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

[Rob Clark]

On Wed, Aug 7, 2019 at 5:38 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Tue, Aug 06, 2019 at 09:31:55AM -0700, Rob Clark wrote:
> > 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()..
>
> AFAICT, that's regular anonymous memory, so there will be a cacheable
> alias in the linear/direct map.

tbh, I'm not 100% sure whether there is a cacheable alias, or whether
any potential linear map is torn down.  My understanding is that a
cacheable alias is "ok", with some caveats.. ie. that the cacheable
alias is not accessed.  I'm not entirely sure about pre-fetch from
access to adjacent pages.  We have been using shmem as a source for
pages since the beginning, and I haven't seen it cause any problems in
the last 6 years.  (This is limited to armv7 and armv8, I'm not really
sure what would happen on armv6, but that is a combo I don't have to
care about.)

BR,
-R

> > 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.
>
> I don't think that's the case, per the example I gave above.
>
> Thanks,
> Mark.