On Tue, Aug 28, 2012 at 5:51 PM, Ben Widawsky <ben at bwidawsk.net> wrote: > On 2012-08-26 23:59, Jani Nikula wrote: >> >> On Fri, 24 Aug 2012, Ben Widawsky <ben at bwidawsk.net> wrote: >>> >>> A designer familiar with the hardware has stated that the forcewake >>> timeout can theoretically be as high as a little over 1ms. Therefore we >>> modify our code to use 2ms (appropriate fudge and because we don't want >>> to round down). >>> >>> Hopefully this can't prevent spurious timeouts. >>> >>> Signed-off-by: Ben Widawsky <ben at bwidawsk.net> >>> --- >>> drivers/gpu/drm/i915/intel_pm.c | 22 +++++++++++----------- >>> 1 file changed, 11 insertions(+), 11 deletions(-) >>> >>> diff --git a/drivers/gpu/drm/i915/intel_pm.c >>> b/drivers/gpu/drm/i915/intel_pm.c >>> index f42c142..2a8468d 100644 >>> --- a/drivers/gpu/drm/i915/intel_pm.c >>> +++ b/drivers/gpu/drm/i915/intel_pm.c >>> @@ -31,7 +31,7 @@ >>> #include "../../../platform/x86/intel_ips.h" >>> #include <linux/module.h> >>> >>> -#define FORCEWAKE_ACK_TIMEOUT_US 500 >>> +#define FORCEWAKE_ACK_TIMEOUT_MS 2 >>> >>> /* FBC, or Frame Buffer Compression, is a technique employed to compress >>> the >>> * framebuffer contents in-memory, aiming at reducing the required >>> bandwidth >>> @@ -3970,15 +3970,15 @@ static void __gen6_gt_force_wake_get(struct >>> drm_i915_private *dev_priv) >>> else >>> forcewake_ack = FORCEWAKE_ACK; >>> >>> - if (wait_for_atomic_us((I915_READ_NOTRACE(forcewake_ack) & 1) == >>> 0, >>> - FORCEWAKE_ACK_TIMEOUT_US)) >>> + if (wait_for_atomic((I915_READ_NOTRACE(forcewake_ack) & 1) == 0, >>> + FORCEWAKE_ACK_TIMEOUT_MS)) >> >> >> Superficially this looks okay, but the implementation of >> wait_for_atomic() not so. As a surprise, this change drops cpu_relax() >> from the busy loop, even thought the timeout is potentially much longer. >> >> The quick fix here would be to just use 2000 us with >> wait_for_atomic_us(), but we should do something about wait_for_atomic() >> too. Luckily it's only ever used at one place. >> >> BR, >> Jani. > > > Hmm, dare I say, I think this is a bug in _wait_for. Without spending too > much brain power on this, I believe the compiler can screw us over here. No > matter the bug, cpu_relax is still probably desirable, unless there is some > newer coolness here? I shall insert a patch before this to do the cpu_relax > in _wait_for. The original idea behind wiat_for_us was that we use udelay and really limit ourselves to that us timeout (since jiffies is too coarse). Now that the timeout for forcewake is 2ms (gawk!) I think we can stop bothering to pretend that this should timeout quickly and drop the _us variant (but still keep the cpu relax imo). -Daniel -- Daniel Vetter daniel.vetter at ffwll.ch - +41 (0) 79 365 57 48 - http://blog.ffwll.ch
