Re: [PATCH v4] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

[Dmitry Baryshkov]

On Sun, 30 Mar 2025 at 20:31, Christopher Obbard
<christopher.obbard@xxxxxxxxxx> wrote:
>
> According to the eDP specification (VESA Embedded DisplayPort Standard
> v1.4b, Section 3.3.10.2), if the value of DP_EDP_PWMGEN_BIT_COUNT is
> less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, the sink is required to use
> the MIN value as the effective PWM bit count.
>
> This commit updates the logic to clamp the reported
> DP_EDP_PWMGEN_BIT_COUNT to the range defined by
> DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN and _CAP_MAX. As part of this change,
> the behavior is modified such that reading _CAP_MIN and _CAP_MAX
> registers is now required to succeed. Before reading these registers
> was optional.
>
> This ensures correct handling of eDP panels that report a zero PWM
> bit count but still provide valid non-zero MIN and MAX capability
> values. Without this clamping, brightness values may be interpreted
> incorrectly, leading to a dim or non-functional backlight.
>
> For example, the Samsung ATNA40YK20 OLED panel used in the Lenovo
> ThinkPad T14s Gen6 (Snapdragon) reports a PWM bit count of 0, but
> supports AUX backlight control and declares a valid 11-bit range.
> Clamping ensures brightness scaling works as intended on such panels.
>
> Co-developed-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> Signed-off-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> Signed-off-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
> ---
> Changes in v4:
> - Remove unrelated whitespace changes.
> - Remove unrelated commit change.
> - Add note to commit message about changing read of PWMGEN_BIT_COUNT_CAP_MIN
>   and _CAP__MAX from optional to required.
> - Link to v3: https://lore.kernel.org/r/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v3-1-156801d97a8a@xxxxxxxxxx
>
> Changes in v3:
> - Properly rebase patch on top of latest version of drm-misc-next.
> - Make patch more generic by clamping PWM bit count to advertised MIN
>   and MAX capabilities (suggested by Dmitry).
> - Link to v2: https://lore.kernel.org/r/20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-v2-1-16dc3ee00276@xxxxxxxxxx
>
> Changes in v2:
> - Split backlight brightness patch from T14s OLED enablement series.
> - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> - Rework commit message to reference eDP spec.
> - Rebase on drm-misc-next.
> - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@xxxxxxxxxx/
> ---
>  drivers/gpu/drm/display/drm_dp_helper.c | 42 +++++++++++++++++++++------------
>  1 file changed, 27 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index e2439c8a7fefe116b04aaa689b557e2387b05540..b5c8a75d036b5d65262417de363409e3ff3f4560 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -28,6 +28,7 @@
>  #include <linux/init.h>
>  #include <linux/iopoll.h>
>  #include <linux/kernel.h>
> +#include <linux/minmax.h>
>  #include <linux/module.h>
>  #include <linux/sched.h>
>  #include <linux/seq_file.h>
> @@ -4035,6 +4036,32 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
>         }
>
>         pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +       ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> +       if (ret != 1) {

NAK. See previous discussion.

> +               drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> +                           aux->name, ret);
> +               return -ENODEV;
> +       }
> +       pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;


-- 
With best wishes
Dmitry