linux-kernel - Re: [patch 2/7] drm/vmgfx: Replace kmap

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Message-ID: <5ea9de05-31a1-855f-ab28-49c0cb6b724c@vmware.com>
Date:   Fri, 5 Mar 2021 16:35:42 +0100
From:   Roland Scheidegger <sroland@...are.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     VMware Graphics <linux-graphics-maintainer@...are.com>,
        Zack Rusin <zackr@...are.com>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Christian Koenig <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Gerd Hoffmann <kraxel@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        spice-devel@...ts.freedesktop.org, Ben Skeggs <bskeggs@...hat.com>,
        nouveau@...ts.freedesktop.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        intel-gfx@...ts.freedesktop.org
Subject: Re: [patch 2/7] drm/vmgfx: Replace kmap_atomic()

On 03.03.21 14:20, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> There is no reason to disable pagefaults and preemption as a side effect of
> kmap_atomic_prot().
> 
> Use kmap_local_page_prot() instead and document the reasoning for the
> mapping usage with the given pgprot.
> 
> Remove the NULL pointer check for the map. These functions return a valid
> address for valid pages and the return was bogus anyway as it would have
> left preemption and pagefaults disabled.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: VMware Graphics <linux-graphics-maintainer@...are.com>
> Cc: Roland Scheidegger <sroland@...are.com>
> Cc: Zack Rusin <zackr@...are.com>
> Cc: David Airlie <airlied@...ux.ie>
> Cc: Daniel Vetter <daniel@...ll.ch>
> Cc: dri-devel@...ts.freedesktop.org
> ---
>  drivers/gpu/drm/vmwgfx/vmwgfx_blit.c |   30 ++++++++++++------------------
>  1 file changed, 12 insertions(+), 18 deletions(-)
> 
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_blit.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_blit.c
> @@ -375,12 +375,12 @@ static int vmw_bo_cpu_blit_line(struct v
>  		copy_size = min_t(u32, copy_size, PAGE_SIZE - src_page_offset);
>  
>  		if (unmap_src) {
> -			kunmap_atomic(d->src_addr);
> +			kunmap_local(d->src_addr);
>  			d->src_addr = NULL;
>  		}
>  
>  		if (unmap_dst) {
> -			kunmap_atomic(d->dst_addr);
> +			kunmap_local(d->dst_addr);
>  			d->dst_addr = NULL;
>  		}
>  
> @@ -388,12 +388,8 @@ static int vmw_bo_cpu_blit_line(struct v
>  			if (WARN_ON_ONCE(dst_page >= d->dst_num_pages))
>  				return -EINVAL;
>  
> -			d->dst_addr =
> -				kmap_atomic_prot(d->dst_pages[dst_page],
> -						 d->dst_prot);
> -			if (!d->dst_addr)
> -				return -ENOMEM;
> -
> +			d->dst_addr = kmap_local_page_prot(d->dst_pages[dst_page],
> +							   d->dst_prot);
>  			d->mapped_dst = dst_page;
>  		}
>  
> @@ -401,12 +397,8 @@ static int vmw_bo_cpu_blit_line(struct v
>  			if (WARN_ON_ONCE(src_page >= d->src_num_pages))
>  				return -EINVAL;
>  
> -			d->src_addr =
> -				kmap_atomic_prot(d->src_pages[src_page],
> -						 d->src_prot);
> -			if (!d->src_addr)
> -				return -ENOMEM;
> -
> +			d->src_addr = kmap_local_page_prot(d->src_pages[src_page],
> +							   d->src_prot);
>  			d->mapped_src = src_page;
>  		}
>  		diff->do_cpy(diff, d->dst_addr + dst_page_offset,
> @@ -436,8 +428,10 @@ static int vmw_bo_cpu_blit_line(struct v
>   *
>   * Performs a CPU blit from one buffer object to another avoiding a full
>   * bo vmap which may exhaust- or fragment vmalloc space.
> - * On supported architectures (x86), we're using kmap_atomic which avoids
> - * cross-processor TLB- and cache flushes and may, on non-HIGHMEM systems
> + *
> + * On supported architectures (x86), we're using kmap_local_prot() which
> + * avoids cross-processor TLB- and cache flushes. kmap_local_prot() will
> + * either map a highmem page with the proper pgprot on HIGHMEM=y systems or
>   * reference already set-up mappings.
>   *
>   * Neither of the buffer objects may be placed in PCI memory
> @@ -500,9 +494,9 @@ int vmw_bo_cpu_blit(struct ttm_buffer_ob
>  	}
>  out:
>  	if (d.src_addr)
> -		kunmap_atomic(d.src_addr);
> +		kunmap_local(d.src_addr);
>  	if (d.dst_addr)
> -		kunmap_atomic(d.dst_addr);
> +		kunmap_local(d.dst_addr);
>  
>  	return ret;
>  }
> 
> 

Seems reasonable to me.
Reviewed-by: Roland Scheidegger <sroland@...are.com>