On 03/10/2017 06:27 AM, Brian Starkey wrote: > 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 Part of the reason I want to remove it is to avoid confusion over callers thinking it will do anything on most heaps. I agree being able to specify a larger granularity would be beneficial but I don't think a dedicated field in the ABI is the right approach. Thanks, Laura
