[Intel-gfx] [PATCH] drm/i915: Make RPS EI/thresholds multiple of 25 on SNB

[Patrik Jakobsson]

On Wed, Apr 20, 2016 at 04:43:56PM +0300, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Somehow my SNB GT1 (Dell XPS 8300) gets very unhappy around
> GPU hangs if the RPS EI/thresholds aren't suitably aligned.
> It seems like scheduling/timer interupts stop working somehow
> and things get stuck eg. in usleep_range().
> 
> I bisected the problem down to
> commit 8a5864377b12 ("drm/i915/skl: Restructured the gen6_set_rps_thresholds function")
> I observed that before all the values were at least multiples of 25,
> but afterwards they are not. And rounding things up to the next multiple
> of 25 does seem to help, so lets' do that. I also tried roundup(..., 5)
> but that wasn't sufficient. Also I have no idea if we might need this sort of
> thing on gen9+ as well.

Do we need to test for performance regressions on stuff like this? And if so,
who do we ping about this?

OTOH impact should be really small and since this fixes a real problem:

Reviewed-by: Patrik Jakobsson <patrik.jakobsson at linux.intel.com>

> 
> These are the original EI/thresholds:
>  LOW_POWER
>   GEN6_RP_UP_EI          12500
>   GEN6_RP_UP_THRESHOLD   11800
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 21250
>  BETWEEN
>   GEN6_RP_UP_EI          10250
>   GEN6_RP_UP_THRESHOLD    9225
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 18750
>  HIGH_POWER
>   GEN6_RP_UP_EI           8000
>   GEN6_RP_UP_THRESHOLD    6800
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 15000
> 
> These are after 8a5864377b12:
>  LOW_POWER
>   GEN6_RP_UP_EI          12500
>   GEN6_RP_UP_THRESHOLD   11875
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 21250
>  BETWEEN
>   GEN6_RP_UP_EI          10156
>   GEN6_RP_UP_THRESHOLD    9140
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 18750
>  HIGH_POWER
>   GEN6_RP_UP_EI           7812
>   GEN6_RP_UP_THRESHOLD    6640
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 15000
> 
> And these are what we have after this patch:
>  LOW_POWER
>   GEN6_RP_UP_EI          12500
>   GEN6_RP_UP_THRESHOLD   11875
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 21250
>  BETWEEN
>   GEN6_RP_UP_EI          10175
>   GEN6_RP_UP_THRESHOLD    9150
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 18750
>  HIGH_POWER
>   GEN6_RP_UP_EI           7825
>   GEN6_RP_UP_THRESHOLD    6650
>   GEN6_RP_DOWN_EI        25000
>   GEN6_RP_DOWN_THRESHOLD 15000
> 
> Cc: stable at vger.kernel.org
> Cc: Akash Goel <akash.goel at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Testcase: igt/kms_pipe_crc_basic/hang-read-crc-pipe-B
> Fixes: 8a5864377b12 ("drm/i915/skl: Restructured the gen6_set_rps_thresholds function")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_reg.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index c21b71c86a6b..08f01f4470cd 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -2948,7 +2948,14 @@ enum skl_disp_power_wells {
>  #define GEN6_RP_STATE_CAP	_MMIO(MCHBAR_MIRROR_BASE_SNB + 0x5998)
>  #define BXT_RP_STATE_CAP        _MMIO(0x138170)
>  
> -#define INTERVAL_1_28_US(us)	(((us) * 100) >> 7)
> +/*
> + * Make these a multiple of magic 25 to avoid SNB (eg. Dell XPS
> + * 8300) freezing up around GPU hangs. Looks as if even
> + * scheduling/timer interrupts start misbehaving if the RPS
> + * EI/thresholds are "bad", leading to a very sluggish or even
> + * frozen machine.
> + */
> +#define INTERVAL_1_28_US(us)	roundup(((us) * 100) >> 7, 25)
>  #define INTERVAL_1_33_US(us)	(((us) * 3)   >> 2)
>  #define INTERVAL_0_833_US(us)	(((us) * 6) / 5)
>  #define GT_INTERVAL_FROM_US(dev_priv, us) (IS_GEN9(dev_priv) ? \
> -- 
> 2.7.4
> 
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