Re: [PATCH 3/4] drm/panthor: Implement reading shader_present from nvmem

[Boris Brezillon <boris.brezillon@collabora.com>]

On Wed, 17 Dec 2025 18:03:29 +0100
Nicolas Frattaroli <nicolas.frattaroli@collabora.com> wrote:

> On some platforms, notably MediaTek MT8196, the shader_present bitmask
> in the Mali GPU register for it has cores enabled that may be faulty.
> The true shader_present bitmask is found in an efuse instead.
> 
> Implement reading shader_present from an nvmem cell if one is present,
> falling back to the Mali register if it's absent. The error codes are
> trickled up through to the probe function so that probe deferral works.
> 
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

> ---
>  drivers/gpu/drm/panthor/panthor_hw.c | 63 ++++++++++++++++++++++++++++++++----
>  1 file changed, 57 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_hw.c b/drivers/gpu/drm/panthor/panthor_hw.c
> index 87ebb7ae42c4..eb44c8b108aa 100644
> --- a/drivers/gpu/drm/panthor/panthor_hw.c
> +++ b/drivers/gpu/drm/panthor/panthor_hw.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0 or MIT
>  /* Copyright 2025 ARM Limited. All rights reserved. */
>  
> +#include <linux/nvmem-consumer.h>
>  #include <drm/drm_print.h>
>  
>  #include "panthor_device.h"
> @@ -109,7 +110,52 @@ static char *get_gpu_model_name(struct panthor_device *ptdev)
>  	return "(Unknown Mali GPU)";
>  }
>  
> -static void panthor_gpu_info_init(struct panthor_device *ptdev)
> +static int overload_shader_present(struct panthor_device *ptdev)
> +{
> +	struct device *dev = ptdev->base.dev;
> +	struct nvmem_cell *cell = nvmem_cell_get(dev, "shader-present");
> +	ssize_t len;
> +	void *buf;
> +	int ret;
> +
> +	if (IS_ERR(cell)) {
> +		/* On platforms without this cell, use the Mali register */
> +		if (PTR_ERR(cell) == -ENOENT)
> +			return 0;
> +
> +		return dev_err_probe(dev, PTR_ERR(cell),
> +				     "Failed to get shader-present nvmem cell\n");
> +	}
> +
> +	buf = nvmem_cell_read(cell, &len);
> +	if (IS_ERR(buf)) {
> +		ret = dev_err_probe(dev, PTR_ERR(buf),
> +				    "Failed to read shader-present nvmem cell\n");
> +		goto err_put_cell;
> +	}
> +
> +	if (!len || len > 8) {
> +		ret = dev_err_probe(dev, -EINVAL, "shader-present cell can't be length %ld\n",
> +				    len);
> +		goto err_free;
> +	}
> +
> +	memcpy(&ptdev->gpu_info.shader_present, buf, len);
> +
> +	kfree(buf);
> +	nvmem_cell_put(cell);
> +
> +	return 0;
> +
> +err_free:
> +	kfree(buf);
> +err_put_cell:
> +	nvmem_cell_put(cell);
> +
> +	return ret;
> +}
> +
> +static int panthor_gpu_info_init(struct panthor_device *ptdev)
>  {
>  	unsigned int i;
>  
> @@ -143,13 +189,18 @@ static void panthor_gpu_info_init(struct panthor_device *ptdev)
>  		ptdev->gpu_info.tiler_present = gpu_read64(ptdev, GPU_TILER_PRESENT);
>  		ptdev->gpu_info.l2_present = gpu_read64(ptdev, GPU_L2_PRESENT);
>  	}
> +
> +	return overload_shader_present(ptdev);
>  }
>  
> -static void panthor_hw_info_init(struct panthor_device *ptdev)
> +static int panthor_hw_info_init(struct panthor_device *ptdev)
>  {
>  	u32 major, minor, status;
> +	int ret;
>  
> -	panthor_gpu_info_init(ptdev);
> +	ret = panthor_gpu_info_init(ptdev);
> +	if (ret)
> +		return ret;
>  
>  	major = GPU_VER_MAJOR(ptdev->gpu_info.gpu_id);
>  	minor = GPU_VER_MINOR(ptdev->gpu_info.gpu_id);
> @@ -172,6 +223,8 @@ static void panthor_hw_info_init(struct panthor_device *ptdev)
>  		 "shader_present=0x%0llx l2_present=0x%0llx tiler_present=0x%0llx",
>  		 ptdev->gpu_info.shader_present, ptdev->gpu_info.l2_present,
>  		 ptdev->gpu_info.tiler_present);
> +
> +	return 0;
>  }
>  
>  static int panthor_hw_bind_device(struct panthor_device *ptdev)
> @@ -218,7 +271,5 @@ int panthor_hw_init(struct panthor_device *ptdev)
>  	if (ret)
>  		return ret;
>  
> -	panthor_hw_info_init(ptdev);
> -
> -	return 0;
> +	return panthor_hw_info_init(ptdev);
>  }
>