{IDF_TARGET_NAME} Memory Management Unit (MMU) is relatively simple. It can do memory address translation between physical memory addresses and virtual memory addresses. So CPU can access physical memories via virtual addresses. There are multiple types of virtual memory addresses, which have different capabilities.
ESP-IDF provides a memory mapping driver that manages the relation between these physical memory addresses and virtual memory addresses, so as to achieve some features such as reading from SPI Flash via a pointer.
Memory mapping driver is actually a capabilities-based virtual memory address allocator that allows apps to make virtual memory address allocations for different purposes. In the following chapters, we call this driver esp_mmap driver.
ESP-IDF also provides a memory synchronisation driver which can be used for potential memory desychronisation scenarios.
Memory mapping driver currently supports mapping to following physical memory types:
- SPI Flash :SOC_SPIRAM_SUPPORTED and not esp32: - PSRAM
- :cpp
MMU_MEM_CAP_EXEC. This capability indicates that the virtual memory address has the execute permission. Note this permission scope is within the MMU hardware. - :cpp
MMU_MEM_CAP_READ. This capability indicates that the virtual memory address has the read permission. Note this permission scope is within the MMU hardware. - :cpp
MMU_MEM_CAP_WRITE. This capability indicates that the virtual memory address has the write permission. Note this permission scope is within the MMU hardware. - :cpp
MMU_MEM_CAP_32BIT. This capability indicates that the virtual memory address allows for 32 bits or multiples of 32 bits access. - :cpp
MMU_MEM_CAP_8BIT. This capability indicates that the virtual memory address allows for 8 bits or multiples of 8 bits access.
esp32
8 MB external memory addresses (from 0x40400000 to 0x40C00000) which have the :cppMMU_MEM_CAP_EXEC and :cppMMU_MEM_CAP_READ capabilities are not avaiable for users to allocate, due to hardware limitations.
esp32s2
4 MB external memory addresses (from 0x40400000 to 0x40800000) which have the :cppMMU_MEM_CAP_EXEC and :cppMMU_MEM_CAP_READ capabilities are not avaiable for users to allocate, due to hardware limitations.
You can call :cppesp_mmu_map_get_max_consecutive_free_block_size to know the largest consecutive mappable block size with certain capabilities.
The virtual memory pool is made up with one or multiple virtual memory regions, see below figure:
- A virtual memory pool stands for the whole virtual address range that can be mapped to physical memory
- A virtual memory region is a range of virtual address with same attributes
- A virtual memory block is a piece of virtual address range that is dynamically mapped.
- A slot is the virtual address range between two virtual memory blocks.
- A physical memory block is a piece of physical address range that is to-be-mapped or already mapped to a virtual memory block.
- Dynamical mapping is done by calling esp_mmap driver API :cpp
esp_mmu_map, this API will map the given physical memory block to a virtual memory block which is allocated by the esp_mmap driver.
When mapping a physical memory block A, block A can have one of the following relations with another previously mapped physical memory block B:
Enclosed: block A is completely enclosed within block B, see figure below:
Identical: block A is completely the same as block B, see figure below:
Note esp_mmap driver will consider the identical scenario the same as the enclosed scenario.
Overlapped: block A is overlapped with block B, see figure below:
There is a special condition, when block A entirely encloses block B, see figure below:
esp_mmap driver will consider this scenario the same as the overlapped scenario.
You can call :cppesp_mmu_map to do a dynamical mapping. This API will allocate a certain size of virtual memory block according to the virtual memory capabilities you selected, then map this virtual memory block to the physical memory block as you requested. The esp_mmap driver supports mapping to one or more types of physical memory, so you should specify the physical memory target when mapping.
By default, physical memory blocks and virtual memory blocks are one-to-one mapped. This means, when calling :cppesp_mmu_map:
- If it's the enclosed scenario, this API will return an :c
ESP_ERR_INVALID_STATE. The out_ptr will be assigned to the start virtual memory address of the previously mapped one which encloses the to-be-mapped one. - If it's the identical scenario, this API will behaves exactly the same as the enclosed scenario.
- If it's the overlapped scenario, this API will by default return an :c
ESP_ERR_INVALID_ARG. This means, esp_mmap driver by default doesn't allow mapping a physical memory address to multiple virtual memory addresses.
Specially, you can use :cESP_MMU_MMAP_FLAG_PADDR_SHARED. This flags stands for one-to-multiple mapping between a physical address and multiple virtual addresses:
- If it's the overlapped scenario, this API will allocate a new virtual memory block as requested, then map to the given physical memory block.
You can call :cppesp_mmu_unmap to unmap a previously mapped memory block. This API will return an :cESP_ERR_NOT_FOUND if you are trying to unmap a virtual memory block that isn't mapped to any physical memory block yet.
The esp_mmap driver provides two helper APIs to do the conversion between virtual memory address and physical memory address.
- :cpp
esp_mmu_vaddr_to_paddr, convert virtual address to physical address. - :cpp
esp_mmu_paddr_to_vaddr, convert physical address to virtual address.
MMU supported physical memories can be accessed by one or multiple methods.
SPI Flash can be accessed by SPI1 (ESP-IDF esp_flash driver APIs), or by pointers. ESP-IDF esp_flash driver APIs have already considered the memory synchronisation, so users don't need to worry about this.
SOC_SPIRAM_SUPPORTED
PSRAM can be accessed by pointers, hardware guarantees the data consistency when PSRAM is only accessed via pointers.
esp32s3
PSRAM can also be accessed by EDMA. Data desynchronisation may happen because hardware does not guarantee the data consistency under such condition. You should call :cppesp_cache_msync to synchronise the Cache and the PSRAM.
APIs in esp_mmu_map.h are not guaranteed to be thread-safe.
APIs in esp_cache.h are guaranteed to be thread-safe.
inc/esp_mmu_map.inc
inc/esp_cache.inc