[PATCH v4 1/3] drm/i915/gt: Disable HW load balancing for CCS

[Matt Roper]

On Wed, Mar 06, 2024 at 02:22:45AM +0100, Andi Shyti wrote:
> The hardware should not dynamically balance the load between CCS
> engines. Wa_14019159160 recommends disabling it across all
> platforms.
> 
> Fixes: d2eae8e98d59 ("drm/i915/dg2: Drop force_probe requirement")
> Signed-off-by: Andi Shyti <andi.shyti at linux.intel.com>
> Cc: Chris Wilson <chris.p.wilson at linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Matt Roper <matthew.d.roper at intel.com>
> Cc: <stable at vger.kernel.org> # v6.2+
> ---
>  drivers/gpu/drm/i915/gt/intel_gt_regs.h     | 1 +
>  drivers/gpu/drm/i915/gt/intel_workarounds.c | 5 +++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_gt_regs.h b/drivers/gpu/drm/i915/gt/intel_gt_regs.h
> index 50962cfd1353..cf709f6c05ae 100644
> --- a/drivers/gpu/drm/i915/gt/intel_gt_regs.h
> +++ b/drivers/gpu/drm/i915/gt/intel_gt_regs.h
> @@ -1478,6 +1478,7 @@
>  
>  #define GEN12_RCU_MODE				_MMIO(0x14800)
>  #define   GEN12_RCU_MODE_CCS_ENABLE		REG_BIT(0)
> +#define   XEHP_RCU_MODE_FIXED_SLICE_CCS_MODE	REG_BIT(1)
>  
>  #define CHV_FUSE_GT				_MMIO(VLV_GUNIT_BASE + 0x2168)
>  #define   CHV_FGT_DISABLE_SS0			(1 << 10)
> diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> index d67d44611c28..a2e78cf0b5f5 100644
> --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> @@ -2945,6 +2945,11 @@ general_render_compute_wa_init(struct intel_engine_cs *engine, struct i915_wa_li
>  
>  		/* Wa_18028616096 */
>  		wa_mcr_write_or(wal, LSC_CHICKEN_BIT_0_UDW, UGM_FRAGMENT_THRESHOLD_TO_3);
> +
> +		/*
> +		 * Wa_14019159160: disable the automatic CCS load balancing

I'm still a bit concerned that this doesn't really match what this
specific workaround is asking us to do.  There seems to be an agreement
on various internal email threads that we need to disable load
balancing, but there's no single specific workaround that officially
documents that decision.

This specific workaround asks us to do a bunch of different things, and
the third item it asks for is to disable load balancing in very specific
cases (i.e., while the RCS is active at the same time as one or more CCS
engines).  Taking this workaround in isolation, it would be valid to
keep load balancing active if you were just using the CCS engines and
leaving the RCS idle, or if balancing was turned on/off by the GuC
scheduler according to engine use at the moment, as the documented
workaround seems to assume will be the case.

So in general I think we do need to disable load balancing based on
other offline discussion, but blaming that entire change on
Wa_14019159160 seems a bit questionable since it's not really what this
specific workaround is asking us to do and someone may come back and try
to "correct" the implementation of this workaround in the future without
realizing there are other factors too.  It would be great if we could
get hardware teams to properly document this expectation somewhere
(either in a separate dedicated workaround, or in the MMIO tuning guide)
so that we'll have a more direct and authoritative source for such a
large behavioral change.


Matt

> +		 */
> +		wa_masked_en(wal, GEN12_RCU_MODE, XEHP_RCU_MODE_FIXED_SLICE_CCS_MODE);
>  	}
>  
>  	if (IS_DG2_G11(i915)) {
> -- 
> 2.43.0
> 

-- 
Matt Roper
Graphics Software Engineer
Linux GPU Platform Enablement
Intel Corporation