Hi Reinhard,
On 05/08/2011 13:23, Reinhard Meyer wrote:
Dear Albert, Aneesh, Eric,
We have a fundamental problem when it comes to invalidating an un-aligned buffer. Either you flush the boundary lines and corrupt your buffer at boundaries OR you invalidate without flushing and corrupt memory around your buffer. Both are not good! The only real solution is to have aligned buffers, if you want to have D-cache enabled and do DMA at the same time.
Plus, there should not be *heavy* modifications; DMA engines tend to use essentially two types of memory-resident objects: data buffers and buffer descriptors. There's only a small handful of places in the driver code to look at to find where these objects are allocated and how.
So I stand by my opinion: since the cache invalidation routine should only be called with cache-aligned objects, there is no requirement to flush the first (resp. last) cache line in case of unaligned start (resp.stop), and I don't want cache operations performed when they are not required.
After considering all issues, any driver that does flush OR invalidate a cache line that it does not fully "own" is prone to cause problems.
At flushing: some DMA might just have put data into the partial line. At invalidating: some Software might have put data, but the writeback had not occured.
So both flush AND invalidate functions should check for this event and emit a proper warning on the console.
Fully agreed.
My 2.7 cents... Reinhard
Amicalement,