Hi,

On Monday, June 18, 2012 9:51 AM Hiroshi Doyu wrote:

On Wed, 6 Jun 2012 15:17:35 +0200
Marek Szyprowski [off-list ref] wrote:

quoted

This is a continuation of the dma-mapping extensions posted in the
following thread:
http://thread.gmane.org/gmane.linux.kernel.mm/78644

We noticed that some advanced buffer sharing use cases usually require
creating a dma mapping for the same memory buffer for more than one
device. Usually also such buffer is never touched with CPU, so the data
are processed by the devices.

From the DMA-mapping perspective this requires to call one of the
dma_map_{page,single,sg} function for the given memory buffer a few
times, for each of the devices. Each dma_map_* call performs CPU cache
synchronization, what might be a time consuming operation, especially
when the buffers are large. We would like to avoid any useless and time
consuming operations, so that was the main reason for introducing
another attribute for DMA-mapping subsystem: DMA_ATTR_SKIP_CPU_SYNC,
which lets dma-mapping core to skip CPU cache synchronization in certain
cases.

I had implemented the similer patch(*1) to optimize/skip the cache
maintanace, but we did this with "dir", not with "attr", making use of
the existing DMA_NONE to skip cache operations. I'm just interested in
why you choose attr for this purpose. Could you enlight me why attr is
used here?

I also thought initially about adding new dma direction for this feature,
but then I realized that there might be cases where the real direction of
the data transfer might be needed (for example to set io read/write
attributes for the mappings) and this will lead us to 3 new dma directions.
The second reason was the compatibility with existing code. There are
already drivers which use DMA_NONE type for their internal stuff. Adding
support for new dma attributes requires changes in all implementations of
dma-mapping for all architectures. DMA attributes are imho better fits
this case. They are by default optional, so other architectures are free
to leave them unimplemented and the drivers should still work correctly.
 
Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center