Memory Management for MMU Supported Memory

Introduction

ESP32-S2 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.

Physical Memory Types

Memory mapping driver currently supports mapping to following physical memory types:

  • SPI Flash

  • PSRAM

Virtual Memory Capabilities

  • 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.

  • 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.

  • 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.

  • MMU_MEM_CAP_32BIT. This capability indicates that the virtual memory address allows for 32 bits or multiples of 32 bits access.

  • MMU_MEM_CAP_8BIT. This capability indicates that the virtual memory address allows for 8 bits or multiples of 8 bits access.

4 MB external memory addresses (from 0x40400000 to 0x40800000) which have the MMU_MEM_CAP_EXEC and MMU_MEM_CAP_READ capabilities are not avaiable for users to allocate, due to hardware limitations.

You can call esp_mmu_map_get_max_consecutive_free_block_size() to know the largest consecutive mappable block size with certain capabilities.

Memory Management Drivers

Driver Concept

Terminology

The virtual memory pool is made up with one or multiple virtual memory regions, see below figure:

../../_images/mem_pool.png

  • 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 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.

Relation between Memory Blocks

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:

    ../../_images/enclosed.png

  • Identical: block A is completely the same as block B, see figure below:

    ../../_images/identical.png

    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:

    ../../_images/overlapped.png

    There is a special condition, when block A entirely encloses block B, see figure below:

    ../../_images/inversed_enclosed.png

    esp_mmap driver will consider this scenario the same as the overlapped scenario.

Driver Behaviour

Memory Map

You can call esp_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 esp_mmu_map():

  • If it's the enclosed scenario, this API will return an 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 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 ESP_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.

Memory Unmap

You can call esp_mmu_unmap() to unmap a previously mapped memory block. This API will return an ESP_ERR_NOT_FOUND if you are trying to unmap a virtual memory block that isn't mapped to any physical memory block yet.

Memory Address Conversion

The esp_mmap driver provides two helper APIs to do the conversion between virtual memory address and physical memory address.

Memory Synchronisation

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.

PSRAM can be accessed by pointers, hardware guarantees the data consistency when PSRAM is only accessed via pointers.

Thread Safety

APIs in esp_mmu_map.h are not guaranteed to be thread-safe.

APIs in esp_cache.h are guaranteed to be thread-safe.

API Reference

API Reference - ESP MMAP Driver

Header File

Functions

esp_err_t esp_mmu_map(esp_paddr_t paddr_start, size_t size, mmu_target_t target, mmu_mem_caps_t caps, int flags, void **out_ptr)

Map a physical memory block to external virtual address block, with given capabilities.

Note

This API does not guarantee thread safety

Parameters

  • paddr_start -- [in] Start address of the physical memory block

  • size -- [in] Size to be mapped. Size will be rounded up by to the nearest multiple of MMU page size

  • target -- [in] Physical memory target you're going to map to, see mmu_target_t

  • caps -- [in] Memory capabilities, see mmu_mem_caps_t

  • flags -- [in] Mmap flags

  • out_ptr -- [out] Start address of the mapped virtual memory

Returns

  • ESP_OK

  • ESP_ERR_INVALID_ARG: Invalid argument, see printed logs

  • ESP_ERR_NOT_SUPPORTED: Only on ESP32, PSRAM is not a supported physical memory target

  • ESP_ERR_NOT_FOUND: No enough size free block to use

  • ESP_ERR_NO_MEM: Out of memory, this API will allocate some heap memory for internal usage

  • ESP_ERR_INVALID_STATE: Paddr is mapped already, this API will return corresponding vaddr_start of the previously mapped block. Only to-be-mapped paddr block is totally enclosed by a previously mapped block will lead to this error. (Identical scenario will behave similarly) new_block_start new_block_end |-----— New Block -----—| |------------— Block ------------—| block_start block_end

esp_err_t esp_mmu_unmap(void *ptr)

Unmap a previously mapped virtual memory block.

Note

This API does not guarantee thread safety

Parameters

ptr -- [in] Start address of the virtual memory

Returns

  • ESP_OK

  • ESP_ERR_INVALID_ARG: Null pointer

  • ESP_ERR_NOT_FOUND: Vaddr is not in external memory, or it's not mapped yet

esp_err_t esp_mmu_map_get_max_consecutive_free_block_size(mmu_mem_caps_t caps, mmu_target_t target, size_t *out_len)

Get largest consecutive free external virtual memory block size, with given capabilities and given physical target.

Parameters

  • caps -- [in] Bitwise OR of MMU_MEM_CAP_* flags indicating the memory block

  • target -- [in] Physical memory target you're going to map to, see mmu_target_t.

  • out_len -- [out] Largest free block length, in bytes.

Returns

  • ESP_OK

  • ESP_ERR_INVALID_ARG: Invalid arguments, could be null pointer

esp_err_t esp_mmu_map_dump_mapped_blocks(FILE *stream)

Dump all the previously mapped blocks

Note

This API shall not be called from an ISR.

Note

This API does not guarantee thread safety

Parameters

stream -- stream to print information to; use stdout or stderr to print to the console; use fmemopen/open_memstream to print to a string buffer.

Returns

  • ESP_OK

esp_err_t esp_mmu_vaddr_to_paddr(void *vaddr, esp_paddr_t *out_paddr, mmu_target_t *out_target)

Convert virtual address to physical address.

Parameters

  • vaddr -- [in] Virtual address

  • out_paddr -- [out] Physical address

  • out_target -- [out] Physical memory target, see mmu_target_t

Returns

  • ESP_OK

  • ESP_ERR_INVALID_ARG: Null pointer, or vaddr is not within external memory

  • ESP_ERR_NOT_FOUND: Vaddr is not mapped yet

esp_err_t esp_mmu_paddr_to_vaddr(esp_paddr_t paddr, mmu_target_t target, mmu_vaddr_t type, void **out_vaddr)

Convert physical address to virtual address.

Parameters

  • paddr -- [in] Physical address

  • target -- [in] Physical memory target, see mmu_target_t

  • type -- [in] Virtual address type, could be either instruction or data

  • out_vaddr -- [out] Virtual address

Returns

  • ESP_OK

  • ESP_ERR_INVALID_ARG: Null pointer

  • ESP_ERR_NOT_FOUND: Paddr is not mapped yet

esp_err_t esp_mmu_paddr_find_caps(const esp_paddr_t paddr, mmu_mem_caps_t *out_caps)

If the physical address is mapped, this API will provide the capabilities of the virtual address where the physical address is mapped to.

Note

: Only return value is ESP_OK(which means physically address is successfully mapped), then caps you get make sense.

Note

This API only check one page (see CONFIG_MMU_PAGE_SIZE), starting from the paddr

Parameters

  • paddr -- [in] Physical address

  • out_caps -- [out] Bitwise OR of MMU_MEM_CAP_* flags indicating the capabilities of a virtual address where the physical address is mapped to.

Returns

  • ESP_OK: Physical address successfully mapped.

  • ESP_ERR_INVALID_ARG: Null pointer

  • ESP_ERR_NOT_FOUND: Physical address is not mapped successfully.

Macros

ESP_MMU_MMAP_FLAG_PADDR_SHARED

Share this mapping.

MMU Memory Mapping Driver APIs for MMU supported memory

Driver Backgrounds:

Type Definitions

typedef uint32_t esp_paddr_t

Physical memory type.

API Reference - ESP MSYNC Driver

Header File

Functions

esp_err_t esp_cache_msync(void *addr, size_t size, int flags)

Memory sync between Cache and external memory.

  • For cache writeback supported chips (you can refer to SOC_CACHE_WRITEBACK_SUPPORTED in soc_caps.h)

    • this API will do a writeback to synchronise between cache and the PSRAM

    • with ESP_CACHE_MSYNC_FLAG_INVALIDATE, this API will also invalidate the values that just written

    • note: although ESP32 is with PSRAM, but cache writeback isn't supported, so this API will do nothing on ESP32

  • For other chips, this API will do nothing. The out-of-sync should be already dealt by the SDK

This API is cache-safe and thread-safe

Note

You should not call this during any Flash operations (e.g. esp_flash APIs, nvs and some other APIs that are based on esp_flash APIs)

Note

If XIP_From_PSRAM is enabled (by enabling both CONFIG_SPIRAM_FETCH_INSTRUCTIONS and CONFIG_SPIRAM_RODATA), you can call this API during Flash operations

Parameters

  • addr -- [in] Starting address to do the msync

  • size -- [in] Size to do the msync

  • flags -- [in] Flags, see ESP_CACHE_MSYNC_FLAG_x

Returns

  • ESP_OK:

    • Successful msync

    • If this chip doesn't support cache writeback, if the input addr is a cache supported one, this API will return ESP_OK

  • ESP_ERR_INVALID_ARG: Invalid argument, not cache supported addr, see printed logs

Macros

ESP_CACHE_MSYNC_FLAG_INVALIDATE

Do an invalidation with the values that just written.

Cache msync flags

ESP_CACHE_MSYNC_FLAG_UNALIGNED

Allow writeback a block that are not aligned to the data cache line size.

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