Date:   Fri, 10 Mar 2017 08:46:12 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Brian Starkey <brian.starkey@....com>,
        Robin Murphy <robin.murphy@....com>
Cc:     devel@...verdev.osuosl.org, Rom Lemarchand <romlem@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Riley Andrews <riandrews@...roid.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Michal Hocko <mhocko@...nel.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        linux-mm@...ck.org,
        Arve Hjønnevåg <arve@...roid.com>,
        Mark Brown <broonie@...nel.org>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [RFC PATCH 00/12] Ion cleanup in preparation for moving out of
 staging

On 03/10/2017 06:27 AM, Brian Starkey wrote:
> On Fri, Mar 10, 2017 at 11:46:42AM +0000, Robin Murphy wrote:
>> On 10/03/17 10:31, Brian Starkey wrote:
>>> Hi,
>>>
>>> On Thu, Mar 09, 2017 at 09:38:49AM -0800, Laura Abbott wrote:
>>>> On 03/09/2017 02:00 AM, Benjamin Gaignard wrote:
>>>
>>> [snip]
>>>
>>>>>
>>>>> For me those patches are going in the right direction.
>>>>>
>>>>> I still have few questions:
>>>>> - since alignment management has been remove from ion-core, should it
>>>>> be also removed from ioctl structure ?
>>>>
>>>> Yes, I think I'm going to go with the suggestion to fixup the ABI
>>>> so we don't need the compat layer and as part of that I'm also
>>>> dropping the align argument.
>>>>
>>>
>>> Is the only motivation for removing the alignment parameter that
>>> no-one got around to using it for something useful yet?
>>> The original comment was true - different devices do have different
>>> alignment requirements.
>>>
>>> Better alignment can help SMMUs use larger blocks when mapping,
>>> reducing TLB pressure and the chance of a page table walk causing
>>> display underruns.
>>
>> For that use-case, though, alignment alone doesn't necessarily help -
>> you need the whole allocation granularity to match your block size (i.e.
>> given a 1MB block size, asking for 17KB and getting back 17KB starting
>> at a 1MB boundary doesn't help much - that whole 1MB needs to be
>> allocated and everyone needs to know it to ensure that the whole lot can
>> be mapped safely). Now, whether it's down to the callers or the heap
>> implementations to decide and enforce that granularity is another
>> question, but provided allocations are at least naturally aligned to
>> whatever the granularity is (which is a reasonable assumption to bake
>> in) then it's all good.
>>
>> Robin.
> 
> Agreed, alignment alone isn't enough. But lets assume that an app
> knows what a "good" granularity is, and always asks for allocation
> sizes which are suitably rounded to allow blocks to be used. Currently
> it looks like a "standard" ION_HEAP_TYPE_CARVEOUT heap would give me
> back just a PAGE_SIZE aligned buffer. So even *if* the caller knows
> its desired block size, there's no way for it to get guaranteed better
> alignment, which wouldn't be a bad feature to have.
> 
> Anyway as Daniel and Rob say, if the interface is designed properly
> this kind of extension would be possible later, or you can have a
> special heap with a larger granule.
> 
> I suppose it makes sense to remove it while there's no-one actually
> implementing it, in case an alternate method proves more usable.
> 
> -Brian

Part of the reason I want to remove it is to avoid confusion over
callers thinking it will do anything on most heaps. I agree being
able to specify a larger granularity would be beneficial but I
don't think a dedicated field in the ABI is the right approach.

Thanks,
Laura

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