Re: [PATCH v3] drm/panfrost: Add AFBC_FEATURES parameter

[Steven Price <steven.price@arm.com>]

On 03/06/2021 22:21, Alyssa Rosenzweig wrote:
> The value of the AFBC_FEATURES register is required by userspace to
> determine AFBC support on Bifrost. A user on our IRC channel (#panfrost)
> reported a workload that raised a fault on one system's Mali G31 but
> worked flawlessly with another system's Mali G31. We determined the
> cause to be missing AFBC support on one vendor's Mali implementation --
> it turns out AFBC is optional on Bifrost!

I was surprised this had been missed, but it turns out the DDK doesn't
care so kbase doesn't expose it either! I guess this is one of those
"recompile the driver to support your hardware" cases with the blob.

> Whether AFBC is supported or not is exposed in the AFBC_FEATURES
> register on Bifrost, which reads back as 0 on Midgard. A zero value
> indicates AFBC is fully supported, provided the architecture itself
> supports AFBC, allowing backwards-compatibility with Midgard. Bits 0 and
> 15 indicate that AFBC support is absent for texturing and rendering
> respectively.
> 
> The user experiencing the fault reports that AFBC_FEATURES reads back
> 0x10001 on their system, confirming the architectural lack of AFBC.
> Userspace needs this this parameter to know to disable AFBC on that
> chip, and perhaps others.
> 
> v2: Fix typo from copy-paste fail.
> 
> v3: Bump the UABI version. This commit was cherry-picked from another
> series so chalking this up to a rebase fail.

I'd like to say third time's the charm, but...

> 
> Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> Cc: Rob Herring <robh@kernel.org>
> Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> Cc: Steven Price <steven.price@arm.com>
> ---
>  drivers/gpu/drm/panfrost/panfrost_device.h | 1 +
>  drivers/gpu/drm/panfrost/panfrost_drv.c    | 3 ++-
>  drivers/gpu/drm/panfrost/panfrost_gpu.c    | 1 +
>  drivers/gpu/drm/panfrost/panfrost_regs.h   | 1 +
>  include/uapi/drm/panfrost_drm.h            | 1 +
>  5 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_device.h b/drivers/gpu/drm/panfrost/panfrost_device.h
> index 597cf1459..f614e9877 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_device.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_device.h
> @@ -45,6 +45,7 @@ struct panfrost_features {
>  	u32 thread_max_workgroup_sz;
>  	u32 thread_max_barrier_sz;
>  	u32 coherency_features;
> +	u32 afbc_features;
>  	u32 texture_features[4];
>  	u32 js_features[16];
>  
> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> index ca07098a6..365ee53f0 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> @@ -63,6 +63,7 @@ static int panfrost_ioctl_get_param(struct drm_device *ddev, void *data, struct
>  		PANFROST_FEATURE(THREAD_MAX_BARRIER_SZ,
>  				thread_max_barrier_sz);
>  		PANFROST_FEATURE(COHERENCY_FEATURES, coherency_features);
> +		PANFROST_FEATURE(AFBC_FEATURES, afbc_features);
>  		PANFROST_FEATURE_ARRAY(TEXTURE_FEATURES, texture_features, 3);
>  		PANFROST_FEATURE_ARRAY(JS_FEATURES, js_features, 15);
>  		PANFROST_FEATURE(NR_CORE_GROUPS, nr_core_groups);
> @@ -559,7 +560,7 @@ static const struct drm_driver panfrost_drm_driver = {

Missing context:

> /*
>  * Panfrost driver version:
>  * - 1.0 - initial interface
>  * - 1.1 - adds HEAP and NOEXEC flags for CREATE_BO
>  */

It would be good to add the new version in this comment to document what
it adds. With that added:

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

Steve

>  	.desc			= "panfrost DRM",
>  	.date			= "20180908",
>  	.major			= 1,
> -	.minor			= 1,
> +	.minor			= 2,
>  
>  	.gem_create_object	= panfrost_gem_create_object,
>  	.prime_handle_to_fd	= drm_gem_prime_handle_to_fd,
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gpu.c b/drivers/gpu/drm/panfrost/panfrost_gpu.c
> index 2aae636f1..0e70e27fd 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gpu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gpu.c
> @@ -228,6 +228,7 @@ static void panfrost_gpu_init_features(struct panfrost_device *pfdev)
>  	pfdev->features.thread_max_workgroup_sz = gpu_read(pfdev, GPU_THREAD_MAX_WORKGROUP_SIZE);
>  	pfdev->features.thread_max_barrier_sz = gpu_read(pfdev, GPU_THREAD_MAX_BARRIER_SIZE);
>  	pfdev->features.coherency_features = gpu_read(pfdev, GPU_COHERENCY_FEATURES);
> +	pfdev->features.afbc_features = gpu_read(pfdev, GPU_AFBC_FEATURES);
>  	for (i = 0; i < 4; i++)
>  		pfdev->features.texture_features[i] = gpu_read(pfdev, GPU_TEXTURE_FEATURES(i));
>  
> diff --git a/drivers/gpu/drm/panfrost/panfrost_regs.h b/drivers/gpu/drm/panfrost/panfrost_regs.h
> index eddaa62ad..dc9df5457 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_regs.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_regs.h
> @@ -82,6 +82,7 @@
>  
>  #define GPU_TEXTURE_FEATURES(n)		(0x0B0 + ((n) * 4))
>  #define GPU_JS_FEATURES(n)		(0x0C0 + ((n) * 4))
> +#define GPU_AFBC_FEATURES		(0x4C)	/* (RO) AFBC support on Bifrost */
>  
>  #define GPU_SHADER_PRESENT_LO		0x100	/* (RO) Shader core present bitmap, low word */
>  #define GPU_SHADER_PRESENT_HI		0x104	/* (RO) Shader core present bitmap, high word */
> diff --git a/include/uapi/drm/panfrost_drm.h b/include/uapi/drm/panfrost_drm.h
> index ec19db1ee..061e700dd 100644
> --- a/include/uapi/drm/panfrost_drm.h
> +++ b/include/uapi/drm/panfrost_drm.h
> @@ -171,6 +171,7 @@ enum drm_panfrost_param {
>  	DRM_PANFROST_PARAM_JS_FEATURES15,
>  	DRM_PANFROST_PARAM_NR_CORE_GROUPS,
>  	DRM_PANFROST_PARAM_THREAD_TLS_ALLOC,
> +	DRM_PANFROST_PARAM_AFBC_FEATURES,
>  };
>  
>  struct drm_panfrost_get_param {
>