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Subject: Re: [PATCH v2] staging: android: ion: Allocate from heap ID directly
 without mask
To:     John Stultz <john.stultz@linaro.org>,
        Brian Starkey <Brian.Starkey@arm.com>
Cc:     "Andrew F. Davis" <afd@ti.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?Q?Arve_Hj=c3=b8nnev=c3=a5g?= <arve@android.com>,
        Christoph Hellwig <hch@infradead.org>,
        Liam Mark <lmark@codeaurora.org>,
        "devel@driverdev.osuosl.org" <devel@driverdev.osuosl.org>,
        lkml <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>, nd <nd@arm.com>
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From:   Laura Abbott <labbott@redhat.com>
Message-ID: <17a3ed06-a46f-75c6-323c-3f486788a3b3@redhat.com>
Date:   Tue, 19 Feb 2019 12:43:47 -0800
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On 2/15/19 11:01 AM, John Stultz wrote:
> On Fri, Feb 15, 2019 at 2:51 AM Brian Starkey <Brian.Starkey@arm.com> wrote:
>>
>> Hi John,
>>
>> On Thu, Feb 14, 2019 at 09:38:29AM -0800, John Stultz wrote:
>>>
>> [snip]
>>
>>> Some thoughts, as this ABI break has the potential to be pretty painful.
>>>
>>> 1) Unfortunately, this ABI is exposed *through* libion via
>>> ion_alloc/ion_alloc_fd out to gralloc implementations. Which means it
>>> will have a wider impact to vendor userland code.
>>
>> I figured libion could fairly easily loop through all the set bits in
>> heap_mask and call the ioctl for each until it succeeds. That
>> preserves the old behaviour from the libion clients' perspective.
> 
> Potentially, though that implicitly still caps the heaps to 32.  So
> I'm not sure what the net benefit would be.
> 
> 
>>> 2) For patches that cause ABI breaks, it might be good to make it
>>> clear in the commit what the userland impact looks like in userspace,
>>> possibly with an example, so the poor folks who bisect down the change
>>> as breaking their system in a year or so have a clear example as to
>>> what they need to change in their code.
>>>
>>> 3) Also, its not clear how a given userland should distinguish between
>>> the different ABIs.  We already have logic in libion to distinguish
>>> between pre-4.12 legacy and post-4.12 implementations (using implicit
>>> ion_free() behavior). I don't see any such check we can make with this
>>> code. Adding another ABI version may require we provide an actual
>>> interface version ioctl.
>>>
>>
>> A slightly fragile/ugly approach might be to attempt a small
>> allocation with a heap_mask of 0xffffffff. On an "old" implementation,
>> you'd expect that to succeed, whereas it would/could be made to fail
>> in the "new" one.
> 
> Yea I think having a proper ION_IOC_VERSION is going to be necessary.
> 
> I'm hoping to send out an ugly first stab at the kernel side for
> switching to per-heap devices (with a config for keeping /dev/ion for
> legacy compat), which I hope will address the core issue this patch
> does (moving away from heap masks to specifically requested heaps).
> 
> thanks
> -john
> 

Arnd/Greg said no to this last time I tried back in 2016

https://lore.kernel.org/lkml/1472769644-11039-4-git-send-email-labbott@redhat.com/T/#u