On Thu, Mar 05, 2026 at 01:48:11PM +0530, Arun R Murthy wrote:
> We at present have drm_dp_Read_lttpr_common_caps to read the LTTPR caps,
> but this function required DPRX caps to be passed. As per the DP2.1 spec
> section 3.6.8.6.1, section 2.12.1, section 2.12.3 (Link Policy) the
> LTTPR caps is to be read first followed by the DPRX capability.
> Hence adding another function to read the LTTPR caps without the need
> for DPRX caps.
>
> In order to handle the issue
> https://gitlab.freedesktop.org/drm/intel/-/issues/4531
> of reading corrupted values for LTTPR caps on few pannels with DP Rev 1.2
> the workaround of reducing the block size to 1 and reading one block at a
> time is done by checking for a valid link rate.
>
> Fixes: 657586e474bd ("drm/i915: Add a DP1.2 compatible way to read LTTPR
> capabilities")
> Signed-off-by: Arun R Murthy <[email protected]>
> ---
> drivers/gpu/drm/display/drm_dp_helper.c | 63
> +++++++++++++++++++++++++++++++++
> include/drm/display/drm_dp_helper.h | 2 ++
> 2 files changed, 65 insertions(+)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c
> b/drivers/gpu/drm/display/drm_dp_helper.c
> index
> a697cc227e28964cd8322803298178e7d788e820..9fe7db73027a43b01c4d12927f1f0e61444658e5
> 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -3050,6 +3050,69 @@ static int drm_dp_read_lttpr_regs(struct drm_dp_aux
> *aux,
> return 0;
> }
>
> +static bool drm_dp_valid_link_rate(u8 link_rate)
> +{
> + switch (link_rate) {
> + case 0x06:
> + case 0x0a:
> + case 0x14:
> + case 0x1e:
> + return true;
> + default:
> + return false;
> + }
> +}
> +
> +/**
> + * drm_dp_read_lttpr_caps - read the LTTPR capabilities
> + * @aux: DisplayPort AUX channel
> + * @caps: buffer to return the capability info in
> + *
> + * Read capabilities common to all LTTPRs.
> + *
> + * Returns 0 on success or a negative error code on failure.
> + */
> +int drm_dp_read_lttpr_caps(struct drm_dp_aux *aux,
> + u8 caps[DP_LTTPR_COMMON_CAP_SIZE])
> +{
> + /*
> + * At least the DELL P2715Q monitor with a DPCD_REV < 0x14 returns
> + * corrupted values when reading from the 0xF0000- range with a block
> + * size bigger than 1.
> + * For DP as per the spec DP2.1 section 3.6.8.6.1, section 2.12.1,
> section
> + * 2.12.3 (Link Policy) the LTTPR caps is to be read first followed by
> the
> + * DPRX capability.
> + * So ideally we dont have DPCD_REV yet to check for the revision,
> instead
> + * check for the correctness of the read value and in found corrupted
> read
> + * block by block.
> + */
> + int block_size;
> + int offset;
> + int ret;
> + int address = DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV;
> + int buf_size = DP_LTTPR_COMMON_CAP_SIZE;
> +
> + ret = drm_dp_dpcd_read_data(aux, address, &caps, buf_size);
> + if (ret < 0)
> + return ret;
> +
> + if (caps[0] == 0x14) {
> + if (!drm_dp_valid_link_rate(caps[1])) {
I don't think the code can depend on what will be in caps[1] (i.e.
DP_MAX_LINK_RATE_PHY_REPEATER / 0xF0001) after the monitor returned a
corrupted value when reading this register. That is the code cannot
depend on this register value being a valid link rate encoding or
some other value.
> + block_size = 1;
> + for (offset = 0; offset < buf_size; offset +=
> block_size) {
> + ret = drm_dp_dpcd_read_data(aux,
> + address + offset,
> + &caps[offset],
> + block_size);
> + if (ret < 0)
> + return ret;
> + }
> + }
> + }
> + return 0;
> +}
> +EXPORT_SYMBOL(drm_dp_read_lttpr_caps);
> +
> /**
> * drm_dp_read_lttpr_common_caps - read the LTTPR common capabilities
> * @aux: DisplayPort AUX channel
> diff --git a/include/drm/display/drm_dp_helper.h
> b/include/drm/display/drm_dp_helper.h
> index
> 1d0acd58f48676f60ff6a07cc6812f72cbb452e8..def145e67011c325b790c807f934b288304260c1
> 100644
> --- a/include/drm/display/drm_dp_helper.h
> +++ b/include/drm/display/drm_dp_helper.h
> @@ -755,6 +755,8 @@ bool drm_dp_read_sink_count_cap(struct drm_connector
> *connector,
> const struct drm_dp_desc *desc);
> int drm_dp_read_sink_count(struct drm_dp_aux *aux);
>
> +int drm_dp_read_lttpr_caps(struct drm_dp_aux *aux,
> + u8 caps[DP_LTTPR_COMMON_CAP_SIZE]);
> int drm_dp_read_lttpr_common_caps(struct drm_dp_aux *aux,
> const u8 dpcd[DP_RECEIVER_CAP_SIZE],
> u8 caps[DP_LTTPR_COMMON_CAP_SIZE]);
>
> --
> 2.25.1
>
