Subject: Re: [RFC PATCH] drm/panfrost: Handle IDVS_GROUP_SIZE feature
To:     Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
        dri-devel@lists.freedesktop.org
Cc:     Rob Herring <robh@kernel.org>,
        Tomeu Vizoso <tomeu.vizoso@collabora.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>, linux-kernel@vger.kernel.org
References: <20220109171254.3183-1-alyssa.rosenzweig@collabora.com>
From:   Steven Price <steven.price@arm.com>
Message-ID: <4628eb5a-b644-47af-a865-73300460a92b@arm.com>
Date:   Mon, 10 Jan 2022 17:10:17 +0000
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On 09/01/2022 17:12, Alyssa Rosenzweig wrote:
> The IDVS group size feature was missing. It is used on some Bifrost and
> Valhall GPUs, and is the last kernel-relevant Bifrost feature we're
> missing.
> 
> This feature adds an extra IDVS group size field to the JM_CONFIG
> register. In kbase, the value is configurable via the device tree; kbase
> uses 0xF as a default if no value is specified. Until we find a device
> demanding otherwise, let's always set the 0xF default on devices which
> support this feature mimicking kbase's behaviour.

This is a performance thing - so I don't think it will break anything if
this is wrong, it just won't be optimal.

> As JM_CONFIG is an undocumented register, it's not clear to me what
> happens if we fail to include this handling. Index-driven vertex shading
> already works on Bifrost boards with this feature without this handling.
> Perhaps this has performance implications? Patch untested for the
> moment, wanted to give Steven a chance to comment.

As it's a performance thing you shouldn't see correctness issues with
not setting it. But 0xF seems to have been chosen as it gave the best
overall performance (although for individual test content this can
vary). AFAICT the performance impact isn't massive either.

> Applies on top of my feature clean up series which should go in first.
> (That's pure cleaunp, this is a behaviour change RFC needing
> discussion.)
> 
> Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

Reviewed-by: Steven Price <steven.price@arm.com>

Since you've tagged this RFC I won't merge it now, but it looks correct
to me.

Thanks,

Steve

> ---
>  drivers/gpu/drm/panfrost/panfrost_features.h | 3 +++
>  drivers/gpu/drm/panfrost/panfrost_gpu.c      | 3 +++
>  drivers/gpu/drm/panfrost/panfrost_regs.h     | 1 +
>  3 files changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_features.h b/drivers/gpu/drm/panfrost/panfrost_features.h
> index 34f2bae1ec8c..36fadcf9634e 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_features.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_features.h
> @@ -20,6 +20,7 @@ enum panfrost_hw_feature {
>  	HW_FEATURE_AARCH64_MMU,
>  	HW_FEATURE_TLS_HASHING,
>  	HW_FEATURE_THREAD_GROUP_SPLIT,
> +	HW_FEATURE_IDVS_GROUP_SIZE,
>  	HW_FEATURE_3BIT_EXT_RW_L2_MMU_CONFIG,
>  };
>  
> @@ -74,6 +75,7 @@ enum panfrost_hw_feature {
>  	BIT_ULL(HW_FEATURE_FLUSH_REDUCTION) | \
>  	BIT_ULL(HW_FEATURE_PROTECTED_MODE) | \
>  	BIT_ULL(HW_FEATURE_PROTECTED_DEBUG_MODE) | \
> +	BIT_ULL(HW_FEATURE_IDVS_GROUP_SIZE) | \
>  	BIT_ULL(HW_FEATURE_COHERENCY_REG))
>  
>  #define hw_features_g76 (\
> @@ -87,6 +89,7 @@ enum panfrost_hw_feature {
>  	BIT_ULL(HW_FEATURE_COHERENCY_REG) | \
>  	BIT_ULL(HW_FEATURE_AARCH64_MMU) | \
>  	BIT_ULL(HW_FEATURE_TLS_HASHING) | \
> +	BIT_ULL(HW_FEATURE_IDVS_GROUP_SIZE) | \
>  	BIT_ULL(HW_FEATURE_3BIT_EXT_RW_L2_MMU_CONFIG))
>  
>  #define hw_features_g31 (\
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gpu.c b/drivers/gpu/drm/panfrost/panfrost_gpu.c
> index bbe628b306ee..50c8922694d7 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gpu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gpu.c
> @@ -145,6 +145,9 @@ static void panfrost_gpu_init_quirks(struct panfrost_device *pfdev)
>  		quirks |= (COHERENCY_ACE_LITE | COHERENCY_ACE) <<
>  			   JM_FORCE_COHERENCY_FEATURES_SHIFT;
>  
> +	if (panfrost_has_hw_feature(pfdev, HW_FEATURE_IDVS_GROUP_SIZE))
> +		quirks |= JM_DEFAULT_IDVS_GROUP_SIZE << JM_IDVS_GROUP_SIZE_SHIFT;
> +
>  	if (quirks)
>  		gpu_write(pfdev, GPU_JM_CONFIG, quirks);
>  
> diff --git a/drivers/gpu/drm/panfrost/panfrost_regs.h b/drivers/gpu/drm/panfrost/panfrost_regs.h
> index 6c5a11ef1ee8..16e776cc82ea 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_regs.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_regs.h
> @@ -208,6 +208,7 @@
>  #define JM_MAX_JOB_THROTTLE_LIMIT	0x3F
>  #define JM_FORCE_COHERENCY_FEATURES_SHIFT 2
>  #define JM_IDVS_GROUP_SIZE_SHIFT	16
> +#define JM_DEFAULT_IDVS_GROUP_SIZE	0xF
>  #define JM_MAX_IDVS_GROUP_SIZE		0x3F
>  
>  
>