We're trying to use the GPMC peripheral as a method to
communicate with an external high-speed processing element, and would
like to use it in a address/data muxed, synchronous mode with support
for bursting transactions.
Before diving into an implementation, I've been spending time trying
to characterize the behaviour of this peripheral, especially with
regards to bursting writes/reads. Based on my conversations with Soren
(another poster here), bursting transactions are determined by the
cache properties of the memory mapped chip select space (by setting
the appropriate Cacheable, Bufferable bits in the page table entry).
I've currently tested in both un-cached and writeback modes using the
L_PTE_MT_UNCACHED, and L_PTE_MT_WRITEBACK protection bits during my
mmap setup. I've also setup the gpmc for synchronous, with
config1.writemultiple and config1.readmultiple set to 1.
However, my experimentation leads me to believe that during writeback
mode, the transactions i've observing on the scope do not entirely
behave as true writeback cache would. Here's an example.
MMAP: Writeback mode (Correct behavior)
1) Read DWORD at location 0 (within GPMC CSx space):
2) Read DWORD immediately after at location 8
I notice a single 32-byte read burst - this confirms a cache line is
MMAP: Writeback mode (Fishy behaviour)
1) Read DWORD at location 0 (within GPMC CSx space)
2) Write DWORD at location 8
3) Read DWORD at location 16
I notice a single 32-byte read burst, then a little while later, a
burst DWORD write transaction and then another 32-byte read burst.
This does not really coincide with my understanding of writeback
cache, which should not persist the three transactions out until i
have invalidated the line, or it has been hit by another memory
Now since i'm working in Linux, and this is set-associative cache, it
could be that that line is being trampled by another location (very
likely) - but i've repeated the tests enough times and get the same
Soooo....what exactly is the relationship between bursting and caching
with respect to the GPMC? Sorry for the long post, but any help here
would be appreciated.
Nuvation Research Corporation (Canada)