The cacheability of a transaction is used by the CCI to determine if a cache look-up is required and if there is a hit, what to do with the read or write data.
The cache and buffer policy for a transaction is encoded in four AxCACHE signals. There are two separate sets of signals for reads and writes. The four AxCACHE signals are driven from register bits and are defined as follows when the bit is set High.
- Bufferable, bit [0]: interconnect can delay transaction before reaching its destination; most relevant to writes
- Cacheable, bit [1]: cacheable transaction indicator
- Read Allocate, bit [2]: a cache miss allocates space for the read data from system memory
- Write Allocate, bit [3]: a cache miss allocates space for the write data from the initiator
The four AxCACHE [3:0] bits are combined together.
-
0000: non-cacheble and non-bufferable -
0001: bufferable only -
0010: cacheable but does not allocate -
0011: cacheable and bufferable, does not allocate -
0110: cacheable write-through, allocates on read -
0111: cacheable write-back, allocates on read -
1010: cacheable write-through, allocates on write -
1011: cacheable write-back, allocates on write -
1110: cacheable write-through, allocates on both read and write -
1111: cacheable write-back, allocates on both read and write
Many PS peripheral transaction hosts can be programmed to route their transactions directly to DDR memory or to the FPD SMMU and the CCI first. The AxCACHE attributes are valid when the transaction is routed through the SMMU and CCI.
- RPU processors
- LPD DMA controllers
- DMA units in the SD, eMMC, OSPI, GEM, DPC controllers