Re: [PATCH 1/2] drm: add cache support for arm64

[Rob Clark]

On Tue, Aug 6, 2019 at 7:35 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Tue, Aug 06, 2019 at 07:11:41AM -0700, Rob Clark wrote:
> > On Tue, Aug 6, 2019 at 1:48 AM Christoph Hellwig <hch@xxxxxx> wrote:
> > >
> > > This goes in the wrong direction.  drm_cflush_* are a bad API we need to
> > > get rid of, not add use of it.  The reason for that is two-fold:
> > >
> > >  a) it doesn't address how cache maintaince actually works in most
> > >     platforms.  When talking about a cache we three fundamental operations:
> > >
> > >         1) write back - this writes the content of the cache back to the
> > >            backing memory
> > >         2) invalidate - this remove the content of the cache
> > >         3) write back + invalidate - do both of the above
> >
> > Agreed that drm_cflush_* isn't a great API.  In this particular case
> > (IIUC), I need wb+inv so that there aren't dirty cache lines that drop
> > out to memory later, and so that I don't get a cache hit on
> > uncached/wc mmap'ing.
>
> Is there a cacheable alias lying around (e.g. the linear map), or are
> these addresses only mapped uncached/wc?
>
> If there's a cacheable alias, performing an invalidate isn't sufficient,
> since a CPU can allocate a new (clean) entry at any point in time (e.g.
> as a result of prefetching or arbitrary speculation).

I *believe* that there are not alias mappings (that I don't control
myself) for pages coming from
shmem_file_setup()/shmem_read_mapping_page()..  digging around at what
dma_sync_sg_* does under the hood, it looks like it is just
arch_sync_dma_for_cpu/device(), so I guess that should be sufficient
for what I need.

There are a few buffers that I vmap for use on kernel side, but like
the userspace mmap's, the vmaps are also writecombine.

BR,
-R