Re: [PATCH 3/7] drm/udl: import prime-fds with proper page-alignment

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jan 20, 2014 at 08:26:25PM +0100, David Herrmann wrote:
> Instead of rounding down to the next lower page-boundary, round up.
> dma-buf guarantees that we can map buffers in multiples of a page, so if
> an exporter does not page-align, do it ourselves.
> 
> This avoids issues if the exported buffer contains an unaligned size and
> we crop it. In this case, the buffer is too small for the UDL CRTC. So we
> round up to page-size now and avoid black borders. Worst case is we end up
> reading out some random kernel memory, but we can never fault as the whole
> page has the same access-rights. And in this case it's an issue of the
> buggy exporting driver, not the importing one.
> 
> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
> ---
>  drivers/gpu/drm/udl/udl_gem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/udl/udl_gem.c b/drivers/gpu/drm/udl/udl_gem.c
> index df963a1..1069e57 100644
> --- a/drivers/gpu/drm/udl/udl_gem.c
> +++ b/drivers/gpu/drm/udl/udl_gem.c
> @@ -227,7 +227,7 @@ static int udl_prime_create(struct drm_device *dev,
>  	struct udl_gem_object *obj;
>  	int npages;
>  
> -	npages = size / PAGE_SIZE;
> +	npages = PAGE_ALIGN(size) >> PAGE_SHIFT;

Imo the proper patch would be to reject exporting anything which isn't
page-aligned as a dma-buf in the driver core (together with a quick review
of the docs to make sure this requirement is clear). This is just a bug in
the exporter.
-Daniel

>  
>  	*obj_p = NULL;
>  	obj = udl_gem_alloc_object(dev, npages * PAGE_SIZE);
> -- 
> 1.8.5.3
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch