Message-ID: <748fdc07-1e0d-b933-f7f3-2e79cda20271@ti.com>
Date:   Fri, 18 Jan 2019 14:43:07 -0600
From:   "Andrew F. Davis" <afd@...com>
To:     Laura Abbott <labbott@...hat.com>, Liam Mark <lmark@...eaurora.org>
CC:     Sumit Semwal <sumit.semwal@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        <devel@...verdev.osuosl.org>, <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 13/14] staging: android: ion: Do not sync CPU cache on
 map/unmap

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@...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?

Thanks,
Andrew

> Thanks,
> Laura

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