Re: [Intel-gfx] [PATCH v7] drm/i915: Enable WaProgramMgsrForCorrectSliceSpecificMmioReads for Gen9

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 22/10/2021 05:14, Cooper Chiou wrote:
> WaProgramMgsrForCorrectSliceSpecificMmioReads applies for Gen9 to
> resolve VP8 hardware encoding system hang up on GT1 sku for
> ChromiumOS projects. This system hang issue can be reproduced on
> Khronos OpenCL conformance tests by test_basic as well, and issue
> can be resolved by this patch on Linux.
> 
> Slice specific MMIO read inaccurate so MGSR needs to be programmed
> appropriately to get correct reads from these slicet-related MMIOs.
> 
> It dictates that before any MMIO read into Slice/Subslice specific
> registers, MCR packet control register(0xFDC) needs to be programmed
> to point to any enabled slice/subslice pair, especially GT1 fused sku
> since this issue can be reproduced on VP8 hardware encoding via ffmpeg
> on ChromiumOS devices.
> When exit PC7, MGSR will reset so that we have to skip fused subslice ID.

Maybe reword the commit message like the below to highlight the OCL impact:

"""
This implements WaProgramMgsrForCorrectSliceSpecificMmioReads which
was omitted by mistake from Gen9 documentation, while it is
actually applicable to fused off parts.

Workaround consists of making sure MCR packet control register is
programmed to point to enabled slice/subslice pair before doing any
MMIO reads from the affected registers.

Failure do to this can result in complete system hangs when running
certain workloads. Two known cases which can cause system hangs are:

1. "test_basic progvar_prog_scope_uninit" test which is part of
    Khronos OpenCL conformance suite
    (https://github.com/KhronosGroup/OpenCL-CTS) with the Intel
    OpenCL driver (https://github.com/intel/compute-runtime).

2. VP8 media hardware encoding using the full-feature build of the
    Intel media-driver (https://github.com/intel/media-driver) and
    ffmpeg.

For the former case patch was verified to fix the hard system hang
when executing the OCL test on Intel Pentium CPU 6405U which
contains fused off GT1 graphics.
"""

Sounds acceptable to you Joonas?
  
> Reference: HSD#1508045018,1405586840, BSID#0575
> 
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: William Tseng <william.tseng@intel.com>
> Cc: Shawn C Lee <shawn.c.lee@intel.com>
> Cc: Pawel Wilma <pawel.wilma@intel.com>
> Signed-off-by: Cooper Chiou <cooper.chiou@intel.com>

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

> ---
>   drivers/gpu/drm/i915/gt/intel_workarounds.c | 41 +++++++++++++++++++++
>   1 file changed, 41 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> index e1f362530889..8ae24da601b0 100644
> --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> @@ -877,11 +877,52 @@ hsw_gt_workarounds_init(struct intel_gt *gt, struct i915_wa_list *wal)
>   	wa_write_clr(wal, GEN7_FF_THREAD_MODE, GEN7_FF_VS_REF_CNT_FFME);
>   }
>   
> +static void
> +gen9_wa_init_mcr(struct drm_i915_private *i915, struct i915_wa_list *wal)
> +{
> +	const struct sseu_dev_info *sseu = &i915->gt.info.sseu;
> +	unsigned int slice, subslice;
> +	u32 mcr, mcr_mask;
> +
> +	GEM_BUG_ON(GRAPHICS_VER(i915) != 9);
> +
> +	/*
> +	 * WaProgramMgsrForCorrectSliceSpecificMmioReads:gen9,glk,kbl,cml
> +	 * Before any MMIO read into slice/subslice specific registers, MCR
> +	 * packet control register needs to be programmed to point to any
> +	 * enabled s/ss pair. Otherwise, incorrect values will be returned.
> +	 * This means each subsequent MMIO read will be forwarded to an
> +	 * specific s/ss combination, but this is OK since these registers
> +	 * are consistent across s/ss in almost all cases. In the rare
> +	 * occasions, such as INSTDONE, where this value is dependent
> +	 * on s/ss combo, the read should be done with read_subslice_reg.
> +	 */
> +	slice = ffs(sseu->slice_mask) - 1;
> +	GEM_BUG_ON(slice >= ARRAY_SIZE(sseu->subslice_mask));
> +	subslice = ffs(intel_sseu_get_subslices(sseu, slice));
> +	GEM_BUG_ON(!subslice);
> +	subslice--;
> +
> +	/*
> +	 * We use GEN8_MCR..() macros to calculate the |mcr| value for
> +	 * Gen9 to address WaProgramMgsrForCorrectSliceSpecificMmioReads
> +	 */
> +	mcr = GEN8_MCR_SLICE(slice) | GEN8_MCR_SUBSLICE(subslice);
> +	mcr_mask = GEN8_MCR_SLICE_MASK | GEN8_MCR_SUBSLICE_MASK;
> +
> +	drm_dbg(&i915->drm, "MCR slice:%d/subslice:%d = %x\n", slice, subslice, mcr);
> +
> +	wa_write_clr_set(wal, GEN8_MCR_SELECTOR, mcr_mask, mcr);
> +}
> +
>   static void
>   gen9_gt_workarounds_init(struct intel_gt *gt, struct i915_wa_list *wal)
>   {
>   	struct drm_i915_private *i915 = gt->i915;
>   
> +	/* WaProgramMgsrForCorrectSliceSpecificMmioReads:glk,kbl,cml,gen9 */
> +	gen9_wa_init_mcr(i915, wal);
> +
>   	/* WaDisableKillLogic:bxt,skl,kbl */
>   	if (!IS_COFFEELAKE(i915) && !IS_COMETLAKE(i915))
>   		wa_write_or(wal,
>