esp32/network_wlan: Reduce RAM usage if SPIRAM fails to initialise.

In ESP-IDF, enabling SPIRAM in menuconfig sets some Kconfig options:

- "Wi-Fi Cache TX Buffers" enabled. By default this tries to allocate 32 of
these when Wi-Fi is initialised, which requires 54,400 bytes of free heap.

- Switches "Type of WiFi TX buffers" from Dynamic to Static. This
pre-allocates all of the Wi-Fi transmit buffers.

Not a problem if PSRAM is initialised, but it's quite a lot of RAM if PSRAM
failed to initialise! As we use the same config for PSRAM & no-PSRAM builds
now, this either causes Wi-Fi to fail to initialise (seen on S2) or will
eat quite a lot of RAM.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/esp32/network_wlan.c

@@ -41,6 +41,7 @@
 
 #include "esp_wifi.h"
 #include "esp_log.h"
+#include "esp_psram.h"
 
 #ifndef NO_QSTR
 #include "mdns.h"
@@ -206,6 +207,23 @@ void esp_initialise_wifi(void) {
         wlan_ap_obj.active = false;
 
         wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();
+        #if CONFIG_SPIRAM_IGNORE_NOTFOUND
+        if (!esp_psram_is_initialized()) {
+            // If PSRAM failed to initialize, disable "Wi-Fi Cache TX Buffers"
+            // (default SPIRAM config ESP32_WIFI_CACHE_TX_BUFFER_NUM==32, this is 54,400 bytes of heap)
+            cfg.cache_tx_buf_num = 0;
+            cfg.feature_caps &= ~CONFIG_FEATURE_CACHE_TX_BUF_BIT;
+
+            // Set some other options back to the non-SPIRAM default values
+            // to save more RAM.
+            //
+            // These can be determined from ESP-IDF components/esp_wifi/Kconfig and the
+            // WIFI_INIT_CONFIG_DEFAULT macro
+            cfg.tx_buf_type = 1; // Dynamic, this "magic number" is defined in IDF KConfig
+            cfg.static_tx_buf_num = 0; // Probably don't need, due to tx_buf_type
+            cfg.dynamic_tx_buf_num = 32; // ESP-IDF default value (maximum)
+        }
+        #endif
         ESP_LOGD("modnetwork", "Initializing WiFi");
         esp_exceptions(esp_wifi_init(&cfg));
         esp_exceptions(esp_wifi_set_storage(WIFI_STORAGE_RAM));