Re: [PATCH v7 3/5] dma-buf: heaps: Add system heap to dmabuf heaps

[John Stultz <john.stultz@linaro.org>]

On Thu, Jul 25, 2019 at 6:02 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> > +struct system_heap {
> > +     struct dma_heap *heap;
> > +} sys_heap;
>
> It seems like this structure could be removed and if would improve
> the code flow.

Good point. We actually keep a few things in the cma version of this,
and I think I copied that over when I started here, but never cleaned
it up.

> > +static struct dma_heap_ops system_heap_ops = {
> > +     .allocate = system_heap_allocate,
> > +};
> > +
> > +static int system_heap_create(void)
> > +{
> > +     struct dma_heap_export_info exp_info;
> > +     int ret = 0;
> > +
> > +     exp_info.name = "system_heap";
> > +     exp_info.ops = &system_heap_ops;
> > +     exp_info.priv = &sys_heap;
> > +
> > +     sys_heap.heap = dma_heap_add(&exp_info);
> > +     if (IS_ERR(sys_heap.heap))
> > +             ret = PTR_ERR(sys_heap.heap);
> > +
> > +     return ret;
>
> The data structures here seem a little odd.  I think you want to:

Yea. There is some awkwardness, and some is due to using the helper
infrastructure, but some is just clutter and I'll revise that.

>  - mark all dma_heap_ops instanes consts, as we generally do that for
>    all structures containing function pointers

Done.

>  - move the name into dma_heap_ops.

I'm not sure this is useful, as there are cases where there are
multiple heaps that use the same ops. Specifically the multiple CMA
heaps.

>  - remove the dma_heap_export_info structure, which is a bit pointless

Andrew and I went back and forth on this a bit. It looks like he just
responded so I'll defer to his answer.

>  - don't bother setting a private data, as you don't need it.
>    If other heaps need private data I'd suggest to switch to embedding
>    the dma_heap structure into containing structure insted so that you
>    can use container_of to get at it.

Fair. There is some cases where we use the priv data, but I'll try to
see if I can minimize it.  And again, I think having the dma_heap
structure be internal/private to the heap implementations made it
difficult to be a contained structure. So it goes back to the
export_info structure point above.

>  - also why is the free callback passed as a callback rather than
>    kept in dma_heap_ops, next to the paired alloc one?

This one is due to the optional heap helpers infrastructure. If a heap
implements its own dma_buf_ops, it can have release directly call the
buffer free function. However, since we tried to minimize the code we
have the heap helpers infrastructure which implements a shared
dma_buf_op, we need some way for the helper release function to call
back to the heap specific free.  We could put it in the dma_heaps_ops
like you suggest, but that brings some confusion as well, as nothing
in the dma-heaps core would call it, it would only be a tool for the
helper infrastructure to trace back to the heap specific free call.
This is why its passed to the heap_helper initializer.  I agree it
feels a little odd, so I'd welcome alternate approaches.

Very much appreciate the review and feedback!  I'll try to address as
much of this as I can in the next revision.

thanks
-john