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Subject: Re: staging: ion: ION allocation fall back order depends on heap
 linkage order
To:     Alexey Skidanov <alexey.skidanov@intel.com>,
        devel@driverdev.osuosl.org
Cc:     linux-kernel@vger.kernel.org
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From:   Laura Abbott <labbott@redhat.com>
Message-ID: <9aab5e55-a2f1-b05d-3568-0a4491ff51f1@redhat.com>
Date:   Wed, 7 Feb 2018 07:32:53 -0800
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On 02/07/2018 07:10 AM, Alexey Skidanov wrote:
> 
> 
> 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
> 

The query ioctl also gives the name which should be different
for each heap. It's not ideal but the name/heap type are the best
way to differentiate between heaps without resorting to hard
coding.

Thanks,
Laura