Re: staging: ion: ION allocation fall back order depends on heap linkage order

[Laura Abbott]

On 02/07/2018 07:10 AM, Alexey Skidanov wrote:
> On 02/07/2018 04:58 PM, Laura Abbott wrote:
> > On 02/06/2018 11:05 PM, Alexey Skidanov wrote:
> > > > Yup, you've hit upon a key problem. Having fallbacks be stable
> > > > was always a problem and the recommendation these days is to
> > > > not rely on them. You can specify a heap at a time and fallback
> > > > manually if you want that behavior.
> > > >
> > > > If you have a proposal to make fallbacks work reliably without
> > > > overly complicating the ABI I'm happy to review it.
> > > >
> > > > Thanks,
> > > > Laura
> > > I think it's possible to "automate" the "manual fallback" behavior. But
> > > the real issues is using heap id to specify the particular heap object.
> > >
> > > Current API (allocation IOCTL) requires to specify the particular heap
> > > object by using heap id. From the other hand, the user space doesn't
> > > control the heaps creation order and heap id assignment. So it may be
> > > tricky, especially when more than one object of the same heap type is
> > > created automatically.
> > >
> > > Thanks,
> > > Alexey
> >
> > The query ioctl is designed to get the heap ID information without
> > needing to rely on the linking order or anything else defined in
> > the kernel.
> >
> > Thanks,
> > Laura
>
> That is true. But if we have 2 *automatically created* heaps of the same
> type, how userspace can distinguish between them?
>
> Thanks,
> Alexey

The query ioctl also gives the name which should be different
for each heap. It's not ideal but the name/heap type are the best
way to differentiate between heaps without resorting to hard
coding.

Thanks,
Laura