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

[Christian König]

Am 08.10.20 um 16:14 schrieb Daniel Vetter:
> 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 ...

Mhm, I don't see a reason why nouveau would need the struct page either.

How about we split that up into two function?

One for converting the sg_table into a linear dma_addr array.

And one for converting the sg_table into a linear struct page array with 
a __deprecated attribute on it?

Christian.

> -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