From 8ed9269b55323cf06dc5b4096e599a0b4da9a67e Mon Sep 17 00:00:00 2001
From: MrScothh <167884257+MrScothh@users.noreply.github.com>
Date: Thu, 8 Oct 2026 09:45:59 +0200
Subject: [PATCH] sdcard: run an SD card on SPI at the fast bus speed when it
 has the bus to itself

---
 src/main/drivers/sdcard/sdcard_spi.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/main/drivers/sdcard/sdcard_spi.c b/src/main/drivers/sdcard/sdcard_spi.c
index 6446ec09164..3ff035737fc 100644
--- a/src/main/drivers/sdcard/sdcard_spi.c
+++ b/src/main/drivers/sdcard/sdcard_spi.c
@@ -48,10 +48,22 @@
 /* Break up 512-byte SD card sectors into chunks of this size to reduce the peak overhead per call to sdcard_poll(). */
 #define SDCARD_BLOCK_CHUNK_SIZE     128
 
+// FAST stays under the 25 MHz of an SD card's default speed on every MCU with an SD card on SPI
 #ifndef SDCARD_BUS_SPEED
-#define SDCARD_BUS_SPEED            BUS_SPEED_STANDARD
+#define SDCARD_BUS_SPEED            BUS_SPEED_FAST
 #endif
 
+// Other devices set their bus speed once, at init, and the bus keeps the last one: a shared bus stays as it was
+static bool sdcardSpi_busShared(void)
+{
+    for (const busDeviceDescriptor_t * descriptor = __busdev_registry_start; descriptor < __busdev_registry_end; descriptor++) {
+        if (descriptor->busType == BUSTYPE_SPI && descriptor->devHwType != DEVHW_SDCARD && descriptor->busdev.spi.spiBus == sdcard.dev->busdev.spi.spiBus) {
+            return true;
+        }
+    }
+    return false;
+}
+
 static void sdcardSpi_select(void)
 {
     busSelectDevice(sdcard.dev);
@@ -104,6 +116,10 @@ static void sdcardSpi_reset(void)
     if (sdcard.failureCount >= SDCARD_MAX_CONSECUTIVE_FAILURES) {
         sdcard.state = SDCARD_STATE_NOT_PRESENT;
     } else {
+        // After an error the card is identified again at the speed it always was, not at the full speed
+        if (sdcard.state >= SDCARD_STATE_READY) {
+            busSetSpeed(sdcard.dev, BUS_SPEED_STANDARD);
+        }
         sdcard.operationStartTime = millis();
         sdcard.state = SDCARD_STATE_RESET;
     }
@@ -535,7 +551,7 @@ static bool sdcardSpi_poll(void)
                 }
 
                 // Now we're done with init and we can switch to the full speed clock (<25MHz)
-                busSetSpeed(sdcard.dev, SDCARD_BUS_SPEED);
+                busSetSpeed(sdcard.dev, sdcardSpi_busShared() ? BUS_SPEED_STANDARD : SDCARD_BUS_SPEED);
 
                 sdcard.multiWriteBlocksRemain = 0;