drivers: Support special QSPI direct-read protocol.

This is useful for interfaces that stay in memory-mapped mode by default.
They can implement this method with a simple `memcpy()`.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

drivers/bus/qspi.h

@@ -46,6 +46,7 @@ typedef struct _mp_qspi_proto_t {
     int (*write_cmd_addr_data)(void *self, uint8_t cmd, uint32_t addr, size_t len, const uint8_t *src);
     int (*read_cmd)(void *self, uint8_t cmd, size_t len, uint32_t *dest);
     int (*read_cmd_qaddr_qdata)(void *self, uint8_t cmd, uint32_t addr, uint8_t num_dummy, size_t len, uint8_t *dest);
+    int (*direct_read)(void *self, uint32_t addr, size_t len, uint8_t *dest); // can be NULL if direct read not supported
 } mp_qspi_proto_t;
 
 typedef struct _mp_soft_qspi_obj_t {

drivers/memory/spiflash.c

@@ -197,6 +197,10 @@ void mp_spiflash_init(mp_spiflash_t *self) {
     } else {
         uint8_t num_dummy = MICROPY_HW_SPIFLASH_QREAD_NUM_DUMMY(self);
         self->config->bus.u_qspi.proto->ioctl(self->config->bus.u_qspi.data, MP_QSPI_IOCTL_INIT, num_dummy);
+        if (self->config->bus.u_qspi.proto->direct_read != NULL) {
+            // A bus with a custom read function should not have any further initialisation done.
+            return;
+        }
     }
 
     mp_spiflash_acquire_bus(self);
@@ -318,6 +322,10 @@ int mp_spiflash_read(mp_spiflash_t *self, uint32_t addr, size_t len, uint8_t *de
     if (len == 0) {
         return 0;
     }
+    const mp_spiflash_config_t *c = self->config;
+    if (c->bus_kind == MP_SPIFLASH_BUS_QSPI && c->bus.u_qspi.proto->direct_read != NULL) {
+        return c->bus.u_qspi.proto->direct_read(c->bus.u_qspi.data, addr, len, dest);
+    }
     mp_spiflash_acquire_bus(self);
     int ret = mp_spiflash_read_data(self, addr, len, dest);
     mp_spiflash_release_bus(self);