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

[John Stultz <john.stultz@linaro.org>]

On Thu, Oct 8, 2020 at 4:51 AM Brian Starkey <brian.starkey@arm.com> wrote:
> On Sat, Oct 03, 2020 at 04:02:57AM +0000, John Stultz wrote:
> > @@ -215,8 +236,12 @@ static void *system_heap_do_vmap(struct system_heap_buffer *buffer)
> >       struct page **pages = vmalloc(sizeof(struct page *) * npages);
> >       struct page **tmp = pages;
> >       struct sg_page_iter piter;
> > +     pgprot_t pgprot = PAGE_KERNEL;
> >       void *vaddr;
> >
> > +     if (buffer->uncached)
> > +             pgprot = pgprot_writecombine(PAGE_KERNEL);
>
> I think this should go after the 'if (!pages)' check. Best to get the
> allocation failure check as close to the allocation as possible IMO.

Sounds good. Changed in my tree.

> > @@ -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...


> > @@ -426,6 +487,16 @@ static int system_heap_create(void)
> >       if (IS_ERR(sys_heap))
> >               return PTR_ERR(sys_heap);
> >
> > +     exp_info.name = "system-uncached";
> > +     exp_info.ops = &system_uncached_heap_ops;
> > +     exp_info.priv = NULL;
> > +
> > +     sys_uncached_heap = dma_heap_add(&exp_info);
> > +     if (IS_ERR(sys_uncached_heap))
> > +             return PTR_ERR(sys_heap);
> > +
>
> In principle, there's a race here between the heap getting registered
> to sysfs and the dma_mask getting updated.
>
> I don't think it would cause a problem in practice, but with the API
> as it is, there's no way to avoid it - so I wonder if the
> dma_heap_get_dev() accessor isn't the right API design.

Hrm.  I guess to address your concern we would need split
dma_heap_add() into something like:
  dma_heap_create()
  dma_heap_add()

Which breaks the creation of the heap with the registering it to the
subsystem, so some attributes can be tweaked inbetween?

I'll see about taking a stab at this, but I'll probably submit my
updated series sooner with this un-addressed so I can get some further
review.

thanks
-john