[PATCH] drm/i915/hdcp: Avoid duplicate HDCP enables

[Ramalingam C]

On 2020-05-20 at 15:47:44 -0400, Sean Paul wrote:
> From: Sean Paul <seanpaul at chromium.org>
> 
> If userspace sets the CP property to DESIRED while it's already ENABLED,
> the driver will try to re-enable HDCP. On some displays, this will
> result in R0' mismatches. I'm guessing this is because the display is
> still sending back Ri instead of re-authenticating.
> 
> At any rate, we can fix this inefficiency easily enough by just nooping
> the DESIRED property set if HDCP is already ENABLED.
Sean,

This will skip the hdcp enable.

But at present too we will be getting below WARN_ON from intel_hdcp_enable,
to indicate userspace is going wrong with request.
        drm_WARN_ON(&dev_priv->drm,
                    hdcp->value == DRM_MODE_CONTENT_PROTECTION_ENABLED);

And if we need to filter this out, could we validate the incoming hdcp request at
drm_atomic_connector_set_property() itself? No point in going into the
atomic commit without a valid request. something like

diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
index a1e5e262bae2..d98b2eeae78d 100644
--- a/drivers/gpu/drm/drm_atomic_uapi.c
+++ b/drivers/gpu/drm/drm_atomic_uapi.c
@@ -746,6 +746,12 @@ static int drm_atomic_connector_set_property(struct drm_connector *connector,
                        DRM_DEBUG_KMS("only drivers can set CP Enabled\n");
                        return -EINVAL;
                }
+               if (config->content_protection_property ==
+                   DRM_MODE_CONTENT_PROTECTION_ENABLED &&
+                   val == DRM_MODE_CONTENT_PROTECTION_DESIRED) {
+                       DRM_DEBUG_KMS("Redundant req for content protection\n");
+                       return -EINVAL;
+               }
                state->content_protection = val;
        } else if (property == config->hdcp_content_type_property) {
                state->hdcp_content_type = val;

-Ram

> 
> Signed-off-by: Sean Paul <seanpaul at chromium.org>
> ---
> 
> I suspect this is the actual root cause I was chasing with
> "drm/i915/hdcp: Add additional R0' wait". I was able to reproduce the
> R0` messages by marking HDCP desired while it was already enabled. This
> _should_ work, but it seems like some displays handle it more graciously
> than others.
> 
> 
>  drivers/gpu/drm/i915/display/intel_hdcp.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_hdcp.c b/drivers/gpu/drm/i915/display/intel_hdcp.c
> index 2cbc4619b4ce..f770fe0c5595 100644
> --- a/drivers/gpu/drm/i915/display/intel_hdcp.c
> +++ b/drivers/gpu/drm/i915/display/intel_hdcp.c
> @@ -2156,12 +2156,16 @@ void intel_hdcp_atomic_check(struct drm_connector *connector,
>  	}
>  
>  	/*
> -	 * Nothing to do if the state didn't change, or HDCP was activated since
> -	 * the last commit. And also no change in hdcp content type.
> +	 * Nothing to do if content type is unchanged and one of:
> +	 *  - state didn't change
> +	 *  - HDCP was activated since the last commit
> +	 *  - attempting to set to desired while already enabled
>  	 */
>  	if (old_cp == new_cp ||
>  	    (old_cp == DRM_MODE_CONTENT_PROTECTION_DESIRED &&
> -	     new_cp == DRM_MODE_CONTENT_PROTECTION_ENABLED)) {
> +	     new_cp == DRM_MODE_CONTENT_PROTECTION_ENABLED) ||
> +	    (old_cp == DRM_MODE_CONTENT_PROTECTION_ENABLED &&
> +	     new_cp == DRM_MODE_CONTENT_PROTECTION_DESIRED)) {
>  		if (old_state->hdcp_content_type ==
>  				new_state->hdcp_content_type)
>  			return;
> -- 
> Sean Paul, Software Engineer, Google / Chromium OS
>