On 02/12/2024 17:49, Geert Uytterhoeven wrote:
On Fri, Oct 18, 2024 at 11:32 AM Geert Uytterhoeven <geert+renesas@xxxxxxxxx> wrote:As I had to test Tomi's WIP patches to add mini-DP output support to Gray Hawk Single, I moved my (old and new) DisplayPort gear to my board farm, and thought this was a good opportunity to test mini-DP output on White Hawk as a baseline (CN5), and add support for the second mini-DP output on the White Hawk BreakOut board (CN15). For testing, I used the following hardware: (A) BenQ BL2420PT desktop display (2560x1440), (B) Joy-It Joy-View 15 portable display (1920x1080), (C) Lindy DisplayPort to 2 Port HDMI MST Hub, (D) Passive mini-DP to HDMI cable, (E) LogiLink Mini DisplayPort to VGA Converter. (A)-(C) are known to work with my Intel desktop. (D)-(E) are known to work with an old Dell XPS13. Software-wise, I used the frame buffer text console, "modetest -M rcar-du -s 86:1920x1080@XR24" or "modetest -M rcar-du -s 86:2560x1440@XR24", and fbtest. White Hawk CN5 -------------- 1. Mini-DP to (A) DP: - 2560x1440 works but flickers (flickering shifts image horizontally; perhaps a cable issue, as 2 below does work?) - 1920x1080 is stableWhile I don't have a second Mini-DP-to-DP-M cable, I tried a few other combos (now on Gray Hawk Single): 7. (D) + HDMI-F-F adapter + passive HDMI-F-to-DP-cable to (A) DP,
D is passive DP->HDMI, it won't work. Are you saying it works, and the result matches the test case 1.?
8. Mini-DP-to-DP-F cable (=X) + plain DP cable (=Y) to (A) DP, unfortunately with the same results. Note that (X) is the same cable
"same results" means same as in 1.?
as used in scenario 2 below, and (Y) works fine with my Intel desktop. However, the maximum cable length for eDP seems to be 30 cm, so that
Where did you read that? I don't think there's such a thing as "eDP cable". In laptops etc. eDP is connected to the panel via custom made cables, and the cable design affects how long it can be. eDP and DP are identical wrt. signaling, so using a DP cable with eDP or DP should behave the same.
That said, I don't think the eDP->DP connector designs we see in these development boards are really made to match what one would expect from a consumer device with a DP output.
may explain why 2 below is the only wiring that works at 2560x1440 (despite cable (X) being 1m, i.e. still too long)?2. Mini-DP to (C) to (A) HDMI: - 2560x1440 OK - 1920x1080 OK
My guess is that this works, because there's an active component there which decodes the DP input and re-encodes it to HDMI output, probably with different timings (read, more standard). So the (C) is more tolerant on not-so-standard DP input than (A) is.
The eDP chip is limited in its capabilities, e.g. back porch registers are 8-bit, so they max out at 255. While those are checked in the driver, I wouldn't be surprised if some other limitations are not (the driver has clearly been written for eDP panels).
You could try different custom timings with kmstest, to find if there's something obvious that makes the case 1. work. For example, if the issue is indeed the cable, probably lowering the pixel clock would fix it. So using the same timings but dropping the pclk in half might get a stable picture (then again, lowering pclk might fix many other kinds of issues too...).
Can you send your monitor's edid blob? I don't have a board up here, but it should be available from somewhere in sysfs. On my desktop it's:
/sys/devices/pci0000:00/0000:00:02.0/drm/card1/card1-DP-2/edid Tomi
