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:   Fri, 30 Oct 2020 12:21:18 -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 Fri, Oct 30, 2020 at 12:51 AM Hillf Danton <hdanton@...a.com> wrote:
> On Thu, 29 Oct 2020 21:04:30 -0700 John Stultz wrote:
> >
> > 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.
>
> If the memory slice(just a page for simple case) from
> dma_alloc_attr(for device-A) does not work for device-B, nor can
> page_alloc do the job imho.

Right. That's why userland chooses which heap to allocate from, as
it's the only one that knows the path the buffer will take.

thanks
-john