Re: [PATCH v3 7/7] dma-buf: system_heap: Add a system-uncached heap re-using the system heap

[Daniel Mentz]

On Fri, Oct 16, 2020 at 12:04 PM John Stultz <john.stultz@xxxxxxxxxx> wrote:
>
> On Thu, Oct 8, 2020 at 4:51 AM Brian Starkey <brian.starkey@xxxxxxx> wrote:
> > On Sat, Oct 03, 2020 at 04:02:57AM +0000, John Stultz wrote:
> > > @@ -393,6 +424,16 @@ static int system_heap_allocate(struct dma_heap *heap,
> > >               /* just return, as put will call release and that will free */
> > >               return ret;
> > >       }
> > > +
> > > +     /*
> > > +      * For uncached buffers, we need to initially flush cpu cache, since
> > > +      * the __GFP_ZERO on the allocation means the zeroing was done by the
> > > +      * cpu and thus it is likely cached. Map (and implicitly flush) it out
> > > +      * now so we don't get corruption later on.
> > > +      */
> > > +     if (buffer->uncached)
> > > +             dma_map_sgtable(dma_heap_get_dev(heap), table, DMA_BIDIRECTIONAL, 0);
> >
> > Do we have to keep this mapping around for the entire lifetime of the
> > buffer?
>
> Yea, I guess we can just map and unmap it right there.  It will look a
> little absurd, but that sort of aligns with your next point.
>
> > Also, this problem (and solution) keeps lingering around. It really
> > feels like there should be a better way to solve "clean the linear
> > mapping all the way to DRAM", but I don't know what that should be.
>
> Yea, something better here would be nice...

In ION, we had a little helper function named
ion_buffer_prep_noncached that called arch_dma_prep_coherent() on all
sg entries like so

for_each_sg(table->sgl, sg, table->orig_nents, i)
        arch_dma_prep_coherent(sg_page(sg), sg->length);

Would that help?