Re: [PATCH v6 4/5] dma-buf: heaps: Add CMA heap to dmabuf heaps

[Andrew F. Davis]

On 7/25/19 8:48 AM, Christoph Hellwig wrote:
> On Wed, Jul 24, 2019 at 07:38:07AM -0400, Laura Abbott wrote:
>> It's not just an optimization for Ion though. Ion was designed to
>> let the callers choose between system and multiple CMA heaps.
> 
> Who cares about ion?  That some out of tree android crap that should
> not be relevant for upstream except as an example for how not to design
> things..
> 

Tell us how you really feel about ION :)

>> On other
>> systems there may be multiple CMA regions dedicated to a specific
>> purpose or placed at a specific address. The callers need to
>> be able to choose exactly whether they want a particular CMA region
>> or discontiguous regions.
> 
> At least in cma is only used either with the global pool or a per-device
> cma pool.  I think if you want to make this new dma-buf API fit in with
> the rest with the kernel you follow that model, and pass in a struct
> device to select the particular cma area, similar how the DMA allocator
> works.
> 

This is a central allocator, it is not tied to any one device. If we
knew the one device ahead of time we would just use the existing dma_alloc.

We might be able to solve some of that with late mapping after all the
devices attach to the buffer, but even then, which device's CMA area
would we chose to use from all the attached devices?

I can agree that allocating from per-device CMA using Heaps doesn't make
much sense, but for global pools I'm not sure I see any way to allow
devices to select which pool is right for a specific use. They don't
have the full use-case information like the application does, the
selection needs to be made from the application.

Andrew