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Disabling I-cache:
- Set CONFIG_SYS_ICACHE_OFF
Disabling D-cache:
- Set CONFIG_SYS_DCACHE_OFF
Enabling I-cache:
- Make sure CONFIG_SYS_ICACHE_OFF is not set and call icache_enable().
Enabling D-cache:
- Make sure CONFIG_SYS_DCACHE_OFF is not set and call dcache_enable().
Enabling Caches at System Startup:
- Implement enable_caches() for your platform and enable the I-cache and
D-cache from this function. This function is called immediately
after relocation.
Guidelines for Working with D-cache:
Memory to Peripheral DMA:
- Flush the buffer after the MPU writes the data and before the DMA is
initiated.
Peripheral to Memory DMA:
- Invalidate the buffer before starting the DMA. In case there are any dirty
lines from the DMA buffer in the cache, subsequent cache-line replacements
may corrupt the buffer in memory while the DMA is still going on. Cache-line
replacement can happen if the CPU tries to bring some other memory locations
into the cache while the DMA is going on.
- Invalidate the buffer after the DMA is complete and before the MPU reads
it. This may be needed in addition to the invalidation before the DMA
mentioned above, because in some processors memory contents can spontaneously
come to the cache due to speculative memory access by the CPU. If this
happens with the DMA buffer while DMA is going on we have a coherency problem.
Buffer Requirements:
- Any buffer that is invalidated(that is, typically the peripheral to
memory DMA buffer) should be aligned to cache-line boundary both at
at the beginning and at the end of the buffer.
- If the buffer is not cache-line aligned invalidation will be restricted
to the aligned part. That is, one cache-line at the respective boundary
may be left out while doing invalidation.
- A suitable buffer can be alloced on the stack using the
ALLOC_CACHE_ALIGN_BUFFER macro.
Cleanup Before Linux:
- cleanup_before_linux() should flush the D-cache, invalidate I-cache, and
disable MMU and caches.
- The following sequence is advisable while disabling d-cache:
1. disable_dcache() - flushes and disables d-cache
2. invalidate_dcache_all() - invalid any entry that came to the cache
in the short period after the cache was flushed but before the
cache got disabled.
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