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

[Minchan Kim <minchan@kernel.org>]

On Wed, Nov 18, 2020 at 07:19:07PM -0800, John Stultz wrote:
> On Wed, Nov 18, 2020 at 5:22 PM Hyesoo Yu <hyesoo.yu@samsung.com> wrote:
> >
> > On Tue, Nov 17, 2020 at 07:00:54PM -0800, John Stultz wrote:
> > > So I suspect Rob will push back on this as he has for other dt
> > > bindings related to ion/dmabuf heaps (I tried to push a similar
> > > solution to exporting multiple CMA areas via dmabuf heaps).
> > >
> > > The proposal he seemed to like best was having an in-kernel function
> > > that a driver would call to initialize the heap (associated with the
> > > CMA region the driver is interested in). Similar to Kunihiko Hayashi's
> > > patch here:
> > >   - https://lore.kernel.org/lkml/1594948208-4739-1-git-send-email-hayashi.kunihiko@socionext.com/
> > >
> > > The one sticking point for that patch (which I think is a good one),
> > > is that we don't have any in-tree users, so it couldn't be merged yet.
> > >
> > > A similar approach might be good here, but again we probably need to
> > > have at least one in-tree user which could call such a registration
> > > function.
> >
> > Thanks for your review.
> >
> > The chunk heap is not considered for device-specific reserved memory and specific driver.
> > It is similar to system heap, but it only collects high-order pages by using specific cma-area for performance.
> 
> So, yes I agree, the chunk heap isn't device specific. It's just that
> the CMA regions usually are tied to devices.
> 
> The main objection to this style of solution has been due to the fact
> that the DTS is supposed to describe the physical hardware (in an OS
> agnostic way), rather than define configuration info for Linux
> software drivers.
> 
> Obviously this can be quibbled about, as the normal way of tying
> devices to CMA has some assumptions of what the driver will use that
> region for, rather than somehow representing a physical tie between a
> memory reservation and a device. Nonetheless, Rob has been hesitant to
> take any sort of ION/DmaBuf Heap DT devices, and has been more
> interested in some device having the memory reservation reference and
> the driver for that doing the registration.
> 
> > It is strange that there is in-tree user who registers chunk heap.
> > (Wouldn't it be strange for some users to register the system heap?)
> 
> Well, as there's no reservation/configuration needed, the system heap
> can register itself.
> 
> 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.

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.

> 
> > 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.

> 
> 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?