Workaround PSRAM cache invalid'm by reading flash

Fixes #2537

While waiting for a working direct cache flush routine, try and force the
cache to evict all PSRAM values by reading a bunch of flash addresses (which
share the XIP cache).  This hurts performance when PSRAM is not used, but
is required for correctness until we have a working XIP flush.

Author: earlephilhower

cores/rp2040/flash_wrapper.cpp

@@ -17,18 +17,37 @@
     License along with this library; if not, write to the Free Software
     Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
-
+#include <Arduino.h>
 #include <hardware/flash.h>
 
 #ifdef PICO_RP2350
 #include <hardware/structs/qmi.h>
 #endif
 
+#if defined(RP2350_PSRAM_CS)
+static volatile uint32_t __wastedsum = 0;
+static void __no_inline_not_in_flash_func(flushcache)() {
+    //for (volatile uint8_t* cache = (volatile uint8_t*)0x18000001; cache < (volatile uint8_t*)(0x18000001 + 2048 * 8); cache += 8) {
+    //    *cache = 0;
+    //}
+    uint32_t sum = 0; // Ignored, just to ensure not optimized out
+    for (volatile uint32_t *flash = (volatile uint32_t *)0x11000000; flash < (volatile uint32_t *)(0x11000000 + 48*1024*4); flash++) {
+        sum += *flash;
+    }
+    __wastedsum += sum;
+}
+#else
+static uint32_t flushcache() {
+    return 0;
+}
+#endif
+
 extern "C" {
     extern void __real_flash_range_erase(uint32_t flash_offs, size_t count);
     void __wrap_flash_range_erase(uint32_t flash_offs, size_t count) {
 #ifdef PICO_RP2350
         auto s = qmi_hw->m[1];
+        flushcache();
 #endif
         __real_flash_range_erase(flash_offs, count);
 #ifdef PICO_RP2350
@@ -41,6 +60,7 @@ extern "C" {
     void __wrap_flash_range_program(uint32_t flash_offs, const uint8_t *data, size_t count) {
 #ifdef PICO_RP2350
         auto s = qmi_hw->m[1];
+        flushcache();
 #endif
         __real_flash_range_program(flash_offs, data, count);
 #ifdef PICO_RP2350
@@ -53,6 +73,7 @@ extern "C" {
     void __wrap_flash_get_unique_id(uint8_t *id_out) {
 #ifdef PICO_RP2350
         auto s = qmi_hw->m[1];
+        flushcache();
 #endif
         __real_flash_get_unique_id(id_out);
 #ifdef PICO_RP2350
@@ -65,6 +86,7 @@ extern "C" {
     void __wrap_flash_do_cmd(const uint8_t *txbuf, uint8_t *rxbuf, size_t count) {
 #ifdef PICO_RP2350
         auto s = qmi_hw->m[1];
+        flushcache();
 #endif
         __real_flash_do_cmd(txbuf, rxbuf, count);
 #ifdef PICO_RP2350