Re: [PATCH] drm/amd/amdgpu: Replace kmap() with kmap_local_page()

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 12.08.22 um 19:57 schrieb Fabio M. De Francesco:
> kmap() is being deprecated in favor of kmap_local_page().
>
> There are two main problems with kmap(): (1) It comes with an overhead as
> mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
>
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and are still valid.
>
> Since its use in amdgpu/amdgpu_ttm.c is safe, it should be preferred.
>
> Therefore, replace kmap() with kmap_local_page() in amdgpu/amdgpu_ttm.c.
>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

For those two use cases that looks good to me. Feel free to add an 
Acked-by: Christian König <christian.koenig@amd.com>

Regards,
Christian.

> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> index 3b4c19412625..c11657b5915f 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> @@ -2301,9 +2301,9 @@ static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
>   		if (p->mapping != adev->mman.bdev.dev_mapping)
>   			return -EPERM;
>   
> -		ptr = kmap(p);
> +		ptr = kmap_local_page(p);
>   		r = copy_to_user(buf, ptr + off, bytes);
> -		kunmap(p);
> +		kunmap_local(ptr);
>   		if (r)
>   			return -EFAULT;
>   
> @@ -2352,9 +2352,9 @@ static ssize_t amdgpu_iomem_write(struct file *f, const char __user *buf,
>   		if (p->mapping != adev->mman.bdev.dev_mapping)
>   			return -EPERM;
>   
> -		ptr = kmap(p);
> +		ptr = kmap_local_page(p);
>   		r = copy_from_user(ptr + off, buf, bytes);
> -		kunmap(p);
> +		kunmap_local(ptr);
>   		if (r)
>   			return -EFAULT;
>