Re: [PATCH] staging: android: ion: Add requested allocation alignment

[Alexey Skidanov]

On 02/12/2018 10:46 PM, Laura Abbott wrote:
> On 02/12/2018 12:22 PM, Alexey Skidanov wrote:
>>
>>
>> On 02/12/2018 09:52 PM, Laura Abbott wrote:
>>> On 02/12/2018 11:11 AM, Alexey Skidanov wrote:
>>>>
>>>> On 02/12/2018 08:42 PM, Laura Abbott wrote:
>>>>> On 02/10/2018 02:17 AM, Alexey Skidanov wrote:
>>>>>> Current ion defined allocation ioctl doesn't allow to specify the
>>>>>> requested
>>>>>> allocation alignment. CMA heap allocates buffers aligned on buffer
>>>>>> size
>>>>>> page order.
>>>>>>
>>>>>> Sometimes, the alignment requirement is less restrictive. In such
>>>>>> cases,
>>>>>> providing specific alignment may reduce the external memory
>>>>>> fragmentation
>>>>>> and in some cases it may avoid the allocation request failure.
>>>>>>
>>>>> I really do not want to bring this back as part of the regular
>>>>> ABI.
>>>> Yes, I know it was removed in 4.12.
>>>> Having an alignment parameter that gets used for exactly
>>>>> one heap only leads to confusion (which is why it was removed
>>>>> from the ABI in the first place).
>>>> You are correct regarding the CMA heap. But, probably it may be used by
>>>> custom heap as well.
>>>
>>> I can think of a lot of instances where it could be used but
>>> ultimately there needs to be an actual in kernel user who wants
>>> it.
>>>
>>>>> The alignment came from the behavior of the DMA APIs. Do you
>>>>> actually need to specify any alignment from userspace or do
>>>>> you only need page size?
>>>> Yes. If CMA gives it for free, I would suggest to let the ion user to
>>>> decide
>>>
>>> I'm really not convinced changing the ABI yet again just to let
>>> the user decide is actually worth it. If we can manage it, I'd
>>> much rather see a proposal that doesn't change the ABI.
>> I didn't actually change the ABI - I just use the "unused" member:
>> struct ion_allocation_data {
>> @@ -80,7 +79,7 @@ struct ion_allocation_data {
>> ÂÂÂÂÂÂÂÂ __u32 heap_id_mask;
>> ÂÂÂÂÂÂÂÂ __u32 flags;
>> ÂÂÂÂÂÂÂÂ __u32 fd;
>> -ÂÂÂÂÂÂ __u32 unused;
>> +ÂÂÂÂÂÂ __u32 align;
>> Â };
>>
> 
> Something that was previously unused is now being used. That's a change
> in how the structure is interpreted which is an ABI change, especially
> since we haven't been checking that the __unused field is set
> to 0.
Yes you are correct. I just realized that it isn't even backward
compatible.
> 
>> As an alternative, I may add __u64 heap_specific_param - but this will
>> change the ABI. But, probably it makes the ABI more generic?
> 
> Why does the ABI need to be more generic? If we change the ABI
> we're stuck with it and I'd like to approach it as the very last
> resort.
> 
> Thanks,
> Laura
It seems, that there is no way to do it without some ABI change?

Thanks,
Alexey