Hi everyone, I have an AOC monitor with the following details: * Product No. 24P2Q. * Model No. 24P2. * Manufactured in March 2021. * Native resolution is 1920x1080. * Input connections: DP, DVI, HDMI, and VGA. * Product website: https://www.aoc.com/uk/monitors/24p2q
The AOC monitor is connected to a custom PC with the following details: * CPU: Intel(R) Celeron(R) N5100 @ 1.10GHz (https://www.intel.com/content/www/us/en/products/sku/212329/intel-celeron-processor-n5100-4m-cache-up-to-2-80-ghz/specifications.html). * Integrated Intel video card VID:PID is 8086:4e61 (JasperLake UHD graphics). * The mainboard has two DP output ports. The monitor and the custom PC are connected with a DP-to-DP cable. The monitor has been working fine with Ubuntu 20.04 LTS in its native resolution: 1920x1080@60. The Ubuntu 20.04 has Linux kernel version 5.15. But, the monitor no longer works in its native resolution after upgrading to Ubuntu 24.04 LTS: the monitor screen remains black. The Ubuntu 24.04 has Linux kernel version 6.14. The monitor can work when the resolution is set to 1024x768@60 using xrandr, but that is not the solution of this problem because I want the monitor to work at its native resolution. The problem remains when using the latest Ubuntu LTS (26.04), which has Linux kernel version 7.0. Different DP-to-DP cables have been tested with Ubuntu 24.04, but the problem remains. The monitor works with Ubuntu 24.04 when either a DP-to-DVI or DP-to-HDMI or DP-to-VGA cable is used, but the monitor cannot work with Ubuntu 24.04 when using a DP-to-DP cable, which again, presents no problem whatsoever when using Ubuntu 20.04 with kernel 5.15. The monitor works with Ubuntu 24.04 using a DP-to-DP cable when connected to a different PC that uses a different Intel processor and a different integrated Intel video card. In other words, the problem is specific to the custom PC described at the beginning with a DP-to-DP connection to the AOC monitor. I have bisected the Linux kernel and found out that the cause of the problem is the following diff-hunk of commit 99cfbed19d06 (https://github.com/torvalds/linux/commit/99cfbed19d06#diff-eb927a6d3f0588695ab1a8f54a0c90a7a5feddcfa8379e49130b2594b819f686), which is found between Linux kernel v6.4 and v6.5-rc1: --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c @@ -699,7 +699,7 @@ intel_dp_prepare_link_train(struct intel_dp *intel_dp, drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_RATE_SET, &rate_select, 1); - link_config[0] = crtc_state->vrr.enable ? DP_MSA_TIMING_PAR_IGNORE_EN : 0; + link_config[0] = crtc_state->vrr.flipline ? DP_MSA_TIMING_PAR_IGNORE_EN : 0; link_config[1] = intel_dp_is_uhbr(crtc_state) ? DP_SET_ANSI_128B132B : DP_SET_ANSI_8B10B; drm_dp_dpcd_write(&intel_dp->aux, DP_DOWNSPREAD_CTRL, link_config, 2); After I rebuilt the Linux kernel at commit 99cfbed19d06 by either reverting the hunk or rewriting the assignment statement to "link_config[0] = 0", the AOC monitor works fine again with the custom PC using a DP-to-DP cable and Ubuntu 24.04 with my custom kernel. Since this problem persists even in the latest state of the Linux kernel as of yesterday (https://github.com/torvalds/linux/commit/44696aa3a489), my question is then as follows: Is the actual problem the i915 driver or the BIOS of the custom PC? Note that I do not blame the firmware of the AOC monitor because the AOC monitor works fine when connected through a DP-to-DP cable to a Ubuntu 24.04 running on a different PC that has a different Intel processor and a different model of the integrated Intel video card. Thank you very much for any help. If you need further information/test, just let me know. -- Best regards, Tadeus Prastowo
