From 613bdcac5824146305e9324582ab2698c0eb6c84 Mon Sep 17 00:00:00 2001
From: Andy Piper <github@andypiper.com>
Date: Fri, 13 Dec 2024 10:13:52 +0000
Subject: [PATCH] AP_Logger: nuance minspace for W25NXX and W25QXX

---
 libraries/AP_Logger/AP_Logger.cpp      |  8 ++++++--
 libraries/AP_Logger/AP_Logger_File.cpp | 12 ++++++++++--
 libraries/AP_Logger/AP_Logger_File.h   |  6 +++++-
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/libraries/AP_Logger/AP_Logger.cpp b/libraries/AP_Logger/AP_Logger.cpp
index 3d834946edf5c..f19a3636f0cd5 100644
--- a/libraries/AP_Logger/AP_Logger.cpp
+++ b/libraries/AP_Logger/AP_Logger.cpp
@@ -58,7 +58,11 @@ extern const AP_HAL::HAL& hal;
 
 #ifndef HAL_LOGGER_MIN_MB_FREE
 #if AP_FILESYSTEM_LITTLEFS_ENABLED
-#define HAL_LOGGER_MIN_MB_FREE 1
+#if AP_FILESYSTEM_LITTLEFS_FLASH_TYPE == AP_FILESYSTEM_FLASH_W25NXX
+#define HAL_LOGGER_MIN_MB_FREE 10
+#else
+#define HAL_LOGGER_MIN_MB_FREE 2
+#endif
 #else
 #define HAL_LOGGER_MIN_MB_FREE 500
 #endif
@@ -143,7 +147,7 @@ const AP_Param::GroupInfo AP_Logger::var_info[] = {
     // @DisplayName: Old logs on the SD card will be deleted to maintain this amount of free space
     // @Description: Set this such that the free space is larger than your largest typical flight log
     // @Units: MB
-    // @Range: 10 1000
+    // @Range: 2 1000
     // @User: Standard
     AP_GROUPINFO("_FILE_MB_FREE",  7, AP_Logger, _params.min_MB_free, HAL_LOGGER_MIN_MB_FREE),
 
diff --git a/libraries/AP_Logger/AP_Logger_File.cpp b/libraries/AP_Logger/AP_Logger_File.cpp
index aab5a15585295..30a53b7a8fad6 100644
--- a/libraries/AP_Logger/AP_Logger_File.cpp
+++ b/libraries/AP_Logger/AP_Logger_File.cpp
@@ -1011,14 +1011,22 @@ void AP_Logger_File::io_timer(void)
         _writebuf.advance(nwritten);
 
         /*
-          fsync on littlefs is extremely expensive (20% CPU on an H7) and not
-          required since the whole point of the filesystem is to avoid corruption
+          fsync on littlefs is extremely expensive (20% CPU on an H7) particularly because the
+          COW architecture can mean you end up copying a load of blocks. instead try and only sync
+          at the end of a block to avoid copying and minimise CPU. fsync is needed for the file
+          metadata (including size) to be updated
          */
 #if CONFIG_HAL_BOARD != HAL_BOARD_SITL && CONFIG_HAL_BOARD_SUBTYPE != HAL_BOARD_SUBTYPE_LINUX_NONE
 #if AP_FILESYSTEM_LITTLEFS_ENABLED
         if (sync_block)
 #endif // AP_FILESYSTEM_LITTLEFS_ENABLED
         {
+            /*
+              the best strategy for minimizing corruption on microSD cards
+              seems to be to write in 4k chunks and fsync the file on each
+              chunk, ensuring the directory entry is updated after each
+              write.
+            */
             last_io_operation = "fsync";
             AP::FS().fsync(_write_fd);
             last_io_operation = "";
diff --git a/libraries/AP_Logger/AP_Logger_File.h b/libraries/AP_Logger/AP_Logger_File.h
index c465b96b3457a..77732b1860008 100644
--- a/libraries/AP_Logger/AP_Logger_File.h
+++ b/libraries/AP_Logger/AP_Logger_File.h
@@ -130,7 +130,11 @@ class AP_Logger_File : public AP_Logger_Backend
     uint32_t _free_space_last_check_time; // milliseconds
     const uint32_t _free_space_check_interval = 1000UL; // milliseconds
 #if AP_FILESYSTEM_LITTLEFS_ENABLED
-    const uint32_t _free_space_min_avail = 4096; // bytes
+#if AP_FILESYSTEM_LITTLEFS_FLASH_TYPE == AP_FILESYSTEM_FLASH_W25NXX
+    const uint32_t _free_space_min_avail = 1024 * 1024; // bytes
+#else
+    const uint32_t _free_space_min_avail = 1024 * 256; // bytes
+#endif
 #else
     const uint32_t _free_space_min_avail = 8388608; // bytes
 #endif