Pico SDK flash memory functions seem unstable

[XZCE]

RE: #raspberrypi/pico-sdk#1984

I'm configuring the rpipicow board for boot2_w25q080_4_padded_checksum (I first noticed my issue at slower flash speed, and sure enough, this is showing it more reliably). I'm also set to 133MHz now - so things should all be pretty standard.

#include "hardware/structs/xip_ctrl.h"
#include "hardware/flash.h"

static uint32_t boot2_copyout[64];

static void __no_inline_not_in_flash_func(flash_enable_xip_via_boot2)(void) {
    ((void (*)(void))((intptr_t)boot2_copyout+1))();
}

void __no_inline_not_in_flash_func(setup) () {
    rom_connect_internal_flash_fn connect_internal_flash_func = (rom_connect_internal_flash_fn)rom_func_lookup_inline(ROM_FUNC_CONNECT_INTERNAL_FLASH);
    rom_flash_exit_xip_fn flash_exit_xip_func = (rom_flash_exit_xip_fn)rom_func_lookup_inline(ROM_FUNC_FLASH_EXIT_XIP);
    rom_flash_flush_cache_fn flash_flush_cache_func = (rom_flash_flush_cache_fn)rom_func_lookup_inline(ROM_FUNC_FLASH_FLUSH_CACHE);

    memcpy (boot2_copyout, (void *)XIP_BASE, sizeof (boot2_copyout));

    __compiler_memory_barrier();

    while (1) {

        connect_internal_flash_func();
        flash_exit_xip_func();
        flash_flush_cache_func(); // Note this is needed to remove CSn IO force as well as cache flushing
        flash_enable_xip_via_boot2();

/*
        xip_ctrl_hw->flush = 1;
        while (!(xip_ctrl_hw->stat & XIP_STAT_FLUSH_READY_BITS))
          tight_loop_contents();
*/
/*
        int i = 0;
        while (i < 256) {
          *(volatile uint32_t *)(XIP_BASE + i) = 0;
          i += sizeof(uint32_t);
        }
*/
        if (memcmp ((void *)XIP_BASE, boot2_copyout, sizeof (boot2_copyout)) != 0)
            break; // Something's wrong!

        digitalWrite (LED_BUILTIN, HIGH);
        delay (100);
        digitalWrite (LED_BUILTIN, LOW);
        delay (100);
    }
}

void __no_inline_not_in_flash_func(loop) () {
}

I'm just looping through the steps of the Pico SDK for the flash functions here, and the LED stops blinking after a while.

The real project I noticed this in actually gets a bunch of 0x00's when reading XIP memory, when the issue occurs.

I can remove all the SDK ROM calls and just do the "->flush = 1;" code and get the same result. When I use the special method to invalidate cache by writing to it, it works fine, but of course I'd have to modify the SDK not to do its cache clear (and so "CSn IO force" would need its own code - whatever that is - so not going to happen).

I know why the LED stops blinking, but I don't know why XIP starts reading as 0x00's when using the SDK flash functions. Can anyone else replicate this? And are the any ideas on why it's happening?