We have 2 laptops, ASUS Z406MA and Acer TravelMate B118, both powered by the same Intel N5000 GemniLake CPU. On the Acer laptop, the panel will blink once during boot which never happens on the ASUS laptop. It caught my attention and I find the difference between them but I need help for more information,
The major difference happens in bxt_sanitize_cdclk() on the following condition check.
if (cdctl == expected)
/* All well; nothing to sanitize */
On the problematic Acer laptop, the value of cdctl is 0x27a while the same cdctl is 0x278 on ASUS machine. Due to the 0x27a is not equal to the expected value 0x278 so it needs to be sanitized by assigning -1 to dev_priv->cdclk.hw.vco. Then the consequent bxt_set_cdclk() will force the full PLL disable and enable. And that's the flicker (blink) we observed during boot.
Although I can't find the definition of the BIT(2) of CDCLK_CTL which causes this difference. I reproduced this bug on latest 5.3 mainline and also kernel built from https://github.com/freedesktop/drm-intel. But I think it's not really about i915 driver if the content from register CDCLK_CTL is unexpected. Can anyone suggest what exactly the problem is and how should we deal with it? Thanks.
Please ref. https://lkml.org/lkml/2019/10/30/240 mail thread here.
Chris, Can you reproduce the issue with drm-tip (https://cgit.freedesktop.org/drm-tip) with kernel parameters drm.debug=0x1e log_buf_len=4M.
If the problem persists attach the full dmesg from boot.
Created attachment 145864 [details]
Dmesg from DRM-TIP
Lakshmi, thanks for your quick response. I can still reproduce this with the kernel built from drm-tip. Please refer to the attached dmesg log.
Apart from a (In reply to Chris Chiu from comment #0)
> We have 2 laptops, ASUS Z406MA and Acer TravelMate B118, both powered by the
> same Intel N5000 GemniLake CPU. On the Acer laptop, the panel will blink
> once during boot which never happens on the ASUS laptop.
Logs seems to be normal and couldn't find any issue.
Apart from blinking once during boot, is there any other impact you notice? Trying to understand the severity of the issue.
Actually, it just causes a blink during boot and no other side effect observed so far. We just wonder whether if the value 0x27a of CDCTL makes sense or not. Is the value given by BIOS and why they differ in different models.
Any suggestions about what we can talk to Acer? They complained this blink issue but I don't really think it's driver related. Maybe the clock setting is made by their BIOS or firmware in early stage and we can make our standpoint to push back to Acer for fix?