Re: [PATCH v2] staging: android: ion: Allocate from heap ID directly without mask

[Laura Abbott]

On 2/15/19 11:01 AM, John Stultz wrote:
> On Fri, Feb 15, 2019 at 2:51 AM Brian Starkey <Brian.Starkey@xxxxxxx> wrote:
> > Hi John,
> >
> > On Thu, Feb 14, 2019 at 09:38:29AM -0800, John Stultz wrote:
> > >
> > [snip]
> >
> > > Some thoughts, as this ABI break has the potential to be pretty painful.
> > >
> > > 1) Unfortunately, this ABI is exposed *through* libion via
> > > ion_alloc/ion_alloc_fd out to gralloc implementations. Which means it
> > > will have a wider impact to vendor userland code.
> >
> > I figured libion could fairly easily loop through all the set bits in
> > heap_mask and call the ioctl for each until it succeeds. That
> > preserves the old behaviour from the libion clients' perspective.
>
> Potentially, though that implicitly still caps the heaps to 32.  So
> I'm not sure what the net benefit would be.
>
>
> > > 2) For patches that cause ABI breaks, it might be good to make it
> > > clear in the commit what the userland impact looks like in userspace,
> > > possibly with an example, so the poor folks who bisect down the change
> > > as breaking their system in a year or so have a clear example as to
> > > what they need to change in their code.
> > >
> > > 3) Also, its not clear how a given userland should distinguish between
> > > the different ABIs.  We already have logic in libion to distinguish
> > > between pre-4.12 legacy and post-4.12 implementations (using implicit
> > > ion_free() behavior). I don't see any such check we can make with this
> > > code. Adding another ABI version may require we provide an actual
> > > interface version ioctl.
> >
> > A slightly fragile/ugly approach might be to attempt a small
> > allocation with a heap_mask of 0xffffffff. On an "old" implementation,
> > you'd expect that to succeed, whereas it would/could be made to fail
> > in the "new" one.
>
> Yea I think having a proper ION_IOC_VERSION is going to be necessary.
>
> I'm hoping to send out an ugly first stab at the kernel side for
> switching to per-heap devices (with a config for keeping /dev/ion for
> legacy compat), which I hope will address the core issue this patch
> does (moving away from heap masks to specifically requested heaps).
>
> thanks
> -john

Arnd/Greg said no to this last time I tried back in 2016

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