On Thu, 05 Jun 2025, Imre Deak <imre.d...@intel.com> wrote:
> Reading DPCD registers has side-effects and some of these can cause a
> problem for instance during link training. Based on this it's better to
> avoid the probing quirk done before each DPCD register read, limiting
> this to the monitor which requires it. Add an EDID quirk for this. Leave
> the quirk enabled by default, allowing it to be disabled after the
> monitor is detected.
>
> v2: Fix lockdep wrt. drm_dp_aux::hw_mutex when calling
> drm_dp_dpcd_set_probe_quirk() with a dependent lock already held.
> v3: Add a helper for determining if DPCD probing is needed. (Jani)
>
> Cc: Ville Syrjälä <ville.syrj...@linux.intel.com>
> Cc: Jani Nikula <jani.nik...@linux.intel.com>
> Signed-off-by: Imre Deak <imre.d...@intel.com>
> ---
> drivers/gpu/drm/display/drm_dp_helper.c | 42 +++++++++++++++++--------
> drivers/gpu/drm/drm_edid.c | 3 ++
> include/drm/display/drm_dp_helper.h | 6 ++++
> include/drm/drm_edid.h | 3 ++
> 4 files changed, 41 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c
> b/drivers/gpu/drm/display/drm_dp_helper.c
> index dc622c78db9d4..d0b9f672d743c 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -691,6 +691,34 @@ void drm_dp_dpcd_set_powered(struct drm_dp_aux *aux,
> bool powered)
> }
> EXPORT_SYMBOL(drm_dp_dpcd_set_powered);
>
> +/**
> + * drm_dp_dpcd_set_dpcd_probe_quirk() - Set whether a probing before DPCD
> access is done
Musing, not sure if this needs to be called "quirk". This is just used
to enable/disable the extra probe. aux->dpcd_probe_disabled doesn't
mention the quirk either, and shouldn't.
Not a big deal.
Reviewed-by: Jani Nikula <jani.nik...@intel.com>
> + * @aux: DisplayPort AUX channel
> + * @enable: Enable the probing if required
> + */
> +void drm_dp_dpcd_set_probe_quirk(struct drm_dp_aux *aux, bool enable)
> +{
> + WRITE_ONCE(aux->dpcd_probe_disabled, !enable);
> +}
> +EXPORT_SYMBOL(drm_dp_dpcd_set_probe_quirk);
> +
> +static bool dpcd_access_needs_probe(struct drm_dp_aux *aux)
> +{
> + /*
> + * 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.
> + */
> + return !aux->is_remote && !READ_ONCE(aux->dpcd_probe_disabled);
> +}
> +
> /**
> * drm_dp_dpcd_read() - read a series of bytes from the DPCD
> * @aux: DisplayPort AUX channel (SST or MST)
> @@ -712,19 +740,7 @@ 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) {
> + if (dpcd_access_needs_probe(aux)) {
> ret = drm_dp_dpcd_probe(aux, DP_LANE0_1_STATUS);
> if (ret < 0)
> return ret;
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 9cca1e6e4736c..5f45820ad62d5 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -248,6 +248,9 @@ static const struct edid_quirk {
> /* OSVR HDK and HDK2 VR Headsets */
> EDID_QUIRK('S', 'V', 'R', 0x1019, BIT(EDID_QUIRK_NON_DESKTOP)),
> EDID_QUIRK('A', 'U', 'O', 0x1111, BIT(EDID_QUIRK_NON_DESKTOP)),
> +
> + /* HP ZR24w DP AUX DPCD access requires probing to prevent corruption.
> */
> + EDID_QUIRK('H', 'W', 'P', 0x2869, BIT(DRM_EDID_QUIRK_DP_DPCD_PROBE)),
> };
>
> /*
> diff --git a/include/drm/display/drm_dp_helper.h
> b/include/drm/display/drm_dp_helper.h
> index e4ca35143ff96..75fa9548aefa0 100644
> --- a/include/drm/display/drm_dp_helper.h
> +++ b/include/drm/display/drm_dp_helper.h
> @@ -523,10 +523,16 @@ struct drm_dp_aux {
> * @no_zero_sized: If the hw can't use zero sized transfers (NVIDIA)
> */
> bool no_zero_sized;
> +
> + /**
> + * @dpcd_probe_disabled: If probing before a DPCD access is disabled.
> + */
> + bool dpcd_probe_disabled;
> };
>
> int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset);
> void drm_dp_dpcd_set_powered(struct drm_dp_aux *aux, bool powered);
> +void drm_dp_dpcd_set_probe_quirk(struct drm_dp_aux *aux, bool enable);
> ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
> void *buffer, size_t size);
> ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
> diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
> index 77fd42608e706..3d1aecfec9b2a 100644
> --- a/include/drm/drm_edid.h
> +++ b/include/drm/drm_edid.h
> @@ -110,6 +110,9 @@ struct detailed_data_string {
> #define DRM_EDID_CVT_FLAGS_REDUCED_BLANKING (1 << 4)
>
> enum drm_edid_quirk {
> + /* Do a dummy read before DPCD accesses, to prevent corruption. */
> + DRM_EDID_QUIRK_DP_DPCD_PROBE,
> +
> DRM_EDID_QUIRK_NUM,
> };
--
Jani Nikula, Intel
