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References: <20221016174158.16638-1-fmdefrancesco@gmail.com>
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From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Mon, 17 Oct 2022 12:53:24 -0400
Message-ID: <CADnq5_MDp6WUYTUO+Lo=XMSLdaYcwu5i2MWid9u1U2hKiyqoYg@mail.gmail.com>
Subject: Re: [RESEND PATCH] drm/amd/amdgpu: Replace kmap() with kmap_local_page()
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Alex Deucher <alexander.deucher@amd.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Hawking Zhang <Hawking.Zhang@amd.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Tao Zhou <tao.zhou1@amd.com>, Jack Xiao <Jack.Xiao@amd.com>,
        amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, Ira Weiny <ira.weiny@intel.com>
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Applied.  Thanks!

On Sun, Oct 16, 2022 at 1:42 PM Fabio M. De Francesco
<fmdefrancesco@gmail.com> wrote:
>
> 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=E2=80=99s pool wraps and it might block when the mapping space is fu=
lly
> 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>
> Acked-by: Christian K=C3=B6nig <christian.koenig@amd.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> ---
>
> I'm resending because I suspect that this patch might have been lost. In
> the meantime I added an "Acked-by" tag from Christian K.. Obviviously,
> there are no further changes in the code.
>
>  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/am=
d/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, ch=
ar __user *buf,
>                 if (p->mapping !=3D adev->mman.bdev.dev_mapping)
>                         return -EPERM;
>
> -               ptr =3D kmap(p);
> +               ptr =3D kmap_local_page(p);
>                 r =3D 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, c=
onst char __user *buf,
>                 if (p->mapping !=3D adev->mman.bdev.dev_mapping)
>                         return -EPERM;
>
> -               ptr =3D kmap(p);
> +               ptr =3D kmap_local_page(p);
>                 r =3D copy_from_user(ptr + off, buf, bytes);
> -               kunmap(p);
> +               kunmap_local(ptr);
>                 if (r)
>                         return -EFAULT;
>
> --
> 2.37.1
>