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 byte array to store the read data.
sector: Start sector number in 32-bit LBA.
count: Number of sectors to read in range of from 1 to 128..

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. If variable sector size is selected (_MIN_SS < MAX_SS), FatFs inquires the sector size with disk_ioctl() after initialization.

The memory address specified by buff is not that always aligned to word boundary because the argument is defined as BYTE*. The misaligned read/write request can occure at direct transfer. If the bus architecture, especially DMA controller, does not allow misaligned 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.
For f_read(), avoid long read request that includes a whole of sector. - Direct transfer will never occure.
For f_read(fp, buff, btr, &br), make sure that (((UINT)buff & 3) == (f_tell(fp) & 3)) is true. - Word aligned direct transfer 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.