drm: simpledrm: fbdev emulation error with CONFIG_DRM_SIMPLEDRM enabled

[Thomas Zimmermann]

Hi,

thanks for the feedback.

Am 24.09.21 um 20:03 schrieb Zoltán Kővágó:
> Hi,
> 
> Did anything happen with this issue? I've updated linux kernel to 5.14.7 
> from 5.13 today, and I've been hit with a very similar problem (even 
> inside qemu), except in my case the height reported in "bad framebuffer 
> height" was twice the actual height, not three times. After some digging 
> around I've figured out that at the end of 
> drm_fb_helper_single_fb_probe, surface_height is multiplied by 
> CONFIG_DRM_FBDEV_OVERALLOC/100, which was somewhy set to 200 in my 
> kernel config. Setting it to 100 fixed that problem. (It looks like the 
> default now, I don't know why was it set to 200 in my config, maybe the 
> old default was different).

I was away for a while and I haven't tried this. But it was my next 
idea. Thanks for confirming.

> 
> However it's still very buggy, even after setting 
> CONFIG_DRM_FBDEV_OVERALLOC to 100. On my notebook (ASUS G551JW, with a 
> 4th gen i7 cpu), this is what I observe:
> * after the kernel boots, the tux logos rapidly flicker at the top of 
> the screen and I only see text at the bottom of the screen. After a few 
> seconds it returns to normal.
> * when starting X, whet it switches to a new VT the tux logos reappear 
> along with some text, but only in the upper left corner. This is also 
> only happens for a second or so, until X overtakes the display.
> * when terminating X, the tux logos with the garbage text remain in the 
> upper left corner, but the rest of the screen is replaced with the 
> correct contents. The garbage in the top left corner remains until 
> something overwrites it (like scrolling the terminal).
> So it works, but buggy as hell and at this point I'm better disabling 
> simpledrm and just relying on the i915 driver, losing early console. I 
> tried booting with drm.debug=0x1ff, but by the time I could run dmesg it 
> generated so many messages that the early boot messages were already lost.

It's all build around core DRM helpers. I don't know why simpledrm seems 
error prone, while the other drivers work with these same helpers. 
Something to explore...

> 
> Now, my desktop is even worse. Motherboard is an ASRock Z87M Extreme4, 
> with a GeForce GT 640 and GTX 980 cards (the integrated GPU in the intel 
> CPU is disabled in UEFI settings). Unfortunately this requires the 
> proprietary nvidia drivers, so it's probably my own problem, but anyway 
> it works until I it loads the nvidia drivers, then afterwards (while 
> still in fbcon) I instead get 4 or 5 small copies of my terminal in the 
> upper part of the screen with wrong stride, so it's completely 
> unreadable. And if I ever start X, I won't have any fbcon until reboot. 
> So 5.14 is basically unusable on this machine.
Ask Nvidia.

> Any ideas what should I try? Everything worked well with 5.13 and its 
> simplefb.

I'd suggest to go back to simplefb for a while and maybe try simpledrm 
occasionally.

Best regards
Thomas

> 
> Thanks,
> Zoltan
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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