[BISECTED] 3.17-rc1 radeon screen corruption due to "Always flush the HDP cache before submitting a CS to the GPU"

[Mikael Pettersson]

Since 3.17-rc1 my radeon card (RV370 / X1050 card) causes screen corruption
after a while in X + firefox.  This still occurs with yesterday's HEAD
of Linus' repo.  3.16 and ealier kernels are fine.

I ran a bisect, which identified:

commit 72a9987edcedb89db988079a03c9b9c65b6ec9ac
Author: Michel Dänzer <michel.daenzer at amd.com>
Date:   Thu Jul 31 18:43:49 2014 +0900

    drm/radeon: Always flush the HDP cache before submitting a CS to the GPU

as the cause of my screen corruption.  Reverting this from 3.17-rc2
(which requires manual intervention due to subsequent changes in
radeon_ring_commit()) eliminates the screen corruption.

User-space is vanilla Fedora 19 / x86_64 with updates.  radeon_drv.so says:

[    62.574] (II) LoadModule: "radeon"
[    62.574] (II) Loading /usr/lib64/xorg/modules/drivers/radeon_drv.so
[    62.574] (II) Module radeon: vendor="X.Org Foundation"
[    62.574]    compiled for 1.14.0, module version = 7.1.99
[    62.574]    Module class: X.Org Video Driver
[    62.574]    ABI class: X.Org Video Driver, version 14.1
...
[    62.585] (--) RADEON(0): Chipset: "ATI Radeon X550 (RV370) 5B63 (PCIE)" (ChipID = 0x5b63)

See also my original report to LKML:
<http://marc.info/?l=linux-kernel&m=140829066726743&w=2>

/Mikael