Re: [PATCH 13/14] staging: android: ion: Do not sync CPU cache on map/unmap

[Andrew F. Davis]

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@xxxxxx>
>>>> ---
>>>>  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).

> 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.

> //buffer is send down the pipeline
> 
> // Usersapce software post processing occurs
> mmap buffer

Perhaps the invalidate should happen here in mmap.

> DMA_BUF_IOCTL_SYNC IOCT with flags DMA_BUF_SYNC_START // No CMO since no 
> devices attached to buffer

And that should be okay, mmap does the sync, and if no devices are
attached nothing could have changed the underlying memory in the
mean-time, DMA_BUF_SYNC_START can safely be a no-op as they are.

> [CPU reads/writes to the buffer]
> DMA_BUF_IOCTL_SYNC IOCTL with flags DMA_BUF_SYNC_END // No CMO since no 
> devices attached to buffer
> munmap buffer
> 
> //buffer is send down the pipeline
> // Buffer is send to video device (who does compression of raw data) and 
> writes to a file
> dma_buf_attach
> dma_map_attachment (buffer needs to be cleaned)
> [video device writes to buffer]
> dma_buf_unmap_attachment 
> dma_buf_detach  (device cannot stay attached because it is being sent down 
> the pipeline and Video doesn't know the end of the use case)
> 
> 
> 
>>> Also ION no longer provides DMA ready memory, so if you are not doing CPU 
>>> access then there is no requirement (that I am aware of) for you to call 
>>> {begin,end}_cpu_access before passing the buffer to the device and if this 
>>> buffer is cached and your device is not IO-coherent then the cache maintenance
>>> in ion_map_dma_buf and ion_unmap_dma_buf is required.
>>>
>>
>> If I am not doing any CPU access then why do I need CPU cache
>> maintenance on the buffer?
>>
> 
> Because ION no longer provides DMA ready memory.
> Take the above example.
> 
> ION allocates memory from buddy allocator and requests zeroing.
> Zeros are written to the cache.
> 
> You pass the buffer to the camera device which is not IO-coherent.
> The camera devices writes directly to the buffer in DDR.
> Since you didn't clean the buffer a dirty cache line (one of the zeros) is 
> evicted from the cache, this zero overwrites data the camera device has 
> written which corrupts your data.
> 

The zeroing *is* a CPU access, therefor it should handle the needed CMO
for CPU access at the time of zeroing.

Andrew

> Liam
> 
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