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

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Date:   Mon, 12 Feb 2018 22:22:02 +0200
From:   Alexey Skidanov <alexey.skidanov@...el.com>
To:     Laura Abbott <labbott@...hat.com>, sumit.semwal@...aro.org,
        gregkh@...uxfoundation.org, arve@...roid.com, tkjos@...roid.com,
        maco@...roid.com, devel@...verdev.osuosl.org
Cc:     linux-kernel@...r.kernel.org, Liam Mark <lmark@...eaurora.org>
Subject: Re: [PATCH] staging: android: ion: Add requested allocation alignment



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 {
        __u32 heap_id_mask;
        __u32 flags;
        __u32 fd;
-       __u32 unused;
+       __u32 align;
 };

As an alternative, I may add __u64 heap_specific_param - but this will
change the ABI. But, probably it makes the ABI more generic?
> 
>>> Thanks,
>>> Laura
>>>
>> Thanks,
>> Alexey
>>
> 

Thanks,
Alexey