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Subject: Re: staging: ion: ION allocation fall back order depends on heap
 linkage order
To:     Laura Abbott <labbott@redhat.com>, devel@driverdev.osuosl.org
Cc:     linux-kernel@vger.kernel.org
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From:   Alexey Skidanov <alexey.skidanov@intel.com>
Message-ID: <3f77b591-6248-da0a-e20d-88ca26a60a95@intel.com>
Date:   Wed, 7 Feb 2018 17:10:36 +0200
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On 02/07/2018 04:58 PM, Laura Abbott wrote:
> On 02/06/2018 11:05 PM, Alexey Skidanov wrote:
>>
>>
>>> Yup, you've hit upon a key problem. Having fallbacks be stable
>>> was always a problem and the recommendation these days is to
>>> not rely on them. You can specify a heap at a time and fallback
>>> manually if you want that behavior.
>>>
>>> If you have a proposal to make fallbacks work reliably without
>>> overly complicating the ABI I'm happy to review it.
>>>
>>> Thanks,
>>> Laura
>>>
>> I think it's possible to "automate" the "manual fallback" behavior. But
>> the real issues is using heap id to specify the particular heap object.
>>
>> Current API (allocation IOCTL) requires to specify the particular heap
>> object by using heap id. From the other hand, the user space doesn't
>> control the heaps creation order and heap id assignment. So it may be
>> tricky, especially when more than one object of the same heap type is
>> created automatically.
>>
>> Thanks,
>> Alexey
>>
>>
> 
> The query ioctl is designed to get the heap ID information without
> needing to rely on the linking order or anything else defined in
> the kernel.
> 
> Thanks,
> Laura

That is true. But if we have 2 *automatically created* heaps of the same
type, how userspace can distinguish between them?

Thanks,
Alexey