[bisected] 5.13-rc2 i915 regression in b12d691ea5e01d "i915: fix remap_io_sg to verify the pgprot"

[Sergei Trofimovich <slyfox@gentoo.org>]

Hi Christoph and i915 maintainers!

vanilla 5.13-rc2 got some rendering regression on gen2(?) i915 chips:

  In gtk apps cursor gets corrupted for a short period of time.

  In firefox text selection and text scrolling shows artifacts for a short
  time (seconds). As if tiny horisontal tiles (1 pixel high, ~20-50 pixels
  long) fail to refresh in time.

  I was not able get a screenshot with artifacts: pictures are always clean.
  I can grab a photo or tiny video if needed.

Bisection was straightforward and landed on:

b12d691ea5e01db42ccf3b4207e57cb3ce7cfe91 is the first bad commit
commit b12d691ea5e01db42ccf3b4207e57cb3ce7cfe91
Author: Christoph Hellwig <hch@lst.de>
Date:   Thu Apr 29 22:57:38 2021 -0700

    i915: fix remap_io_sg to verify the pgprot

    remap_io_sg claims that the pgprot is pre-verified using an io_mapping,
    but actually does not get passed an io_mapping and just uses the pgprot in
    the VMA.  Remove the apply_to_page_range abuse and just loop over
    remap_pfn_range for each segment.

    Note: this could use io_mapping_map_user by passing an iomap to
    remap_io_sg if the maintainers can verify that the pgprot in the iomap in
    the only caller is indeed the desired one here.

    Link: https://lkml.kernel.org/r/20210326055505.1424432-5-hch@lst.de
    Signed-off-by: Christoph Hellwig <hch@lst.de>
    Cc: Chris Wilson <chris@chris-wilson.co.uk>
    Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
    Cc: Jani Nikula <jani.nikula@linux.intel.com>
    Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

 drivers/gpu/drm/i915/i915_mm.c | 73 +++++++++++++-----------------------------
 1 file changed, 23 insertions(+), 50 deletions(-)

System:
    $ Linux sf 5.12.0-11146-g8ca5297e7e38 #302 SMP PREEMPT Thu Apr 29 23:30:52 BST 2021 x86_64 Intel(R) Core(TM) i7-2700K CPU @ 3.50GHz GenuineIntel GNU/Linux
    $ lspci -v
    00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09) (prog-if 00 [VGA controller])
        Subsystem: Gigabyte Technology Co., Ltd 2nd Generation Core Processor Family Integrated Graphics Controller
        Flags: bus master, fast devsel, latency 0, IRQ 27
        Memory at f7800000 (64-bit, non-prefetchable) [size=4M]
        Memory at e0000000 (64-bit, prefetchable) [size=256M]
        I/O ports at f000 [size=64]
        Expansion ROM at 000c0000 [virtual] [disabled] [size=128K]
        Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
        Capabilities: [d0] Power Management version 2
        Capabilities: [a4] PCI Advanced Features
        Kernel driver in use: i915

Software:
    mesa-20.3.5 : i965 driver
      glamor X acceleration enabled on Mesa DRI Intel(R) HD Graphics 3000 (SNB GT2)
    xorg-server-1.20.11

Thanks!

-- 

  Sergei