Re: [Intel-gfx] [PATCH v5 8/9] drm/i915/dg2: Maintain backward-compatible nested batch behavior

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

On 05/08/2021 17:36, Matt Roper wrote:
> For tgl+, the per-context setting of MI_MODE[12] determines whether
> the bits of a nested MI_BATCH_BUFFER_START instruction should be
> interpreted in the traditional manner or whether they should
> instead use a new tgl+ meaning that breaks backward compatibility, but
> allows nesting into 3rd-level batchbuffers.  For previous platforms,
> the hardware default for this register bit is to maintain
> backward-compatible behavior unless a context intentionally opts into
> the new behavior; however Xe_HPG flips the hardware default behavior.
> 
>>From a SW perspective, we want to maintain the backward-compatible
> behavior for userspace, so we'll apply a fake workaround to set it back
> to the legacy behavior on platforms where the hardware default is to
> break compatibility.  At the moment there is no Linux userspace that
> utilizes third-level batchbuffers, so this will avoid userspace from
> needing to make any changes.  using the legacy meaning is the correct
> thing to do.  If/when we have userspace consumers that want to utilize
> third-level batch nesting, we can provide a context parameter to allow
> them to opt-in.
> 
> Bspec: 45974, 45718
> Cc: John Harrison <John.C.Harrison@Intel.com>
> Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_workarounds.c | 39 +++++++++++++++++++--
>   drivers/gpu/drm/i915/i915_reg.h             |  1 +
>   2 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> index aae609d7d85d..97b3cd81b721 100644
> --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> @@ -644,6 +644,37 @@ static void dg1_ctx_workarounds_init(struct intel_engine_cs *engine,
>   		     DG1_HZ_READ_SUPPRESSION_OPTIMIZATION_DISABLE);
>   }
>   
> +static void fakewa_disable_nestedbb_mode(struct intel_engine_cs *engine,
> +					 struct i915_wa_list *wal)
> +{
> +	/*
> +	 * This is a "fake" workaround defined by software to ensure we
> +	 * maintain reliable, backward-compatible behavior for userspace with
> +	 * regards to how nested MI_BATCH_BUFFER_START commands are handled.
> +	 *
> +	 * The per-context setting of MI_MODE[12] determines whether the bits
> +	 * of a nested MI_BATCH_BUFFER_START instruction should be interpreted
> +	 * in the traditional manner or whether they should instead use a new
> +	 * tgl+ meaning that breaks backward compatibility, but allows nesting
> +	 * into 3rd-level batchbuffers.  When this new capability was first
> +	 * added in TGL, it remained off by default unless a context
> +	 * intentionally opted in to the new behavior.  However Xe_HPG now
> +	 * flips this on by default and requires that we explicitly opt out if
> +	 * we don't want the new behavior.
> +	 *
> +	 * From a SW perspective, we want to maintain the backward-compatible
> +	 * behavior for userspace, so we'll apply a fake workaround to set it
> +	 * back to the legacy behavior on platforms where the hardware default
> +	 * is to break compatibility.  At the moment there is no Linux
> +	 * userspace that utilizes third-level batchbuffers, so this will avoid
> +	 * userspace from needing to make any changes.  using the legacy
> +	 * meaning is the correct thing to do.  If/when we have userspace
> +	 * consumers that want to utilize third-level batch nesting, we can
> +	 * provide a context parameter to allow them to opt-in.
> +	 */
> +	wa_masked_dis(wal, RING_MI_MODE(engine->mmio_base), TGL_NESTED_BB_EN);
> +}
> +
>   static void
>   __intel_engine_init_ctx_wa(struct intel_engine_cs *engine,
>   			   struct i915_wa_list *wal,
> @@ -651,11 +682,15 @@ __intel_engine_init_ctx_wa(struct intel_engine_cs *engine,
>   {
>   	struct drm_i915_private *i915 = engine->i915;
>   
> +	wa_init_start(wal, name, engine->name);
> +
> +	/* Applies to all engines */
> +	if (GRAPHICS_VER_FULL(i915) >= IP_VER(12, 55))
> +		fakewa_disable_nestedbb_mode(engine, wal);
> +
>   	if (engine->class != RENDER_CLASS)
>   		return;

Is it intentional to skip wa_init_finish on non-render engines? Would be 
a bit odd although granted no significant functional difference apart 
from not logging and maybe not trimming the list storage.

Regards,

Tvrtko

>   
> -	wa_init_start(wal, name, engine->name);
> -
>   	if (IS_DG1(i915))
>   		dg1_ctx_workarounds_init(engine, wal);
>   	else if (GRAPHICS_VER(i915) == 12)
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 77f6dcaba2b9..269685955fbd 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -2821,6 +2821,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
>   #define MI_MODE		_MMIO(0x209c)
>   # define VS_TIMER_DISPATCH				(1 << 6)
>   # define MI_FLUSH_ENABLE				(1 << 12)
> +# define TGL_NESTED_BB_EN				(1 << 12)
>   # define ASYNC_FLIP_PERF_DISABLE			(1 << 14)
>   # define MODE_IDLE					(1 << 9)
>   # define STOP_RING					(1 << 8)
>