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.
> DRM_ERROR("Force wake wait timed out\n");
>
> I915_WRITE_NOTRACE(FORCEWAKE, 1);
> POSTING_READ(FORCEWAKE);
>
> - if (wait_for_atomic_us((I915_READ_NOTRACE(forcewake_ack) & 1),
> - FORCEWAKE_ACK_TIMEOUT_US))
> + if (wait_for_atomic((I915_READ_NOTRACE(forcewake_ack) & 1),
> + FORCEWAKE_ACK_TIMEOUT_MS))
> DRM_ERROR("Force wake wait timed out\n");
>
> __gen6_gt_wait_for_thread_c0(dev_priv);
> @@ -3993,15 +3993,15 @@ static void __gen6_gt_force_wake_mt_get(struct drm_i915_private *dev_priv)
> else
> forcewake_ack = FORCEWAKE_MT_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))
> DRM_ERROR("Force wake wait timed out\n");
>
> I915_WRITE_NOTRACE(FORCEWAKE_MT, _MASKED_BIT_ENABLE(1));
> POSTING_READ(FORCEWAKE_MT);
>
> - if (wait_for_atomic_us((I915_READ_NOTRACE(forcewake_ack) & 1),
> - FORCEWAKE_ACK_TIMEOUT_US))
> + if (wait_for_atomic((I915_READ_NOTRACE(forcewake_ack) & 1),
> + FORCEWAKE_ACK_TIMEOUT_MS))
> DRM_ERROR("Force wake wait timed out\n");
>
> __gen6_gt_wait_for_thread_c0(dev_priv);
> @@ -4088,8 +4088,8 @@ static void vlv_force_wake_get(struct drm_i915_private *dev_priv)
> I915_WRITE_NOTRACE(FORCEWAKE_VLV, 0xffffffff);
> POSTING_READ(FORCEWAKE_VLV);
>
> - if (wait_for_atomic_us((I915_READ_NOTRACE(FORCEWAKE_ACK_VLV) & 1),
> - FORCEWAKE_ACK_TIMEOUT_US))
> + if (wait_for_atomic((I915_READ_NOTRACE(FORCEWAKE_ACK_VLV) & 1),
> + FORCEWAKE_ACK_TIMEOUT_MS))
> DRM_ERROR("Force wake wait timed out\n");
>
> __gen6_gt_wait_for_thread_c0(dev_priv);
> --
> 1.7.12
>
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> Intel-gfx at lists.freedesktop.org
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