External memory support

[wdx04]

Hi,

Many boards are equipped with external memories that can operate in memory-mapped mode. For example, the STM32F429I-DISCO is equipped with 8MB of SDRAM, while the B-U585I-IOT02A is equipped with 8MB of OSPI PSRAM and 64MB of QSPI Flash.

External memories are typically used in applications with high storage requirements, such as graphics and HMI applications.

Currently Mbed has no support for external memories. For example, to use external memories on STM32 discovery boards, we have to copy the external memory drivers from the STM32Cube package. This is inconvenient for writing portable graphics and HMI applications for different boards.

I think it would be nice to have a uniform way to use external memories in memory-mapped mode:

• memory-mapping of external memories can be turned on/off via configuration options (map-external-ram/map-external-flash)
• if memory-mapping is enabled, the configuration of external memories can be done in the TargetBSP_Init function
• the address and size of mapped external memories are provided as macros (MBED_EXTERNAL_RAM_START/TMBED_EXTERNAL_RAM_SIZE, etc)
• the external RAM can be used freely, or we can provide an option to use the external RAM for additional heap

Do you think it would be useful to add this feature to Mbed CE?

[multiplemonomials]

This sounds like a great thing to add! And some Mbed targets already do support external memories, e.g. MIMXRT105x and its optional external SDRAM. The only real issue is that adding external memories generally requires changes to the memory bank configuration (so Mbed knows about them) and the linker script (so that the toolchain knows what can be put in there). So support would have to be implemented on a per-target basis (as the linker script would need to be modified).

One potential workaround would be to use the external memory as purely heap and then instantiate an allocator within it. So basically you'd call like ext_mem_malloc(n) to allocate n bytes from external RAM. That way the toolchain and OS wouldn't really have to know about it. But then only user code using this function could access the space, not code in Mbed or global variables.