CVS commit: src/sys/external/bsd/drm2/dist/drm/i915

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Wed Feb 25 13:30:02 UTC 2015

Modified Files:
        src/sys/external/bsd/drm2/dist/drm/i915: intel_ringbuffer.c

Log Message:
Back-port attempt [1/2] to fix i915drmkms on G4X, from Linux.

commit ece4a17d237a79f63fbfaf3f724a12b6d500555c
Author: Jiri Kosina <jkosina%suse.cz@localhost>
Date:   Thu Aug 7 16:29:53 2014 +0200

    drm/i915: read HEAD register back in init_ring_common() to enforce ordering

    Withtout this, ring initialization fails reliabily during resume with

        [drm:init_ring_common] *ERROR* render ring initialization failed ctl 000
1f001 head ffffff8804 tail 00000000 start 000e4000

    This is not a complete fix, but it is verified to make the ring
    initialization failures during resume much less likely.

    We were not able to root-cause this bug (likely HW-specific to Gen4 chips)
    yet. This is therefore used as a ducttape before problem is fully
    understood and proper fix created, so that people don't suffer from
    completely unusable systems in the meantime.

    The discussion and debugging is happening at

        https://bugs.freedesktop.org/show_bug.cgi?id=76554


To generate a diff of this commit:
cvs rdiff -u -r1.4 -r1.5 \
    src/sys/external/bsd/drm2/dist/drm/i915/intel_ringbuffer.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.