Cache Control

This section contains definitions for supporting control of the caches on the CPU.

These definitions are usually found in the header file cyg/hal/hal_cache.h. This file may be defined in the architecture, variant or platform HAL, depending on where the caches are implemented for the target. Often there will be a generic implementation of the cache control macros in the architecture HAL with the ability to override or undefine them in the variant or platform HAL. Even when the implementation of the cache macros is in the architecture HAL, the cache dimensions will be defined in the variant or platform HAL. As with other files, the variant or platform specific definitions are usually found in cyg/hal/var_cache.h and cyg/hal/plf_cache.h respectively. These files are include automatically by this header, so need not be included explicitly.

There are versions of the macros defined here for both the Data and Instruction caches. these are distinguished by the use of either DCACHE or ICACHE in the macro names. Some architectures have a unified cache, where both data and instruction share the same cache. In these cases the control macros use UCACHE and the DCACHE and ICACHE macros will just be calls to the UCACHE version. In the following descriptions, XCACHE is used to stand for any of these. Where there are issues specific to a particular cache, this will be explained in the text.

There might be target specific restrictions on the use of some of the macros which it is the user's responsibility to comply with. Such restrictions are documented in the header file with the macro definition.

Note that destructive cache macros should be used with caution. Preceding a cache invalidation with a cache synchronization is not safe in itself since an interrupt may happen after the synchronization but before the invalidation. This might cause the state of dirty data lines created during the interrupt to be lost.

Depending on the architecture's capabilities, it may be possible to temporarily disable the cache while doing the synchronization and invalidation which solves the problem (no new data would be cached during an interrupt). Otherwise it is necessary to disable interrupts while manipulating the cache which may take a long time.

Some platform HALs now support a pair of cache state query macros: HAL_ICACHE_IS_ENABLED( x ) and HAL_DCACHE_IS_ENABLED( x ) which set the argument to true if the instruction or data cache is enabled, respectively. Like most cache control macros, these are optional, because the capabilities of different targets and boards can vary considerably. Code which uses them, if it is to be considered portable, should test for their existence first by means of #ifdef. Be sure to include <cyg/hal/hal_cache.h> in order to do this test and (maybe) use the macros.

Cache Dimensions

HAL_XCACHE_SIZE
HAL_XCACHE_LINE_SIZE
HAL_XCACHE_WAYS
HAL_XCACHE_SETS

These macros define the size and dimensions of the Instruction and Data caches.

HAL_XCACHE_SIZE

Defines the total size of the cache in bytes.

HAL_XCACHE_LINE_SIZE

Defines the cache line size in bytes.

HAL_XCACHE_WAYS

Defines the number of ways in each set and defines its level of associativity. This would be 1 for a direct mapped cache, 2 for a 2-way cache, 4 for 4-way and so on.

HAL_XCACHE_SETS

Defines the number of sets in the cache, and is calculated from the previous values.

Global Cache Control

HAL_XCACHE_ENABLE()
HAL_XCACHE_DISABLE()
HAL_XCACHE_INVALIDATE_ALL()
HAL_XCACHE_SYNC()
HAL_XCACHE_BURST_SIZE( size )
HAL_DCACHE_WRITE_MODE( mode )
HAL_XCACHE_LOCK( base, size )
HAL_XCACHE_UNLOCK( base, size )
HAL_XCACHE_UNLOCK_ALL()

These macros affect the state of the entire cache, or a large part of it.

HAL_XCACHE_ENABLE() and HAL_XCACHE_DISABLE()

Enable and disable the cache.

HAL_XCACHE_INVALIDATE_ALL()

Causes the entire contents of the cache to be invalidated. Depending on the hardware, this may require the cache to be disabled during the invalidation process. If so, the implementation must use HAL_XCACHE_IS_ENABLED() to save and restore the previous state.

Note: If this macro is called after HAL_XCACHE_SYNC() with the intention of clearing the cache (invalidating the cache after writing dirty data back to memory), you must prevent interrupts from happening between the two calls:

 ...
 HAL_DISABLE_INTERRUPTS(old);
 HAL_XCACHE_SYNC();
 HAL_XCACHE_INVALIDATE_ALL();
 HAL_RESTORE_INTERRUPTS(old);
 ...

Since the operation may take a very long time, real-time responsiveness could be affected, so only do this when it is absolutely required and you know the delay will not interfere with the operation of drivers or the application.

HAL_XCACHE_SYNC()

Causes the contents of the cache to be brought into synchronization with the contents of memory. In some implementations this may be equivalent to HAL_XCACHE_INVALIDATE_ALL().

HAL_XCACHE_BURST_SIZE()

Allows the size of cache to/from memory bursts to be controlled. This macro will only be defined if this functionality is available.

HAL_DCACHE_WRITE_MODE()

Controls the way in which data cache lines are written back to memory. There will be definitions for the possible modes. Typical definitions are HAL_DCACHE_WRITEBACK_MODE and HAL_DCACHE_WRITETHRU_MODE. This macro will only be defined if this functionality is available.

HAL_XCACHE_LOCK()

Causes data to be locked into the cache. The base and size arguments define the memory region that will be locked into the cache. It is architecture dependent whether more than one locked region is allowed at any one time, and whether this operation causes the cache to cease acting as a cache for addresses outside the region during the duration of the lock. This macro will only be defined if this functionality is available.

HAL_XCACHE_UNLOCK()

Cancels the locking of the memory region given. This should normally correspond to a region supplied in a matching lock call. This macro will only be defined if this functionality is available.

HAL_XCACHE_UNLOCK_ALL()

Cancels all existing locked memory regions. This may be required as part of the cache initialization on some architectures. This macro will only be defined if this functionality is available.

Cache Line Control

HAL_DCACHE_ALLOCATE( base , size )
HAL_DCACHE_FLUSH( base , size )
HAL_XCACHE_INVALIDATE( base , size )
HAL_DCACHE_STORE( base , size )
HAL_DCACHE_READ_HINT( base , size )
HAL_DCACHE_WRITE_HINT( base , size )
HAL_DCACHE_ZERO( base , size )

All of these macros apply a cache operation to all cache lines that match the memory address region defined by the base and size arguments. These macros will only be defined if the described functionality is available. Also, it is not guaranteed that the cache function will only be applied to just the described regions, in some architectures it may be applied to the whole cache.

HAL_DCACHE_ALLOCATE()

Allocates lines in the cache for the given region without reading their contents from memory, hence the contents of the lines is undefined. This is useful for preallocating lines which are to be completely overwritten, for example in a block copy operation.

HAL_DCACHE_FLUSH()

Invalidates all cache lines in the region after writing any dirty lines to memory.

HAL_XCACHE_INVALIDATE()

Invalidates all cache lines in the region. Any dirty lines are invalidated without being written to memory.

HAL_DCACHE_STORE()

Writes all dirty lines in the region to memory, but does not invalidate any lines.

HAL_DCACHE_READ_HINT()

Hints to the cache that the region is going to be read from in the near future. This may cause the region to be speculatively read into the cache.

HAL_DCACHE_WRITE_HINT()

Hints to the cache that the region is going to be written to in the near future. This may have the identical behavior to HAL_DCACHE_READ_HINT().

HAL_DCACHE_ZERO()

Allocates and zeroes lines in the cache for the given region without reading memory. This is useful if a large area of memory is to be cleared.

2017-02-09
Documentation license for this page: Open Publication License