| Message ID | 20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-v2-1-16dc3ee00276@linaro.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] drm/dp: fallback to minimum when PWM bit count is zero | expand |
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.silva@linaro.org> > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org> > Signed-off-by: Christopher Obbard <christopher.obbard@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-e9bc7c9d30cc@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. > + > bl->max = (1 << pn) - 1; > if (!driver_pwm_freq_hz) > return 0;
Hi Dmitry, On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov <dmitry.baryshkov@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.silva@linaro.org> > > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org> > > Signed-off-by: Christopher Obbard <christopher.obbard@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-e9bc7c9d30cc@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;
On Thu, 27 Mar 2025 at 20:19, Christopher Obbard <christopher.obbard@linaro.org> wrote: > > Hi Dmitry, > > On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov > <dmitry.baryshkov@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.silva@linaro.org> > > > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org> > > > Signed-off-by: Christopher Obbard <christopher.obbard@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-e9bc7c9d30cc@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 ;-). Okay, no worries. It should be pretty easy: pn = clamp(pn_min, pn_max); No need to check for pn being non-zero, etc. > I do appreciate your continued reviews though, however painful it must be. It's not, no worries.
On Fri, 28 Mar 2025 at 11:25, Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com> wrote: > > On Thu, 27 Mar 2025 at 20:19, Christopher Obbard > <christopher.obbard@linaro.org> wrote: > > > > Hi Dmitry, > > > > On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov > > <dmitry.baryshkov@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.silva@linaro.org> > > > > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org> > > > > Signed-off-by: Christopher Obbard <christopher.obbard@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-e9bc7c9d30cc@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 ;-). > > Okay, no worries. It should be pretty easy: > > pn = clamp(pn_min, pn_max); > > No need to check for pn being non-zero, etc. I think you probably mean: pn = clamp(pn, pn_min, pn_max); I will look to use this macro in the next version later today & make it more generic; thanks ! PS: I also found someone who tried to fix this in 2019; I will add the author to CC of the next version. https://lore.kernel.org/r/dri-devel/a766c3498d3754b598a8bf66f59a76e78ec57080.camel@intel.com/T/
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; + bl->max = (1 << pn) - 1; if (!driver_pwm_freq_hz) return 0; @@ -4122,21 +4153,6 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf * - FxP is within 25% of desired value. * Note: 25% is arbitrary value and may need some tweak. */ - 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; - } - 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_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK; - pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK; - /* Ensure frequency is within 25% of desired value */ fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4); fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);