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Subject: Re: [PATCH] staging: android: ion: Add requested allocation alignment
To:     Alexey Skidanov <alexey.skidanov@intel.com>,
        sumit.semwal@linaro.org, gregkh@linuxfoundation.org,
        arve@android.com, tkjos@android.com, maco@android.com,
        devel@driverdev.osuosl.org
Cc:     linux-kernel@vger.kernel.org, Liam Mark <lmark@codeaurora.org>
References: <1518257863-6903-1-git-send-email-alexey.skidanov@intel.com>
 <ae413008-6c8b-5e53-9251-d11f4ae9713a@redhat.com>
 <8284b2ba-a532-23fd-4c52-7ac556d63918@intel.com>
From:   Laura Abbott <labbott@redhat.com>
Message-ID: <ed842c91-4c4c-7dc0-3957-7e2684e24e32@redhat.com>
Date:   Mon, 12 Feb 2018 11:52:03 -0800
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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.

>> Thanks,
>> Laura
>>
> Thanks,
> Alexey
>