[REGRESSION][BISECTED] 5.9-rc4 disables console on radeon

[Christian König]

Yes, that is an already known issue.

We are most likely going to revert the patch ASAP.

Christian.

Am 08.09.20 um 18:27 schrieb Mikael Pettersson:
> Starting with linux-5.9-rc4, the Dell monitor on my desktop PC goes
> black during boot
> when the kernel activates the framebuffer console, except for this
> error message shown
> in the center of the screen:
>
> "Dell U2412M
>   The current input timing is not supported by the monitor display. Please
>   change your input timing to 1920x1200 at 60Hz or any other monitor
>   listed timing as per the monitor specifications.
>   <DVI symbol>"
>
> The monitor remains black until I reboot.
>
> All kernels up to and including 5.9-rc3 were Ok.  A git bisect identified
>
> # first bad commit: [fc8c70526bd30733ea8667adb8b8ffebea30a8ed]
> drm/radeon: Prefer lower feedback dividers
>
> as the culprit, and reverting that from -rc4 makes the console work again.
>
> Adding a bit of debugging code to that function shows:
>
> avivo_get_fb_ref_div: fb_div_new 136 fb_div_old 137
> avivo_get_fb_ref_div: fb_div_new 142 fb_div_old 143
> avivo_get_fb_ref_div: fb_div_new 136 fb_div_old 137
> avivo_get_fb_ref_div: fb_div_new 119 fb_div_old 120
> avivo_get_fb_ref_div: fb_div_new 136 fb_div_old 137
>
> during boot, where "new" is what the commit above changed the code to compute,
> and "old" is the value computed by the working code from rc3.
>
> The graphics card is a Radeon HD6450 silent model:
> 01:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
> [AMD/ATI] Caicos [Radeon HD 6450/7450/8450 / R5 230 OEM] (prog-if 00
> [VGA controller])
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