linux-kernel - Re: [PATCH 2/4] drm/prime: document that use the page array is deprecated

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Date:   Thu, 8 Oct 2020 16:09:14 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, dri-devel@...ts.freedesktop.org,
        linux-media@...r.kernel.org, chris@...is-wilson.co.uk,
        airlied@...hat.com, akpm@...ux-foundation.org, daniel@...ll.ch,
        sumit.semwal@...aro.org
Subject: Re: [PATCH 2/4] drm/prime: document that use the page array is
 deprecated

On Thu, Oct 08, 2020 at 01:23:40PM +0200, Christian König wrote:
> We have reoccurring requests on this so better document that
> this approach doesn't work and dma_buf_mmap() needs to be used instead.
> 
> Signed-off-by: Christian König <christian.koenig@....com>
> ---
>  drivers/gpu/drm/drm_prime.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 4910c446db83..16fa2bfc271e 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -956,7 +956,7 @@ EXPORT_SYMBOL(drm_gem_prime_import);
>  /**
>   * drm_prime_sg_to_page_addr_arrays - convert an sg table into a page array
>   * @sgt: scatter-gather table to convert
> - * @pages: optional array of page pointers to store the page array in
> + * @pages: deprecated array of page pointers to store the page array in
>   * @addrs: optional array to store the dma bus address of each page
>   * @max_entries: size of both the passed-in arrays
>   *
> @@ -965,6 +965,11 @@ EXPORT_SYMBOL(drm_gem_prime_import);
>   *
>   * Drivers can use this in their &drm_driver.gem_prime_import_sg_table
>   * implementation.
> + *
> + * Specifying the pages array is deprecated and strongly discouraged for new
> + * drivers. The pages array is only useful for page faults and those can
> + * corrupt fields in the struct page if they are not handled by the exporting
> + * driver.
>   */

I'd make this a _lot_ stronger: Aside from amdgpu and radeon all drivers
using this only need it for the pages array. Imo just open-code the sg
table walking loop in amdgpu/radeon (it's really not much code), and then
drop the dma_addr_t parameter from this function here (it's set to NULL by
everyone else).

And then deprecate this entire function here with a big warning that a)
dma_buf_map_attachment is allowed to leave the struct page pointers NULL
and b) this breaks mmap, users must call dma_buf_mmap instead.

Also maybe make it an uppercase DEPRECATED or something like that :-)
-Daniel

>  int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
>  				     dma_addr_t *addrs, int max_entries)
> -- 
> 2.17.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch