linux-kernel - Re: [PATCH 07/10] drm/panthor: remove unnecessary mmu_hw_wait

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Message-ID: <74e2f1a8-0410-4a5e-bbf3-29d5d5d55308@arm.com>
Date: Thu, 2 Oct 2025 11:41:38 +0100
From: Steven Price <steven.price@....com>
To: Chia-I Wu <olvaffe@...il.com>,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Grant Likely <grant.likely@...aro.org>, Heiko Stuebner <heiko@...ech.de>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/10] drm/panthor: remove unnecessary mmu_hw_wait_ready
 calls

On 16/09/2025 22:08, Chia-I Wu wrote:
> No need to call mmu_hw_wait_ready after panthor_gpu_flush_caches or
> before returning from mmu_hw_flush_caches.

Why is there no need? If we attempt to send a command when the hardware
is busy then the command will be dropped (so the cache flush won't
happen), and if we don't wait for the unlock command to complete then
then we don't know that the flush is complete.

Thanks,
Steve

> Signed-off-by: Chia-I Wu <olvaffe@...il.com>
> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 373871aeea9f4..c223e3fadf92e 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -669,12 +669,9 @@ static int mmu_hw_flush_caches(struct panthor_device *ptdev, int as_nr, u64 iova
>  	 * at the end of the GPU_CONTROL cache flush command, unlike
>  	 * AS_COMMAND_FLUSH_MEM or AS_COMMAND_FLUSH_PT.
>  	 */
> -	ret = mmu_hw_wait_ready(ptdev, as_nr);
> -	if (!ret)
> -		mmu_hw_cmd_unlock(ptdev, as_nr);
> +	mmu_hw_cmd_unlock(ptdev, as_nr);
>  
> -	/* Wait for the unlock command to complete */
> -	return mmu_hw_wait_ready(ptdev, as_nr);
> +	return 0;
>  }
>  
>  static int mmu_hw_do_operation(struct panthor_vm *vm,