hi Albert,
On Tue Jul 09, 2013 at 10:28:13AM +0200, Albert ARIBAUD wrote:
The arm926ej-s data cache does not have a single fixed policy, and does not have a bypass-on-write policy, only write-through and copy-back.
Other, more complex, policies may be defined, but at the MMU, not cache, level, and those are not constant for all arm926ej-s based SoCs; not even constant for a given SoC as they are configurable at run-time to fit the chosen system addressing map.
Can you please elucidate on these policies. Based on my reading of the arm developers manual and the arm926ejs trm, the mmu makes a particular region cacheable and/or write bufferable. I did not find mention of any other policies. Maybe pointers or links to the documents would help.
You are correct re the other policies of the DDI0198E (ARM926EJ-S TRM) MMU -- page 3-11, bits 3-2 of the section descriptor. Note however that you may have to refer to your specific SoC's TRM or equivalent, as the SoC designer may have defined its own system-level cache and MMU architecture.
Note in any case that none of the policies mentioned in DDI0198E is described as read-allocate (let alone "read-allocate only" where writes would bypass the enabled cache); on the contrary, the only cache policies mentioned are write-through and write-back, both of which contradict cache bypass on write.
I was referring to the cache allocation policy mentioned in section 4.1 in the DDI0198E document -- this is also mentioned in table 12.1 in chapter 12 of the arm developers guide.
(Besides, bypassing the cache for writes and not reads is of little interest for plain DDR caching.)
Again, afaik this is independent of the target interface that is being cached(if i've missed something, can you please point me to the document). Thanks.
Sorry, I don't understand this last comment of yours wrt my point on the (lack of) interest of bypassing cache for DDR caching.
What i meant to state was that i did not find any mention that the cache real allocate policy did not apply for DDR caching.
-sughosh