Re: [RFC PATCH 00/12] Ion cleanup in preparation for moving out of staging

[Brian Starkey]

On Fri, Mar 10, 2017 at 11:46:42AM +0000, Robin Murphy wrote:
> On 10/03/17 10:31, Brian Starkey wrote:
> > Hi,
> >
> > On Thu, Mar 09, 2017 at 09:38:49AM -0800, Laura Abbott wrote:
> > > On 03/09/2017 02:00 AM, Benjamin Gaignard wrote:
> >
> > [snip]
> >
> > > > For me those patches are going in the right direction.
> > > >
> > > > I still have few questions:
> > > > - since alignment management has been remove from ion-core, should it
> > > > be also removed from ioctl structure ?
> > >
> > > Yes, I think I'm going to go with the suggestion to fixup the ABI
> > > so we don't need the compat layer and as part of that I'm also
> > > dropping the align argument.
> >
> > Is the only motivation for removing the alignment parameter that
> > no-one got around to using it for something useful yet?
> > The original comment was true - different devices do have different
> > alignment requirements.
> >
> > Better alignment can help SMMUs use larger blocks when mapping,
> > reducing TLB pressure and the chance of a page table walk causing
> > display underruns.
>
> For that use-case, though, alignment alone doesn't necessarily help -
> you need the whole allocation granularity to match your block size (i.e.
> given a 1MB block size, asking for 17KB and getting back 17KB starting
> at a 1MB boundary doesn't help much - that whole 1MB needs to be
> allocated and everyone needs to know it to ensure that the whole lot can
> be mapped safely). Now, whether it's down to the callers or the heap
> implementations to decide and enforce that granularity is another
> question, but provided allocations are at least naturally aligned to
> whatever the granularity is (which is a reasonable assumption to bake
> in) then it's all good.
>
> Robin.

Agreed, alignment alone isn't enough. But lets assume that an app
knows what a "good" granularity is, and always asks for allocation
sizes which are suitably rounded to allow blocks to be used. Currently
it looks like a "standard" ION_HEAP_TYPE_CARVEOUT heap would give me
back just a PAGE_SIZE aligned buffer. So even *if* the caller knows
its desired block size, there's no way for it to get guaranteed better
alignment, which wouldn't be a bad feature to have.

Anyway as Daniel and Rob say, if the interface is designed properly
this kind of extension would be possible later, or you can have a
special heap with a larger granule.

I suppose it makes sense to remove it while there's no-one actually
implementing it, in case an alternate method proves more usable.

-Brian

> > -Brian
> >
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