Re: [PATCHv2 1/4] staging: android: ion: Drop heap type masks

[Brian Starkey]

Hi,

On Fri, Sep 02, 2016 at 12:36:25PM -0700, Laura Abbott wrote:
> On 09/02/2016 06:41 AM, Brian Starkey wrote:
> > Hi Laura,
> >
> > On Thu, Sep 01, 2016 at 03:40:41PM -0700, Laura Abbott wrote:
> > > There is no advantage to having heap types be a mask. The ion client has
> > > long since dropped the mask. Drop the notion of heap type masks as well.
> >
> > I know this is the same patch you sent last time, so sorry for not
> > picking this up then - but I'm curious what "The" ion client is here?
>
> ion_client_create used to take a mask to indicate what heap types it
> could allocate from. This hasn't been the case since 2bb9f5034ec7
> ("gpu: ion: Remove heapmask from client"). "The ion client" probably
> should have been "struct ion_client"

Ah I see, the in-kernel ion_client. Sorry, I completely forgot that
even existed (because it's totally useless - how is a driver meant to
find the global ion_device?)

> > Our ion client(s) certainly still use these masks, and it's still
> > used as a mask within ion itself - even if the relationship between a
> > mask and a heap type has been somewhat lost.
>
> Where is it used in Ion? I don't see it in tree unless I missed something
> and I'm not eager to keep this around for out of tree code. What's the
> actual use for this?

You're certainly right that these heap-ID-to-allocation-mask macros
are unused in the kernel, but I don't really see the reason for
removing them - they are convenient (for now).

Example: I'm using the dummy ion driver, and I want to allocate from
the SYSTEM_CONTIG heap - the ION_HEAP_SYSTEM_CONTIG_MASK gives me the
exact mask I need for that.

It seems your opinion is that heap-IDs are already, and should be,
completely decoupled from their type. That sounds like a good idea to
me, but it's not true (yet) - again check out the dummy driver.

At the moment, heap-IDs are assigned by ion drivers in any way they
see fit. For as long as that stays the case there's always going to
be heap-masks hard-coded in UAPI kernel headers (in-tree or not), so
removing these particular masks seems a bit fruitless.

I'd rather see driver-assigned heap-IDs disappear completely, and have
them assigned by ion core from an idr or something. At that point
these macros really *are* meaningless, and I'd be totally fine with
removing them (and userspace won't be able to depend on hard-coded
allocation masks any more - it will have to use the query ioctl,
which I assume is the whole point?).

IMO it's not the right time to remove these macros, because they still
have meaning and usefulness.

Cheers,
Brian

> > Thanks,
> > Brian
> >
> > > Signed-off-by: Laura Abbott <labbott@xxxxxxxxxx>
> > > ---
> > > drivers/staging/android/uapi/ion.h | 6 ------
> > > 1 file changed, 6 deletions(-)
> > >
> > > diff --git a/drivers/staging/android/uapi/ion.h b/drivers/staging/android/uapi/ion.h
> > > index 0a8e40f..a9c4e8b 100644
> > > --- a/drivers/staging/android/uapi/ion.h
> > > +++ b/drivers/staging/android/uapi/ion.h
> > > @@ -44,14 +44,8 @@ enum ion_heap_type {
> > >                   * must be last so device specific heaps always
> > >                   * are at the end of this enum
> > >                   */
> > > -    ION_NUM_HEAPS = 16,
> > > };
> > >
> > > -#define ION_HEAP_SYSTEM_MASK        (1 << ION_HEAP_TYPE_SYSTEM)
> > > -#define ION_HEAP_SYSTEM_CONTIG_MASK    (1 << ION_HEAP_TYPE_SYSTEM_CONTIG)
> > > -#define ION_HEAP_CARVEOUT_MASK        (1 << ION_HEAP_TYPE_CARVEOUT)
> > > -#define ION_HEAP_TYPE_DMA_MASK        (1 << ION_HEAP_TYPE_DMA)
> > > -
> > > #define ION_NUM_HEAP_IDS        (sizeof(unsigned int) * 8)
> > >
> > > /**
> > > --
> > > 2.7.4