[PATCH 2/2] drm/tidss: Fix sync-lost issue with two displays

[Aradhya Bhatia]

On 13/02/24 13:46, Tomi Valkeinen wrote:
> A sync lost issue can be observed with two displays, when moving a plane
> from one disabled display to an another disabled display, and then
> enabling the display to which the plane was moved to. The exact
> requirements for this to trigger are not clear.
> 
> It looks like the issue is that the layers are left enabled in the first
> display's OVR registers. Even if the corresponding VP is disabled, it
> still causes an issue, as if the disabled VP and its OVR would still be
> in use, leading to the same VID being used by two OVRs. However, this is
> just speculation based on testing the DSS behavior.
> 
> Experimentation shows that as a workaround, we can disable all the
> layers in the OVR when disabling a VP. There should be no downside to
> this, as the OVR is anyway effectively disabled if its VP is disabled,
> and it seems to solve the sync lost issue.
> 
> However, there may be a bigger issue in play here, related to J721e
> erratum i2097 ("DSS: Disabling a Layer Connected to Overlay May Result
> in Synclost During the Next Frame"). Experimentation also shows that the
> OVR's CHANNELIN field has similar issue. So we may need to revisit this
> when we find out more about the core issue.
> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
> Fixes: 32a1795f57ee ("drm/tidss: New driver for TI Keystone platform Display SubSystem")

Reviewed-by: Aradhya Bhatia <a-bhatia1 at ti.com>

> ---
>  drivers/gpu/drm/tidss/tidss_crtc.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/tidss/tidss_crtc.c b/drivers/gpu/drm/tidss/tidss_crtc.c
> index 5f838980c7a1..94f8e3178df5 100644
> --- a/drivers/gpu/drm/tidss/tidss_crtc.c
> +++ b/drivers/gpu/drm/tidss/tidss_crtc.c
> @@ -265,6 +265,16 @@ static void tidss_crtc_atomic_disable(struct drm_crtc *crtc,
>  
>  	reinit_completion(&tcrtc->framedone_completion);
>  
> +	/*
> +	 * If a layer is left enabled when the videoport is disabled, and the
> +	 * vid pipeline that was used for the layer is taken into use on
> +	 * another videoport, the DSS will report sync lost issues. Disable all
> +	 * the layers here as a work-around.
> +	 */
> +	for (u32 layer = 0; layer < tidss->feat->num_planes; layer++)
> +		dispc_ovr_enable_layer(tidss->dispc, tcrtc->hw_videoport, layer,
> +				       false);
> +
>  	dispc_vp_disable(tidss->dispc, tcrtc->hw_videoport);
>  
>  	if (!wait_for_completion_timeout(&tcrtc->framedone_completion,
>