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Subject: Re: [PATCH 13/14] staging: android: ion: Do not sync CPU cache on
 map/unmap
To:     "Andrew F. Davis" <afd@ti.com>, Liam Mark <lmark@codeaurora.org>
Cc:     Sumit Semwal <sumit.semwal@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?Q?Arve_Hj=c3=b8nnev=c3=a5g?= <arve@android.com>,
        devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org,
        dri-devel <dri-devel@lists.freedesktop.org>
References: <20190111180523.27862-1-afd@ti.com>
 <20190111180523.27862-14-afd@ti.com>
 <alpine.DEB.2.10.1901140910130.23879@lmark-linux.qualcomm.com>
 <79eb70f6-00b0-2939-5ec9-65e196ab4987@ti.com>
 <alpine.DEB.2.10.1901150937280.4046@lmark-linux.qualcomm.com>
 <cfcd3daf-bd1d-0dbc-7b5c-8e504d179564@ti.com>
 <b6cb5596-6beb-73e0-0d8c-d9a13466d626@redhat.com>
 <99ca0b08-02bd-64fd-d43c-c330f0d11639@ti.com>
 <alpine.DEB.2.10.1901161441280.10666@lmark-linux.qualcomm.com>
 <7620534f-b749-76f9-0f53-f73e3f12e9a9@ti.com>
 <fd33986f-ec26-c028-32c2-19c8cc0aba15@redhat.com>
 <748fdc07-1e0d-b933-f7f3-2e79cda20271@ti.com>
From:   Laura Abbott <labbott@redhat.com>
Message-ID: <41c5c506-8c45-9062-a26d-79a9e5428f65@redhat.com>
Date:   Fri, 18 Jan 2019 12:46:33 -0800
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On 1/18/19 12:43 PM, Andrew F. Davis wrote:
> On 1/18/19 2:31 PM, Laura Abbott wrote:
>> On 1/17/19 8:13 AM, Andrew F. Davis wrote:
>>> On 1/16/19 4:48 PM, Liam Mark wrote:
>>>> On Wed, 16 Jan 2019, Andrew F. Davis wrote:
>>>>
>>>>> On 1/15/19 1:05 PM, Laura Abbott wrote:
>>>>>> On 1/15/19 10:38 AM, Andrew F. Davis wrote:
>>>>>>> On 1/15/19 11:45 AM, Liam Mark wrote:
>>>>>>>> On Tue, 15 Jan 2019, Andrew F. Davis wrote:
>>>>>>>>
>>>>>>>>> On 1/14/19 11:13 AM, Liam Mark wrote:
>>>>>>>>>> On Fri, 11 Jan 2019, Andrew F. Davis wrote:
>>>>>>>>>>
>>>>>>>>>>> Buffers may not be mapped from the CPU so skip cache maintenance
>>>>>>>>>>> here.
>>>>>>>>>>> Accesses from the CPU to a cached heap should be bracketed with
>>>>>>>>>>> {begin,end}_cpu_access calls so maintenance should not be needed
>>>>>>>>>>> anyway.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Andrew F. Davis <afd@ti.com>
>>>>>>>>>>> ---
>>>>>>>>>>>     drivers/staging/android/ion/ion.c | 7 ++++---
>>>>>>>>>>>     1 file changed, 4 insertions(+), 3 deletions(-)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/drivers/staging/android/ion/ion.c
>>>>>>>>>>> b/drivers/staging/android/ion/ion.c
>>>>>>>>>>> index 14e48f6eb734..09cb5a8e2b09 100644
>>>>>>>>>>> --- a/drivers/staging/android/ion/ion.c
>>>>>>>>>>> +++ b/drivers/staging/android/ion/ion.c
>>>>>>>>>>> @@ -261,8 +261,8 @@ static struct sg_table
>>>>>>>>>>> *ion_map_dma_buf(struct
>>>>>>>>>>> dma_buf_attachment *attachment,
>>>>>>>>>>>           table = a->table;
>>>>>>>>>>>     -    if (!dma_map_sg(attachment->dev, table->sgl,
>>>>>>>>>>> table->nents,
>>>>>>>>>>> -            direction))
>>>>>>>>>>> +    if (!dma_map_sg_attrs(attachment->dev, table->sgl,
>>>>>>>>>>> table->nents,
>>>>>>>>>>> +                  direction, DMA_ATTR_SKIP_CPU_SYNC))
>>>>>>>>>>
>>>>>>>>>> Unfortunately I don't think you can do this for a couple reasons.
>>>>>>>>>> You can't rely on {begin,end}_cpu_access calls to do cache
>>>>>>>>>> maintenance.
>>>>>>>>>> If the calls to {begin,end}_cpu_access were made before the
>>>>>>>>>> call to
>>>>>>>>>> dma_buf_attach then there won't have been a device attached so the
>>>>>>>>>> calls
>>>>>>>>>> to {begin,end}_cpu_access won't have done any cache maintenance.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> That should be okay though, if you have no attachments (or all
>>>>>>>>> attachments are IO-coherent) then there is no need for cache
>>>>>>>>> maintenance. Unless you mean a sequence where a non-io-coherent
>>>>>>>>> device
>>>>>>>>> is attached later after data has already been written. Does that
>>>>>>>>> sequence need supporting?
>>>>>>>>
>>>>>>>> Yes, but also I think there are cases where CPU access can happen
>>>>>>>> before
>>>>>>>> in Android, but I will focus on later for now.
>>>>>>>>
>>>>>>>>> DMA-BUF doesn't have to allocate the backing
>>>>>>>>> memory until map_dma_buf() time, and that should only happen
>>>>>>>>> after all
>>>>>>>>> the devices have attached so it can know where to put the
>>>>>>>>> buffer. So we
>>>>>>>>> shouldn't expect any CPU access to buffers before all the
>>>>>>>>> devices are
>>>>>>>>> attached and mapped, right?
>>>>>>>>>
>>>>>>>>
>>>>>>>> Here is an example where CPU access can happen later in Android.
>>>>>>>>
>>>>>>>> Camera device records video -> software post processing -> video
>>>>>>>> device
>>>>>>>> (who does compression of raw data) and writes to a file
>>>>>>>>
>>>>>>>> In this example assume the buffer is cached and the devices are not
>>>>>>>> IO-coherent (quite common).
>>>>>>>>
>>>>>>>
>>>>>>> This is the start of the problem, having cached mappings of memory
>>>>>>> that
>>>>>>> is also being accessed non-coherently is going to cause issues one
>>>>>>> way
>>>>>>> or another. On top of the speculative cache fills that have to be
>>>>>>> constantly fought back against with CMOs like below; some coherent
>>>>>>> interconnects behave badly when you mix coherent and non-coherent
>>>>>>> access
>>>>>>> (snoop filters get messed up).
>>>>>>>
>>>>>>> The solution is to either always have the addresses marked
>>>>>>> non-coherent
>>>>>>> (like device memory, no-map carveouts), or if you really want to use
>>>>>>> regular system memory allocated at runtime, then all cached
>>>>>>> mappings of
>>>>>>> it need to be dropped, even the kernel logical address (area as
>>>>>>> painful
>>>>>>> as that would be).
>>>>>>>
>>>>>>
>>>>>> I agree it's broken, hence my desire to remove it :)
>>>>>>
>>>>>> The other problem is that uncached buffers are being used for
>>>>>> performance reason so anything that would involve getting
>>>>>> rid of the logical address would probably negate any performance
>>>>>> benefit.
>>>>>>
>>>>>
>>>>> I wouldn't go as far as to remove them just yet.. Liam seems pretty
>>>>> adamant that they have valid uses. I'm just not sure performance is one
>>>>> of them, maybe in the case of software locks between devices or
>>>>> something where there needs to be a lot of back and forth interleaved
>>>>> access on small amounts of data?
>>>>>
>>>>
>>>> I wasn't aware that ARM considered this not supported, I thought it was
>>>> supported but they advised against it because of the potential
>>>> performance
>>>> impact.
>>>>
>>>
>>> Not sure what you mean by "this" being not supported, do you mean mixed
>>> attribute mappings? If so, it will certainly cause problems, and the
>>> problems will change from platform to platform, avoid at all costs is my
>>> understanding of ARM's position.
>>>
>>>> This is after all supported in the DMA APIs and up until now devices
>>>> have
>>>> been successfully commercializing with this configurations, and I think
>>>> they will continue to commercialize with these configurations for
>>>> quite a
>>>> while.
>>>>
>>>
>>> Use of uncached memory mappings are almost always wrong in my experience
>>> and are used to work around some bug or because the user doesn't want to
>>> implement proper CMOs. Counter examples welcome.
>>>
>>>> It would be really unfortunate if support was removed as I think that
>>>> would drive clients away from using upstream ION.
>>>>
>>>
>>> I'm not petitioning to remove support, but at very least lets reverse
>>> the ION_FLAG_CACHED flag. Ion should hand out cached normal memory by
>>> default, to get uncached you should need to add a flag to your
>>> allocation command pointing out you know what you are doing.
>>>
>>
>> I thought about doing that, the problem is it becomes an ABI break for
>> existing users which I really didn't want to do again. If it
>> ends up being the last thing we do before moving out of staging,
>> I'd consider doing it.
>>
>>>>>>>> ION buffer is allocated.
>>>>>>>>
>>>>>>>> //Camera device records video
>>>>>>>> dma_buf_attach
>>>>>>>> dma_map_attachment (buffer needs to be cleaned)
>>>>>>>
>>>>>>> Why does the buffer need to be cleaned here? I just got through
>>>>>>> reading
>>>>>>> the thread linked by Laura in the other reply. I do like +Brian's
>>>>>>> suggestion of tracking if the buffer has had CPU access since the
>>>>>>> last
>>>>>>> time and only flushing the cache if it has. As unmapped heaps
>>>>>>> never get
>>>>>>> CPU mapped this would never be the case for unmapped heaps, it
>>>>>>> solves my
>>>>>>> problem.
>>>>>>>
>>>>>>>> [camera device writes to buffer]
>>>>>>>> dma_buf_unmap_attachment (buffer needs to be invalidated)
>>>>>>>
>>>>>>> It doesn't know there will be any further CPU access, it could get
>>>>>>> freed
>>>>>>> after this for all we know, the invalidate can be saved until the CPU
>>>>>>> requests access again.
>>>>>>>
>>>>>>>> dma_buf_detach  (device cannot stay attached because it is being
>>>>>>>> sent
>>>>>>>> down
>>>>>>>> the pipeline and Camera doesn't know the end of the use case)
>>>>>>>>
>>>>>>>
>>>>>>> This seems like a broken use-case, I understand the desire to keep
>>>>>>> everything as modular as possible and separate the steps, but at this
>>>>>>> point no one owns this buffers backing memory, not the CPU or any
>>>>>>> device. I would go as far as to say DMA-BUF should be free now to
>>>>>>> de-allocate the backing storage if it wants, that way it could get
>>>>>>> ready
>>>>>>> for the next attachment, which may change the required backing memory
>>>>>>> completely.
>>>>>>>
>>>>>>> All devices should attach before the first mapping, and only let go
>>>>>>> after the task is complete, otherwise this buffers data needs
>>>>>>> copied off
>>>>>>> to a different location or the CPU needs to take ownership
>>>>>>> in-between.
>>>>>>>
>>>>>>
>>>>>> Maybe it's broken but it's the status quo and we spent a good
>>>>>> amount of time at plumbers concluding there isn't a great way
>>>>>> to fix it :/
>>>>>>
>>>>>
>>>>> Hmm, guess that doesn't prove there is not a great way to fix it
>>>>> either.. :/
>>>>>
>>>>> Perhaps just stronger rules on sequencing of operations? I'm not saying
>>>>> I have a good solution either, I just don't see any way forward without
>>>>> some use-case getting broken, so better to fix now over later.
>>>>>
>>>>
>>>> I can see the benefits of Android doing things the way they do, I would
>>>> request that changes we make continue to support Android, or we find
>>>> a way
>>>> to convice them to change, as they are the main ION client and I assume
>>>> other ION clients in the future will want to do this as well.
>>>>
>>>
>>> Android may be the biggest user today (makes sense, Ion come out of the
>>> Android project), but that can change, and getting changes into Android
>>> will be easier that the upstream kernel once Ion is out of staging.
>>>
>>> Unlike some other big ARM vendors, we (TI) do not primarily build mobile
>>> chips targeting Android, our core offerings target more traditional
>>> Linux userspaces, and I'm guessing others will start to do the same as
>>> ARM tries to push more into desktop, server, and other spaces again.
>>>
>>>> I am concerned that if you go with a solution which enforces what you
>>>> mention above, and bring ION out of staging that way, it will make it
>>>> that
>>>> much harder to solve this for Android and therefore harder to get
>>>> Android clients to move to the upstream ION (and get everybody off their
>>>> vendor modified Android versions).
>>>>
>>>
>>> That would be an Android problem, reducing functionality in upstream to
>>> match what some evil vendor trees do to support Android is not the way
>>> forward on this. At least for us we are going to try to make all our
>>> software offerings follow proper buffer ownership (including our Android
>>> offering).
>>>
>>
>> I don't think this is reducing functionality, it's about not breaking
>> what already works. There is some level of Android testing on a mainline
>> tree (hikey boards). I would say if we can come to an agreement on
>> a correct API, we could always merge the 'correct' version out of
>> staging and keep a legacy driver around for some time as a transition.
>>
> 
> I'm not sure that is what staging should be for, but I can certainly see
> why you would want that (I help maintain our Android offering and every
> kernel migration I get to go fixup libion and all its users..).
> 
> I'm sure we all know the API will get broken to get this out of staging,
> so maybe we need to start a list (or update the TODO) with all the
> things we agree need changed during the last step before destaging.
> Sounds like you agree about the ION_FLAG_CACHED reversal for starters. I
> think direct heap managed dma_buf_ops will be needed.
> 
> What's left, do we have any current proposals for the heap query
> floating around that can go up for review?
> 

I was hoping the last time I broke the API would be the last time
we would need to break it again. The query ioctl is already merged
and I haven't seen any other counter proposals around for discussion.
The TODO list could probably use some updating though.

> Thanks,
> Andrew
> 
>> Thanks,
>> Laura