Re: [PATCH 13/14] staging: android: ion: Do not sync CPU cache on map/unmap
From: Laura Abbott
Date: Tue Jan 15 2019 - 14:05:53 EST
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@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).
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.
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 :/
//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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