FatFs - disk

disk_read

The disk_read function reads sector(s) from the storage device.

DRESULT disk_read (
  BYTE pdrv,     /* [IN] Physical drive number */
  BYTE* buff,    /* [OUT] Pointer to the read data buffer */
  DWORD sector,  /* [IN] Start sector number */
  UINT count     /* [IN] Number of sectros to read */
);

Parameters

pdrv: Physical drive number to identify the target device.
buff: Pointer to the first item of the byte array to store read data.
sector: Start sector number in 32-bit LBA.
count: Number of sectors to read.

Return Value

RES_OK (0): The function succeeded.
RES_ERROR: Any hard error occured during the read operation and could not recover it.
RES_PARERR: Invalid parameter.
RES_NOTRDY: The device has not been initialized.

Description

The data read/write operation to the storage devices is done in unit of sector. FatFs supports the sector size in range of from 512 to 4096 bytes. When FatFs is configured to fixed sector size (_MIN_SS == MAX_SS, this will be the most case), the read/write function must work at that sector size. When FatFs is configured to variable sector size (_MIN_SS != MAX_SS), sector size is inquired with disk_ioctl function following disk_initialize function.

The memory address specified by buff is not that always aligned to word boundary because the argument is defined as BYTE*. The unaligned read/write request can occure at direct transfer. If the bus architecture, especially DMA controller, does not allow unaligned memory access, it should be solved in this function. There are some workarounds described below to avoid this issue.

Convert word transfer to byte transfer in this function if needed. - Recommended.
On the f_read calls, avoid long read request that includes a whole of sector. - Any direct transfer never occures.
On the f_read calls, make sure that (((UINT)data & 3) == (f_tell(fp) & 3)) is true. - Word alignment of buff is guaranteed.

Generally, a multiple sector transfer request must not be split into single sector transactions to the storage device, or you will not get good read throughput.