Re: [PATCH] drm/amd/display: fix NULL deref when opening a closed DDC

[Alex Hung]

Hi Arthur,

When AUX already holds the DDC, dal_ddc_open() fails with
GPIO_RESULT_ALREADY_OPENED, and the HPD path then calls dal_ddc_close()
on pins it does not own.

Please try whether the following solution fix your KWin issue?


diff --git 
drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c 
drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c
index ad1a499b0d9d..1f53b33fc875 100644
--- drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c
+++ drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c
@@ -2604,8 +2604,6 @@ bool dp_is_sink_present(struct dc_link *link)

 	if (dal_ddc_open(ddc, GPIO_MODE_INPUT,
 			 GPIO_DDC_CONFIG_TYPE_MODE_I2C) != GPIO_RESULT_OK) {
-		dal_ddc_close(ddc);
-
 		return present;
 	}



On 10/7/26 11:47, Arthur Liberman wrote:
> dal_ddc_open() uses gpio->pin after dal_gpio_service_open() installs
> it. Close clears that pointer. HPD sink detection and a KWin atomic
> check can both open the same DDC: the check reads DPCD for DSC while
> the HPD worker closes the pin. The load of hw_gpio.store.en then
> oopses at offset 0x28 and kwin_wayland exits with IRQs disabled.
>
> If either pin is gone after open, close what this call holds and
> return GPIO_RESULT_NULL_HANDLE so the AUX transfer fails instead.
> This does not serialize the two threads.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Cursor:xai-grok-4.7
> Signed-off-by: Arthur Liberman <arthur_liberman@xxxxxxxxxxx>
> ---
>   drivers/gpu/drm/amd/display/dc/gpio/gpio_service.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/display/dc/gpio/gpio_service.c b/drivers/gpu/drm/amd/display/dc/gpio/gpio_service.c
> index 95f8b7c7d657..a3145cd82d8b 100644
> --- a/drivers/gpu/drm/amd/display/dc/gpio/gpio_service.c
> +++ b/drivers/gpu/drm/amd/display/dc/gpio/gpio_service.c
> @@ -577,6 +577,17 @@ enum gpio_result dal_ddc_open(
>   		goto failure;
>   	}
>   
> +	/*
> +	 * dal_gpio_service_open() installs gpio->pin, and close clears it.
> +	 * MST bring-up can AUX-read for DSC while HPD teardown closes the
> +	 * same DDC. pin is NULL again in that window; do not dereference it.
> +	 */
> +	if (!ddc->pin_data->pin || !ddc->pin_clock->pin) {
> +		dal_gpio_close(ddc->pin_clock);
> +		dal_gpio_close(ddc->pin_data);
> +		return GPIO_RESULT_NULL_HANDLE;
> +	}
> +
>   	/* DDC clock and data pins should belong
>   	 * to the same DDC block id,
>   	 * we use the data pin to set the pad mode. */
>
> base-commit: 41505ac433cc4c5179deee8860d197b04f6d1c1d