Re: [PATCH] drm/amd/display: Increase the HDMI HPD disconnect filter time

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] drm/amd/display: HDMI HPD disconnect filter masks valid KVM pulses
--

commit 56a856b6f885a6969be27edf739c5c17d243d79b
Author: Wang Jiang <jiangwang@kylinos.cn>

drm/amd/display: Increase the HDMI HPD disconnect filter time

This commit raises the DVI and HDMI HPD disconnect filter time from 100ms to
800ms to hide bouncing hotplug signals when sinks reconfigure.

> diff --git a/drivers/gpu/drm/amd/display/dc/link/protocols/link_hpd.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_hpd.c
> --- a/drivers/gpu/drm/amd/display/dc/link/protocols/link_hpd.c
> +++ b/drivers/gpu/drm/amd/display/dc/link/protocols/link_hpd.c
> @@ -82,8 +82,12 @@ bool program_hpd_filter(const struct dc_link *link)
>  	case SIGNAL_TYPE_HDMI_TYPE_A:
>  	case SIGNAL_TYPE_HDMI_FRL:
>  		/* Program hpd filter */
> +		/* Some HDMI/DVI sinks drive HPD low for a few hundred ms while
> +		 * they reconfigure; 100ms is not enough to hide that bounce,
> +		 * which is then reported as a fake unplug/plug pair.
> +		 */
>  		delay_on_connect_in_ms = 500;
> -		delay_on_disconnect_in_ms = 100;
> +		delay_on_disconnect_in_ms = 800;

[Severity: Medium]
Will this 800ms filter in program_hpd_filter() mask legitimate hotplug pulses
from KVM switches or AV receivers?

Standard HPD low pulses used by these devices to indicate a configuration
change are typically between 100ms and 500ms. Because this filter is programmed
in hardware, an 800ms threshold completely hides these valid pulses from the
CPU, meaning the HPD interrupt never fires.

Could this cause the driver to miss valid hotplug events when users switch
inputs, resulting in blank screens or incorrect video modes because EDID and
HDCP are not renegotiated?

>  		break;
>  	case SIGNAL_TYPE_DISPLAY_PORT:
>  	case SIGNAL_TYPE_DISPLAY_PORT_MST:

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/tencent_B4299278015532212E7D2E624FA23C09C90A@qq.com?part=1