PR2040: libblock: Flash disk documentation

1 files changed, 119 insertions, 28 deletions

diff --git a/cpukit/libblock/include/rtems/flashdisk.h b/cpukit/libblock/include/rtems/flashdisk.h
index 31b65007de..28e21bbb5e 100644
--- a/cpukit/libblock/include/rtems/flashdisk.h
+++ b/cpukit/libblock/include/rtems/flashdisk.h
@@ -1,5 +1,7 @@
 /**
- * @file rtems/flashdisk.h
+ * @file
+ *
+ * @ingroup RTEMSFDisk
  *
  * This file defines the interface to a flash disk block device.
  */
@@ -23,13 +25,84 @@
 #include <rtems.h>
 
 /**
- * The base name of the flash disks.
- */
-#define RTEMS_FLASHDISK_DEVICE_BASE_NAME "/dev/fdd"
-
-/**
- * Flash disk specific ioctl request types. To use open the
- * device and issue the ioctl call.
+ * @defgroup RTEMSFDisk Flash Disk Device
+ *
+ * @ingroup rtems_blkdev
+ *
+ * Flash disk driver for RTEMS provides support for block based
+ * file systems on flash devices. The driver is not a flash file
+ * system nor does it try to compete with flash file systems. It
+ * currently does not journal how-ever block sequence numbering
+ * could be added to allow recovery of a past positions if
+ * a power down occurred while being updated.
+ *
+ * This flash driver provides block device support for most flash
+ * devices. The driver has been tested on NOR type devices such
+ * as the AMLV160 or M28W160. Support for NAND type devices may
+ * require driver changes to allow speedy recover of the block
+ * mapping data and to also handle the current use of word programming.
+ * Currently the page descriptors are stored in the first few pages
+ * of each segment.
+ *
+ * The driver supports devices, segments and pages. You provide
+ * to the driver the device descriptions as a table of device
+ * descriptors. Each device descriptor contain a table of
+ * segment descriptions or segment descriptors. The driver uses
+ * this information to manage the devices.
+ *
+ * A device is made up of segments. These are also called
+ * sectors or blocks. It is the smallest erasable part of a device.
+ * A device can have differing size segments at different
+ * offsets in the device. The segment descriptors support repeating
+ * segments that are continuous in the device. The driver breaks the
+ * segments up into pages. The first pages of a segment contain
+ * the page descriptors. A page descriptor hold the page flags,
+ * a CRC for the page of data and the block number the page
+ * holds. The block can appear in any order in the devices. A
+ * page is active if it hold a current block of data. If the
+ * used bit is set the page is counted as used. A page moves
+ * from erased to active to used then back to erased. If a block
+ * is written that is already in a page, the block is written to
+ * a new page the old page is flagged as used.
+ *
+ * At initialization time each segment's page descriptors are
+ * read into memory and scanned to determine the active pages,
+ * the used pages and the bad pages. If a segment has any erased
+ * pages it is queue on the available queue. If the segment has
+ * no erased pages it is queue on the used queue.
+ *
+ * The available queue is sorted from the least number available
+ * to the most number of available pages. A segment that has just
+ * been erased will placed at the end of the queue. A segment that
+ * has only a few available pages will be used sooner and once
+ * there are no available pages it is queued on the used queue.
+ * The used queue hold segments that have no available pages and
+ * is sorted from the least number of active pages to the most
+ * number of active pages.
+ *
+ * The driver is required to compact segments. Compacting takes
+ * the segment with the most number of available pages from the
+ * available queue then takes segments with the least number of
+ * active pages from the used queue until it has enough pages
+ * to fill the empty segment. As the active pages are moved
+ * they flagged as used and once the segment has only used pages
+ * it is erased.
+ *
+ * A flash block driver like this never knows if a page is not
+ * being used by the file-system. A typical file system is not
+ * design with the idea of erasing a block on a disk once it is
+ * not being used. The file-system will normally use a flag
+ * or a location as a marker to say that part of the disk is
+ * no longer in use. This means a number of blocks could be
+ * held in active pages but are no in use by the file system.
+ * The file system may also read blocks that have never been
+ * written to disk. This complicates the driver and may make
+ * the wear, usage and erase patterns harsher than a flash
+ * file system. The driver may also suffer from problems if
+ * power is lost.
+ *
+ * There are some flash disk specific IO control request types.
+ * To use open the device and issue the ioctl() call.
  *
  * @code
  *  int fd = open ("/dev/flashdisk0", O_WRONLY, 0);
@@ -45,7 +118,15 @@
  *  }
  *  close (fd);
  * @endcode
+ *
+ * @{
+ */
+
+/**
+ * @brief The base name of the flash disks.
  */
+#define RTEMS_FLASHDISK_DEVICE_BASE_NAME "/dev/fdd"
+
 #define RTEMS_FDISK_IOCTL_ERASE_DISK   _IO('B', 128)
 #define RTEMS_FDISK_IOCTL_COMPACT      _IO('B', 129)
 #define RTEMS_FDISK_IOCTL_ERASE_USED   _IO('B', 130)
@@ -54,7 +135,7 @@
 #define RTEMS_FDISK_IOCTL_PRINT_STATUS _IO('B', 133)
 
 /**
- * Flash Disk Monitoring Data allows a user to obtain
+ * @brief Flash Disk Monitoring Data allows a user to obtain
  * the current status of the disk.
  */
 typedef struct rtems_fdisk_monitor_data
@@ -78,10 +159,9 @@ typedef struct rtems_fdisk_monitor_data
 } rtems_fdisk_monitor_data;
 
 /**
- * Flash Segment Descriptor holds, number of continuous segments in the
- * device of this type, the base segment number in the device, the
- * address offset of the base segment in the device, and the size of
- * segment.
+ * @brief Flash Segment Descriptor holds, number of continuous segments in the
+ * device of this type, the base segment number in the device, the address
+ * offset of the base segment in the device, and the size of segment.
  *
  * Typically this structure is part of a table of segments in the
  * device which is referenced in the flash disk configuration table.
@@ -97,7 +177,7 @@ typedef struct rtems_fdisk_segment_desc
 } rtems_fdisk_segment_desc;
 
 /**
- * Return the number of kilo-bytes.
+ * @brief Return the number of kilo-bytes.
  */
 #define RTEMS_FDISK_KBYTES(_k) (UINT32_C(1024) * (_k))
 
@@ -107,8 +187,8 @@ typedef struct rtems_fdisk_segment_desc
 struct rtems_fdisk_device_desc;
 
 /**
- * Flash Low Level driver handlers.
-
+ * @brief Flash Low Level driver handlers.
+ *
  * Typically this structure is part of a table of handlers in the
  * device which is referenced in the flash disk configuration table.
  * The reference is kept in the driver and used all the time to
@@ -249,10 +329,10 @@ typedef struct rtems_fdisk_driver_handlers
 } rtems_fdisk_driver_handlers;
 
 /**
- * Flash Device Descriptor holds the segments in a device. The
- * placing of the segments in a device decriptor allows the
- * low level driver to share the segment descriptors for a
- * number of devices.
+ * @brief Flash Device Descriptor holds the segments in a device.
+ *
+ * The placing of the segments in a device decriptor allows the low level
+ * driver to share the segment descriptors for a number of devices.
  *
  * Typically this structure is part of a table of segments in the
  * device which is referenced in the flash disk configuration table.
@@ -267,7 +347,7 @@ typedef struct rtems_fdisk_device_desc
 } rtems_fdisk_device_desc;
 
 /**
- * RTEMS Flash Disk configuration table used to initialise the
+ * @brief RTEMS Flash Disk configuration table used to initialise the
  * driver.
  *
  * The unavailable blocks count is the number of blocks less than the
@@ -302,13 +382,22 @@ typedef struct rtems_flashdisk_config
   const rtems_fdisk_device_desc* devices;        /**< The device descriptions. */
   uint32_t                       flags;          /**< Set of flags to control
                                                       driver. */
-  uint32_t                       unavail_blocks; /**< Number of blocks not
-                                                      available to the file sys. */
+  /**
+   * Number of blocks not available to the file system.  This number must be
+   * greater than or equal to the number of blocks in the largest segment to
+   * avoid starvation of erased blocks.
+   */
+  uint32_t                       unavail_blocks;
+
   uint32_t                       compact_segs;   /**< Max number of segs to
                                                       compact in one pass. */
-  uint32_t                       avail_compact_segs; /**< The number of segments
-                                                          when compaction occurs
-                                                          when writing. */
+  /**
+   * The number of segments when compaction occurs when writing.  In case the
+   * number of segments in the available queue is less than or equal to this
+   * number the compaction process will be triggered.  The available queue
+   * contains all segments with erased blocks.
+   */
+  uint32_t                       avail_compact_segs;
   uint32_t                       info_level;     /**< Default info level. */
 } rtems_flashdisk_config;
 
@@ -356,15 +445,17 @@ rtems_fdisk_initialize (rtems_device_major_number major,
                         void*                     arg);
 
 /**
- * External reference to the configuration. Please supply.
+ * @brief External reference to the configuration. Please supply.
  * Support is present in confdefs.h for providing this variable.
  */
 extern const rtems_flashdisk_config rtems_flashdisk_configuration[];
 
 /**
- * External reference to the number of configurations. Please supply.
+ * @brief External reference to the number of configurations. Please supply.
  * Support is present in confdefs.h for providing this variable.
  */
 extern uint32_t rtems_flashdisk_configuration_size;
 
+/** @} */
+
 #endif