GIET-VM / FAT32 Handler

The fat32.c​ and fat32.h​ files define the data structures and functions that are used to access a FAT32 file system stored on a mass storage block device (such as BDV, HBA, SDC, or RDK peripherals).

All the block device peripherals support a polling mode on the peripheral status register to detect the completion of a read or write access to the device, but some peripheral (BDV and HBA) support a descheduling mode for the calling task, with an interrupt to signal transfer completion.

The GIET-VM handler supports the 4 peripheral types (only one type in a given platform), and most of the FAT access functions support a specific argument to activate the descheduling/IRQ mode when it is supported by the hardware. As a general rule the GIET-VM boot-loader uses only the polling mode, and the GIET-VM kernel uses the descheduling + IRQ mode to handle the user system calls.

All these functions are prefixed by _ to remind that they can only be executed by a processor in kernel mode.

Kernel-level FAT32 access functions

1) int _fat_init( unsigned int use_irq )

This function initialises the kernel _fat structure describing the FAT32 file system, including the set of open files, and the FAT cache (only one single 512 bytes block at the moment). The use_irq argument define the requested access mode (polling / descheduling), but both the GIET-VM boot-loader and the GIET-VM kernel use the polling mode for FAT initialisation. Return 0 in case of success, exit if error.

2) int _fat_open( unsigned int use_irq , char* pathname , unsigned int create )

This function open a file, register it in the set of open files, and return a file descriptor index (fd_id) that must be used by all other access functions.

• use_irq : Boolean => request to use the descheduling mode when non zero.
• pathname : file absolute path name from the root directory.
• create : Boolean => request to create the file if it does not exist (Not supported yet).

Returns fd_id if success, returns -1 if file not found).

3) int _fat_read( unsigned int use_irq , unsigned int fd_id , void* buffer , unsigned int count , unsigned int offset )

This function transfer an integer number of blocks from an open file (identified by the fd_id) to a memory buffer, after checking the arguments. If the number of requested sectors exceeds the file size, this number of loaded sectors is reduced.

• use_irq : Boolean => request to use the descheduling mode when non zero.
• fd_id : file descriptor index
• buffer : memory buffer virtual base address
• count : number of blocks to transfer
• offset : number of blocks to skip in file

Returns number of sectors transfered if success, returns -1 if error.

4) int _fat_write( unsigned int use_irq , unsigned int fd_id , void* buffer , unsigned int count , unsigned int offset )

This function transfer an integer number of blocks from a memory buffer to an open file (identified by the fd_id), after checking the arguments. It allocates new clusters on the block device if (offset + count) is larger than the current file size.

• use_irq : Boolean => request to use the descheduling mode when non zero.
• fd_id : file descriptor index
• buffer : memory buffer virtual base address
• count : number of blocks to transfer
• offset : number of blocks to skip in file

Returns number of sectors written if success, returns -1 if error.

5) int _fat_fstat( unsigned int fd_id )

Return the file size (bytes) for a file identified by the fd_id.

6) int fat_close( unsigned int fd_id )

This function removes a file identified by the fd_id from the set of open files.

7) int _fat_ioc_access( unsigned int use_irq , unsigned int to_mem , unsigned int lba , unsigned int buf_vaddr , unsigned int count )

All the previous functions uses this function to access the block device. It computes the buffer physical address, and call the proper block device driver, as specified in the mapping.

• use_irq : Boolean => request to use the descheduling mode when non zero.
• to_mem : Boolean => to memory transfer when non zero.
• lba : first block index on the block device.
• buf_vaddr : memory buffer virtual base address
• count : number of blocks to transfer

Returns 0 if success, returns -1 if error.

User level FAT32 access functions

These functions are used by the kernel to handle the user system calls. They should be modified to respect the UNIX specification.

1) int _fat_user_open( char* pathname , unsigned int flags )

2) int _fat_user_read( unsigned int fd , void* buffer , unsigned int count , unsigned int offset )

3) int _fat_user_write( unsigned int fd , void* buffer , unsigned int count , unsigned int offset )

4) int _fat_user_lseek( unsigned int fd , unsigned int offset , unsigned int whence )