linux-kernel - Re: [PATCH v4 7/7] dma-buf: system_heap: Add a system-uncached heap re-using the system heap

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Date:   Thu, 29 Oct 2020 21:04:30 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Robin Murphy <robin.murphy@....com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Simon Ser <contact@...rsion.fr>,
        Christoph Hellwig <hch@...radead.org>,
        James Jones <jajones@...dia.com>,
        linux-media <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 7/7] dma-buf: system_heap: Add a system-uncached heap
 re-using the system heap

On Thu, Oct 29, 2020 at 7:48 PM Hillf Danton <hdanton@...a.com> wrote:
> On Thu, 29 Oct 2020 15:28:34 -0700 John Stultz wrote:
> > On Thu, Oct 29, 2020 at 12:10 AM Hillf Danton <hdanton@...a.com> wrote:
> > > On Thu, 29 Oct 2020 00:16:24 +0000 John Stultz wrote:
> > > > @@ -194,6 +210,9 @@ static int system_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> > > >       struct sg_page_iter piter;
> > > >       int ret;
> > > >
> > > > +     if (buffer->uncached)
> > > > +             vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> > > > +
> > >
> > > Wonder why you turn back to dma_mmap_wc() and friends?
> >
> > Sorry, can you expand on what you are proposing here instead?  I'm not
> > sure I see how dma_alloc/mmap/*_wc() quite fits here.
>
> I just wondered if *_wc() could save you two minutes or three. Can you
> shed some light on your concerns about their unfitness?

Sorry, I feel a bit daft here. I'm still not exactly sure what you're
proposing, and your reply of saving minutes doesn't really clarify
things.
So I'm not sure it's a good use of time to try to (most likely,
incorrectly) refute all the possible things you might be suggesting.
:)

But I'll try to share my thoughts:

So the system heap allows for allocation of non-contiguous buffers
(currently allocated from page_alloc), which we keep track using
sglists.
Since the resulting dmabufs are shared between multiple devices, we
want to provide a *specific type of memory* (in this case
non-contiguous system memory), rather than what the underlying
dma_alloc_attr() allocates for a specific device.

My sense is dma_mmap_wc() likely ought to be paired with switching to
using dma_alloc_wc() as well, which calls down to dma_alloc_attr().
Maybe one could use dma_alloc_attr against the heap device to allocate
chunks that we track in the sglist. But I'm not sure how that saves us
much other than possibly swapping dma_mmap_wc() for remap_pfn_range()?

But again, I suspect I've mischaracterized what you're actually
suggesting. So please let me know what you're thinking and I'm happy
to consider it.

thanks
-john