Re: [PATCH 4/4] dma-heap: Devicetree binding for chunk heap

[Minchan Kim <minchan@kernel.org>]

On Thu, Dec 10, 2020 at 12:15:15AM -0800, John Stultz wrote:
> On Wed, Dec 9, 2020 at 3:53 PM Minchan Kim <minchan@kernel.org> wrote:
> > On Wed, Nov 18, 2020 at 07:19:07PM -0800, John Stultz wrote:
> > > The CMA heap currently only registers the default CMA heap, as we
> > > didn't want to expose all CMA regions and there's otherwise no way to
> > > pick and choose.
> >
> > Yub.
> >
> > dma-buf really need a way to make exclusive CMA area. Otherwise, default
> > CMA would be shared among drivers and introduce fragmentation easily
> > since we couldn't control other drivers. In such aspect, I don't think
> > current cma-heap works if userspace needs big memory chunk.
> 
> Yes, the default CMA region is not always optimal.
> 
> That's why I was hopeful for Kunihiko Hayashi's patch to allow for
> exposing specific cma regions:
>   https://lore.kernel.org/lkml/1594948208-4739-1-git-send-email-hayashi.kunihiko@socionext.com/
> 
> I think it would be a good solution, but all we need is *some* driver
> which can be considered the primary user/owner of the cma region which
> would then explicitly export it via the dmabuf heaps.
> 
> > Here, the problem is there is no in-kernel user to bind the specific
> > CMA area because the owner will be random in userspace via dma-buf
> > interface.
> 
> Well, while I agree that conceptually the dmabuf heaps allow for
> allocations for multi-device pipelines, and thus are not tied to
> specific devices. I do think that the memory types exposed are likely
> to have specific devices/drivers in the pipeline that it matters most
> to. So I don't see a big issue with the in-kernel driver registering a
> specific CMA region as a dmabuf heap.

Then, I am worry about that we spread out dma_heap_add_cma to too many
drivers since kernel doesn't how userspace will use it.
For example, system 1 could have device A-B-C pipeline so they added
it A driver. After that, system 2 could have device B-C-D pipeline
so they add dma_heap_add_cma into B device.

> 
> > > > Is there a reason to use dma-heap framework to add cma-area for specific device ?
> > > >
> > > > Even if some in-tree users register dma-heap with cma-area, the buffers could be allocated in user-land and these could be shared among other devices.
> > > > For exclusive access, I guess, the device don't need to register dma-heap for cma area.
> > > >
> > >
> > > It's not really about exclusive access. More just that if you want to
> > > bind a memory reservation/region (cma or otherwise), at least for DTS,
> > > it needs to bind with some device in DT.
> > >
> > > Then the device driver can register that region with a heap driver.
> > > This avoids adding new Linux-specific software bindings to DT. It
> > > becomes a driver implementation detail instead. The primary user of
> > > the heap type would probably be a practical pick (ie the display or
> > > isp driver).
> >
> > If it's the only solution, we could create some dummy driver which has
> > only module_init and bind it from there but I don't think it's a good
> > idea.
> 
> Yea, an un-upstreamable dummy driver is maybe what it devolves to in
> the worst case. But I suspect it would be cleaner for a display or ISP
> driver that benefits most from the heap type to add the reserved
> memory reference to their DT node, and on init for them to register
> the region with the dmabuf heap code.

As I mentioned above, it could be a display at this moment but it could
be different driver next time. If I miss your point, let me know.

Thanks for the review, John.

> 
> 
> > > The other potential solution Rob has suggested is that we create some
> > > tag for the memory reservation (ie: like we do with cma: "reusable"),
> > > which can be used to register the region to a heap. But this has the
> > > problem that each tag has to be well defined and map to a known heap.
> >
> > Do you think that's the only solution to make progress for this feature?
> > Then, could you elaborate it a bit more or any other ideas from dma-buf
> > folks?
> 
> I'm skeptical of that DT tag approach working out, as we'd need a new
> DT binding for the new tag name, and we'd have to do so for each new
> heap type that needs this (so non-default cma, your chunk heap,
> whatever other similar heap types that use reserved regions folks come
> up with).  Having *some* driver take ownership for the reserved region
> and register it with the appropriate heap type seems much
> cleaner/flexible and avoids mucking with the DT ABI.
> 
> thanks
> -john