Hi Dmitry,
On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov
<dmitry.barysh...@oss.qualcomm.com> wrote:
>
> On 27/03/2025 19:25, Christopher Obbard wrote:
> > According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
> > if 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 bit count.
> >
> > Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
> > valid non-zero MIN and MAX capability values. This patch updates the logic
> > to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
> > backlight brightness values.
> >
> > This improves compatibility with panels like the Samsung ATNA40YK20 used
> > on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
> > bit count of 0 but declares an 11-bit PWM capability range.
> >
> > Co-developed-by: Rui Miguel Silva <rui.si...@linaro.org>
> > Signed-off-by: Rui Miguel Silva <rui.si...@linaro.org>
> > Signed-off-by: Christopher Obbard <christopher.obb...@linaro.org>
> > ---
> > 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-e9bc7c9d3...@linaro.org/
> > ---
> > drivers/gpu/drm/display/drm_dp_helper.c | 50
> > ++++++++++++++++++++++-----------
> > 1 file changed, 33 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c
> > b/drivers/gpu/drm/display/drm_dp_helper.c
> > index
> > dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9
> > 100644
> > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > @@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux,
> > struct drm_edp_backlight_inf
> > {
> > int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
> > int ret;
> > - u8 pn, pn_min, pn_max;
> > + u8 pn, pn_min, pn_max, bl_caps;
> >
> > if (!bl->aux_set)
> > return 0;
> > @@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux,
> > struct drm_edp_backlight_inf
> > aux->name, ret);
> > return -ENODEV;
> > }
> > -
> > pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > +
> > + ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
> > &pn_min);
> > + if (ret != 1) {
> > + drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit
> > count cap min: %d\n",
> > + aux->name, ret);
> > + return 0;
> > + }
> > + pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > +
> > + ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX,
> > &pn_max);
> > + if (ret != 1) {
> > + drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit
> > count cap max: %d\n",
> > + aux->name, ret);
> > + return 0;
> > + }
> > + pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > +
> > + ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP,
> > &bl_caps);
> > + if (ret != 1) {
> > + bl_caps = 0;
> > + drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight
> > adjustment cap: %d\n",
> > + aux->name, ret);
> > + }
> > +
> > + /*
> > + * Some eDP panels report brightness byte count support, but the byte
> > count
> > + * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
> > + */
> > + if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> > + && pn_min)
> > + pn = pn_min;
>
> I wonder, what stops you from implementing this part according to the
> standard, rather than adding a hack for 0 value.
I am simply quite dense, I do not know this spec or the DRM code well
at all ;-).
I do appreciate your continued reviews though, however painful it must be.
Before I send v3, can I clarify a proper solution?
I think you are saying that I can remove the section where I read the
capabilities and instead simply have something like (in pseudocode):
/* Determine effective bit count according to eDP spec */
u8 effective_bit_count = pwm_bit_count;
if (pwm_bit_count == 0 || pwm_bit_count < cap_min) {
effective_bit_count = cap_min;
}
I would like to introduce this new variable to make the code easier to read.
If my understanding is correct, I'll send a v3 later.
> > +
> > bl->max = (1 << pn) - 1;
> > if (!driver_pwm_freq_hz)
> > return 0;
