linux-kernel - Re: [PATCH v2] drm/panthor: Simplify mmu_hw_do_operation

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Message-ID: <ee996a62-bcbf-4702-837e-85f93feb7240@arm.com>
Date: Fri, 15 Aug 2025 15:01:56 +0100
From: Karunika Choo <karunika.choo@....com>
To: Steven Price <steven.price@....com>,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Liviu Dudau <liviu.dudau@....com>, Daniel Stone <daniel@...ishbar.org>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 Chia-I Wu <olvaffe@...il.com>, nd@....com
Subject: Re: [PATCH v2] drm/panthor: Simplify mmu_hw_do_operation_locked

On 15/08/2025 14:42, Steven Price wrote:
> The only callers to mmu_hw_do_operation_locked() pass an 'op' of either
> AS_COMAND_FLUSH_MEM or AS_COMMAND_FLUSH_PT. This means the code paths
> after that are dead. Removing those paths means the
> mmu_hw_do_flush_on_gpu_ctrl() function might has well be inlined.
> 
> Simplify everything by having a switch statement for the type of 'op'
> (warning if we get an unexpected value) and removing the dead cases.
> 
> Suggested-by: Daniel Stone <daniel@...ishbar.org>
> Signed-off-by: Steven Price <steven.price@....com>
> ---
> Changes from v1:
>  * As well as removing dead code, inline mmu_hw_do_flush_on_gpu_ctrl
> 
>  drivers/gpu/drm/panthor/panthor_mmu.c | 57 ++++++++++++---------------
>  1 file changed, 26 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 367c89aca558..9d77e7c16ed2 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -569,15 +569,37 @@ static void lock_region(struct panthor_device *ptdev, u32 as_nr,
>  	write_cmd(ptdev, as_nr, AS_COMMAND_LOCK);
>  }
>  
> -static int mmu_hw_do_flush_on_gpu_ctrl(struct panthor_device *ptdev, int as_nr,
> -				       u32 op)
> +static int mmu_hw_do_operation_locked(struct panthor_device *ptdev, int as_nr,
> +				      u64 iova, u64 size, u32 op)
>  {
>  	const u32 l2_flush_op = CACHE_CLEAN | CACHE_INV;
> -	u32 lsc_flush_op = 0;
> +	u32 lsc_flush_op;
>  	int ret;
>  
> -	if (op == AS_COMMAND_FLUSH_MEM)
> +	lockdep_assert_held(&ptdev->mmu->as.slots_lock);
> +
> +	switch (op) {
> +	case AS_COMMAND_FLUSH_MEM:
>  		lsc_flush_op = CACHE_CLEAN | CACHE_INV;
> +		break;
> +	case AS_COMMAND_FLUSH_PT:
> +		lsc_flush_op = 0;
> +		break;
> +	default:
> +		drm_WARN(&ptdev->base, 1, "Unexpected AS_COMMAND: %d", op);
> +		return -EINVAL;
> +	}
> +
> +	if (as_nr < 0)
> +		return 0;
> +

Hi Steve,

Thanks for pushing this patch. I was planning to address Daniel's
comment next week.

One small nit, would it be better to move the (as_nr < 0) check just
after the lockdep_assert_held() (above the switch case)?

Looks good to me otherwise.

Kind regards,
Karunika

> +	/*
> +	 * If the AS number is greater than zero, then we can be sure
> +	 * the device is up and running, so we don't need to explicitly
> +	 * power it up
> +	 */
> +
> +	lock_region(ptdev, as_nr, iova, size);
>  
>  	ret = wait_ready(ptdev, as_nr);
>  	if (ret)
> @@ -598,33 +620,6 @@ static int mmu_hw_do_flush_on_gpu_ctrl(struct panthor_device *ptdev, int as_nr,
>  	return wait_ready(ptdev, as_nr);
>  }
>  
> -static int mmu_hw_do_operation_locked(struct panthor_device *ptdev, int as_nr,
> -				      u64 iova, u64 size, u32 op)
> -{
> -	lockdep_assert_held(&ptdev->mmu->as.slots_lock);
> -
> -	if (as_nr < 0)
> -		return 0;
> -
> -	/*
> -	 * If the AS number is greater than zero, then we can be sure
> -	 * the device is up and running, so we don't need to explicitly
> -	 * power it up
> -	 */
> -
> -	if (op != AS_COMMAND_UNLOCK)
> -		lock_region(ptdev, as_nr, iova, size);
> -
> -	if (op == AS_COMMAND_FLUSH_MEM || op == AS_COMMAND_FLUSH_PT)
> -		return mmu_hw_do_flush_on_gpu_ctrl(ptdev, as_nr, op);
> -
> -	/* Run the MMU operation */
> -	write_cmd(ptdev, as_nr, op);
> -
> -	/* Wait for the flush to complete */
> -	return wait_ready(ptdev, as_nr);
> -}
> -
>  static int mmu_hw_do_operation(struct panthor_vm *vm,
>  			       u64 iova, u64 size, u32 op)
>  {