Re: [PATCH 2/4] drm/prime: document that use the page array is deprecated
From: Daniel Vetter
Date: Thu Oct 08 2020 - 10:14:27 EST
On Thu, Oct 08, 2020 at 04:09:14PM +0200, Daniel Vetter wrote:
> 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@xxxxxxx>
> > ---
> > 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 :-)
OK I just realized I missed nouveau. That would be 3 places where we need
to stuff the dma_addr_t list into something ttm can take. Still feels
better than this half-deprecated function kludge ...
-Daniel
> -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
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch