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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