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Thu, 24 Jul 2025 02:08:44 -0700 (PDT) Message-ID: <6612cd14-8353-4d3a-a248-5d32e0d3ca23@linaro.org> Date: Thu, 24 Jul 2025 11:08:43 +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@linaro.org Reply-To: Neil Armstrong Subject: Re: [PATCH v6] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities To: Johan Hovold , Christopher Obbard Cc: Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Dmitry Baryshkov , 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> 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 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)); ? 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 >