8MB psram question

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Joined: Mon Oct 14, 2019 10:50 am

8MB psram question

Postby glarty » Tue Oct 15, 2019 4:43 pm

I have been toying with an AI Thinker module which has 8MB psram.

soc.h defines -
#define SOC_EXTRAM_DATA_LOW 0x3F800000
#define SOC_EXTRAM_DATA_HIGH 0x3FC00000

With psram installed it maps the 1st 4MB from SOC_DIRAM_DRAM_LOW (to 0x3FBFFFFF i think)
The rest of the ram in unused unless something like himem is added. (which reduces the end memory address (0x3FBC00000) to allow bit to allow for bank switching )

SOC_EXTRAM_DATA_HIGH memory address isn't used from what I can guess for this module.

So the idea is simply map the 1st 4MB to SOC_EXTRAM_DATA_LOW, and 2nd 4MB to SOC_EXTRAM_DATA_HIGH

Would the code changes indicated below be enough, or am missing something?

looking at spirim.c

Code: Select all

void IRAM_ATTR esp_spiram_init_cache()
    //Enable external RAM in MMU
    cache_sram_mmu_set( 0, 0, SOC_EXTRAM_DATA_LOW, 0, 32, 128 );
    //Flush and enable icache for APP CPU
    cache_sram_mmu_set( 1, 0, SOC_EXTRAM_DATA_LOW, 0, 32, 128 );

esp_err_t esp_spiram_add_to_heapalloc()
    //Add entire external RAM region to heap allocator. Heap allocator knows the capabilities of this type of memory, so there's
    //no need to explicitly specify them.
    ESP_EARLY_LOGI(TAG, "Adding pool of %dK of external SPI memory to heap allocator", (spiram_size_usable_for_malloc() - (&_ext_ram_bss_end - &_ext_ram_bss_start))/1024);
    return heap_caps_add_region((intptr_t)&_ext_ram_bss_end, (intptr_t)SOC_EXTRAM_DATA_LOW + spiram_size_usable_for_malloc()-1);
    ESP_EARLY_LOGI(TAG, "Adding pool of %dK of external SPI memory to heap allocator", spiram_size_usable_for_malloc()/1024);
    return heap_caps_add_region((intptr_t)SOC_EXTRAM_DATA_LOW, (intptr_t)SOC_EXTRAM_DATA_LOW + spiram_size_usable_for_malloc()-1);
Would the following 2 changes work, and if yes, would there any "badness" if it did?

ADD IN -> void IRAM_ATTR esp_spiram_init_cache()
cache_sram_mmu_set( 0, 0, SOC_EXTRAM_DATA_HIGH, 0, 32, 128 );

ADD IN (or create 2nd function for high bank )-> esp_err_t esp_spiram_add_to_heapalloc()
heap_caps_add_region((intptr_t)SOC_EXTRAM_DATA_HIGH, (intptr_t)SOC_EXTRAM_DATA_HIGH + spiram_size_usable_for_malloc()-1);

Not important, just interested if this approach has any merit, before pursuing further,
If something like this would work, you could have a total heap with > 8MB :)


Posts: 2956
Joined: Tue Dec 01, 2015 7:35 am

Re: 8MB psram question

Postby WiFive » Wed Oct 16, 2019 4:41 am

SOC_EXTRAM_DATA_HIGH means SOC_EXTRAM_DATA_END so there is no extra address space there

Posts: 3
Joined: Mon Oct 14, 2019 10:50 am

Re: 8MB psram question

Postby glarty » Wed Oct 16, 2019 8:10 am

Thanx for the clarification.

If I could bother you just 1 more time....
Is this possible/sensible?

Add to soc.h ->
#define SOC_EXTRAM_DATA_LOW2 0x3FC00000
#define SOC_EXTRAM_DATA_HIGH2 0x3FF00000
I can't find any refs in soc.h for this range and being new to the esp platform there I have no idea where else i need to look.

Then map ~3MB more (the range above 0x3FF00000 being already allocated minus 1MB, hence 3MB limit) as a separate chunk from the same psram module to this address space eg-
cache_sram_mmu_set( 0, 0, SOC_EXTRAM_DATA_LOW2, XXXX, 32, 128 );
where XXXX is 4*1024*1024+1

Posts: 2181
Joined: Sun May 08, 2016 4:11 am

Re: 8MB psram question

Postby ESP_Angus » Thu Oct 17, 2019 3:07 am

Hi glarty,

This is a hardware limitation. You can't concurrently map more than 4MB of PSRAM memory into the address space of an ESP32 (this limit is raised in the forthcoming ESP32-S2 Beta).

Working around this limitation is why the Himem API that you mentioned was created:
https://docs.espressif.com/projects/esp ... himem.html

Essentially a part of the 4MB space is set aside for use as "bank switching" space, and then the Himem API controls what part of the PSRAM is mapped there.

If it was possible to increase the address space window beyond 4MB then we would have done this instead of adding Himem!


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