[Intel-gfx] [PATCH 3.0-rc3] i915: Fix gen6 (SNB) GPU stalling

[Daniel J Blueman]

On 15 June 2011 12:43, Ben Widawsky <ben at bwidawsk.net> wrote:
> On Wed, Jun 15, 2011 at 12:51:47AM +0800, Daniel J Blueman wrote:
>> On 14 June 2011 13:23, Eric Anholt <eric at anholt.net> wrote:
>> > On Tue, 14 Jun 2011 12:18:36 +0800, Daniel J Blueman <daniel.blueman at gmail.com> wrote:
>> >> Hi Eric,
>> >>
>> >> The frequent ~1.5s pauses I hit with SNB hardware in the gnome3 UI (eg
>> >> whenever you hit the top-left of the screen to show all windows) are
>> >> nicely addressed by your recent wake patch [1] (ported to -rc3). Thus
>> >> I see no 'missed IRQ' kernel messages.
>> >>
>> >> As this addresses a significant usability regression, are you happy to
>> >> add it to the 3.0-rc queue? I think it has very good value in -stable
>> >> also (assuming correctness). What do you think?
>> >
>> > This one had significant performance impacts, and later hacks in this
>> > series worked around the problem to approximately the same level of
>> > success with less impact, and we don't actually have a justification of
>> > why any of them work.  We were still hoping to come up with some clue,
>> > and haven't yet.
>>
>> True; that is quite heavy handed delay looping.
>>
>> It's a pity the usual Intel font didn't make it to the programmer's
>> reference manuals. Anyway, unmasking the blitter user interrupt in the hardware
>> status mask register addresses the root cause. Out of reset it's FFFFFFFFh,
>> so we don't need to read it here.
>>
>> It would be good to get this into -rc4. -stable probably needs some additional
>> tweaks.
>>
>> Signed-off-by: Daniel J Blueman <daniel.blueman at gmail.com>
>> ---
>>  drivers/gpu/drm/i915/i915_irq.c |    6 ++++++
>>  1 files changed, 6 insertions(+), 0 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
>> index b9fafe3..9a98c1b 100644
>> --- a/drivers/gpu/drm/i915/i915_irq.c
>> +++ b/drivers/gpu/drm/i915/i915_irq.c
>> @@ -1827,6 +1827,12 @@ int ironlake_irq_postinstall(struct drm_device *dev)
>>               ironlake_enable_display_irq(dev_priv, DE_PCU_EVENT);
>>       }
>>
>> +     if (IS_GEN6(dev))
>> +             /* allow blitter user interrupt to generate a MSI write from
>> +                the ISR */
>> +             I915_WRITE(GEN6_BLITTER_HWSTAM,
>> +                     0xffffffff & ~GEN6_BLITTER_USER_INTERRUPT);
>> +
>>       return 0;
>>  }
>>
>
> I wish the docs said that that this hwstam unmasked MI_USER_INTERRUPTS
> parsed by the Blitter Command Parser, instead of the Render Command
> parser.
>
> I was just about to write an email about how this is just making the
> same interrupt happen twice, when I realized that the docs make no
> sense, and this must be a copy-paste doc bug.
>
> This patch sounds good to me. Two small comments:
> 1. The HWSTAM is touched in preinstall already, why not move this there?
> 2. I'd prefer you read the register even though as you say it isn't
> necessary. It just makes the code self-documented by doing it that way.

The render HWSTAM is tweaked in preinstall, but we need to tweak the
blitter HWSTAM (new to gen6).

To me, it makes sense to reset the blitter HWSTAM register to what the
driver expects, in case anything before the i915 module loads and
adjusts it for a particular purpose (including debug); the render
HWSTAM is set this way too. I could add a comment to both perhaps?

Updating the blitter HWSTAM in the postinstall was a marginally safer
choice, as there'll not be any potential race with a blitter user
interrupt getting emitted before we're ready (which wouldn't have been
tested), but the risk is probably so low that it could just go into
the preinstall.

What do you think?

Daniel
-- 
Daniel J Blueman