Arnd,
On 21 April 2011 20:29, Arnd Bergmann arnd@arndb.de wrote:
I think the recent discussions on linaro-mm-sig and the BoF last week at ELC have been quite productive, and at least my understanding of the missing pieces has improved quite a bit. This is a list of things that I think need to be done in the kernel. Please complain if any of these still seem controversial:
- Fix the arm version of dma_alloc_coherent. It's in use today and
is broken on modern CPUs because it results in both cached and uncached mappings. Rebecca suggested different approaches how to get there.
It's not broken since we moved to using Normal non-cacheable memory for the coherent DMA buffers (as long as you flush the cacheable alias before using the buffer, as we already do). The ARM ARM currently says unpredictable for such situations but this is being clarified in future updates and the Normal non-cacheable vs cacheable aliases can be used (given correct cache maintenance before using the buffer).
- Implement dma_alloc_noncoherent on ARM. Marek pointed out
that this is needed, and it currently is not implemented, with an outdated comment explaining why it used to not be possible to do it.
As Russell pointed out, there are 4 main combinations with iommu and some coherency support (i.e. being able to snoop the CPU caches). But in an SoC you can have different devices with different iommu and coherency configurations. Some of them may even be able to see the L2 cache but not the L1 (in which case it would help if we can get an inner non-cacheable outer cacheable mapping).
Anyway, we end up with different DMA ops per device via dev_archdata.