  	else

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat