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Thu, 24 Jul 2025 02:42:40 -0700 (PDT) Message-ID: Date: Thu, 24 Jul 2025 11:42:38 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Neil Armstrong Reply-To: Neil Armstrong Subject: Re: [PATCH v6] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities To: Dmitry Baryshkov Cc: Johan Hovold , Christopher Obbard , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Rui Miguel Silva , Abel Vesa References: <20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v6-1-84ad1cd1078a@linaro.org> <6612cd14-8353-4d3a-a248-5d32e0d3ca23@linaro.org> Content-Language: en-US, fr Autocrypt: addr=neil.armstrong@linaro.org; keydata= xsBNBE1ZBs8BCAD78xVLsXPwV/2qQx2FaO/7mhWL0Qodw8UcQJnkrWmgTFRobtTWxuRx8WWP GTjuhvbleoQ5Cxjr+v+1ARGCH46MxFP5DwauzPekwJUD5QKZlaw/bURTLmS2id5wWi3lqVH4 BVF2WzvGyyeV1o4RTCYDnZ9VLLylJ9bneEaIs/7cjCEbipGGFlfIML3sfqnIvMAxIMZrvcl9 qPV2k+KQ7q+aXavU5W+yLNn7QtXUB530Zlk/d2ETgzQ5FLYYnUDAaRl+8JUTjc0CNOTpCeik 80TZcE6f8M76Xa6yU8VcNko94Ck7iB4vj70q76P/J7kt98hklrr85/3NU3oti3nrIHmHABEB AAHNKk5laWwgQXJtc3Ryb25nIDxuZWlsLmFybXN0cm9uZ0BsaW5hcm8ub3JnPsLAkQQTAQoA OwIbIwULCQgHAwUVCgkICwUWAgMBAAIeAQIXgBYhBInsPQWERiF0UPIoSBaat7Gkz/iuBQJk Q5wSAhkBAAoJEBaat7Gkz/iuyhMIANiD94qDtUTJRfEW6GwXmtKWwl/mvqQtaTtZID2dos04 YqBbshiJbejgVJjy+HODcNUIKBB3PSLaln4ltdsV73SBcwUNdzebfKspAQunCM22Mn6FBIxQ GizsMLcP/0FX4en9NaKGfK6ZdKK6kN1GR9YffMJd2P08EO8mHowmSRe/ExAODhAs9W7XXExw UNCY4pVJyRPpEhv373vvff60bHxc1k/FF9WaPscMt7hlkbFLUs85kHtQAmr8pV5Hy9ezsSRa GzJmiVclkPc2BY592IGBXRDQ38urXeM4nfhhvqA50b/nAEXc6FzqgXqDkEIwR66/Gbp0t3+r yQzpKRyQif3OwE0ETVkGzwEIALyKDN/OGURaHBVzwjgYq+ZtifvekdrSNl8TIDH8g1xicBYp QTbPn6bbSZbdvfeQPNCcD4/EhXZuhQXMcoJsQQQnO4vwVULmPGgtGf8PVc7dxKOeta+qUh6+ SRh3vIcAUFHDT3f/Zdspz+e2E0hPV2hiSvICLk11qO6cyJE13zeNFoeY3ggrKY+IzbFomIZY 4yG6xI99NIPEVE9lNBXBKIlewIyVlkOaYvJWSV+p5gdJXOvScNN1epm5YHmf9aE2ZjnqZGoM Mtsyw18YoX9BqMFInxqYQQ3j/HpVgTSvmo5ea5qQDDUaCsaTf8UeDcwYOtgI8iL4oHcsGtUX oUk33HEAEQEAAcLAXwQYAQIACQUCTVkGzwIbDAAKCRAWmrexpM/4rrXiB/sGbkQ6itMrAIfn M7IbRuiSZS1unlySUVYu3SD6YBYnNi3G5EpbwfBNuT3H8//rVvtOFK4OD8cRYkxXRQmTvqa3 3eDIHu/zr1HMKErm+2SD6PO9umRef8V82o2oaCLvf4WeIssFjwB0b6a12opuRP7yo3E3gTCS KmbUuLv1CtxKQF+fUV1cVaTPMyT25Od+RC1K+iOR0F54oUJvJeq7fUzbn/KdlhA8XPGzwGRy 4zcsPWvwnXgfe5tk680fEKZVwOZKIEuJC3v+/yZpQzDvGYJvbyix0lHnrCzq43WefRHI5XTT QbM0WUIBIcGmq38+OgUsMYu4NzLu7uZFAcmp6h8g Organization: Linaro In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 24/07/2025 11:32, Dmitry Baryshkov wrote: > On Thu, 24 Jul 2025 at 12:08, wrote: >> >> On 20/05/2025 10:06, Johan Hovold wrote: >>> Hi Chris, >>> >>> On Fri, Apr 04, 2025 at 02:24:32PM +0100, Christopher Obbard wrote: >>>> On Fri, 4 Apr 2025 at 09:54, Johan Hovold wrote: >>>>> On Fri, Apr 04, 2025 at 08:54:29AM +0100, Christopher Obbard wrote: >>>>>> On Mon, 31 Mar 2025 at 09:33, Johan Hovold wrote: >>>>>>>> @@ -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 < 0) { >>>>>>>> + 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; >>>>>>>> + >>>>>>>> + ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max); >>>>>>>> + if (ret < 0) { >>>>>>>> + drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n", >>>>>>>> + aux->name, ret); >>>>>>>> + return -ENODEV; >>>>>>>> + } >>>>>>>> + pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK; >>>>>>>> + >>>>>>>> + /* >>>>>>>> + * Per VESA eDP Spec v1.4b, section 3.3.10.2: >>>>>>>> + * If DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, >>>>>>>> + * the sink must use the MIN value as the effective PWM bit count. >>>>>>>> + * Clamp the reported value to the [MIN, MAX] capability range to ensure >>>>>>>> + * correct brightness scaling on compliant eDP panels. >>>>>>>> + */ >>>>>>>> + pn = clamp(pn, pn_min, pn_max); >>>>>>> >>>>>>> You never make sure that pn_min <= pn_max so you could end up with >>>>>>> pn < pn_min on broken hardware here. Not sure if it's something you need >>>>>>> to worry about at this point. >> >> I'm trying to figure out what would be the behavior in this case ? >> >> - Warn ? >> - pn_max = pn_min ? >> - use BIT_COUNT as-is and ignore MIN/MAX ? >> - pm_max = max(pn_min, pn_max); pm_min = min(pn_min, pn_max); ? >> - reverse clamp? clamp(pn, pn_max, pn_min); ? >> - generic clamp? clamp(pn, min(pn_min, pn_max), max(pn_min, pn_max)); ? > > Per the standard, the min >= 1 and max >= min. We don't need to bother > about anything here. Yeah, I agree. But I think a: if (likely(pn_min <= pn_max)) is simple and doesn't cost much... > > On the other hand, I think the patch needs to be updated a bit. If the > pn value changed after clamping, it makes sense to write it back to > the DP_EDP_PWMGEN_BIT_COUNT register by jumping to the tail of the > drm_edp_backlight_probe_max() function You're right, we need to write it back, but we can't jump to the tail of the function since it has all the pwmgen logic in the middle. Neil > >> >> Or just bail out ? >> >> Neil >> >>>>>> >>>>>> I am honestly not sure. I would hope that devices follow the spec and >>>>>> there is no need to be too paranoid, but then again we do live in the >>>>>> real world where things are... not so simple ;-). >>>>>> I will wait for further feedback from someone who has more experience >>>>>> with eDP panels than I have. >>>>> >>>>> There's always going to be buggy devices and input should always be >>>>> sanitised so I suggest adding that check before calling clamp() (which >>>>> expects min <= max) so that the result here is well-defined. >>>> >>>> Makes sense, I will do so in the next revision. >>> >>> It seems you never got around to respinning this one so sending a >>> reminder. >>> >>> Johan >>> >> > >