FatFs - disk

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disk_read

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The disk_read function is called to read data from the sector(s) of storage device.

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+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 */
+);
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Parameters

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

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Return Value

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RES_OK (0): The function succeeded.
RES_ERROR: An unrecoverable hard error occured during the read operation.
RES_PARERR: Invalid parameter.
RES_NOTRDY: The device has not been initialized.

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Description

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Read/write operation to the generic storage devices, such as memory card, hadddisk and optical disk, is done in unit of block of data bytes called sector. FatFs supports the sector size in range of 512 to 4096 bytes. When FatFs is configured for fixed sector size (FF_MIN_SS == FF_MAX_SS, this is the most case), the read/write function must work at that sector size. When FatFs is configured for variable sector size (FF_MIN_SS < FF_MAX_SS), the sector size of medium is inquired with disk_ioctl function immediately following disk_initialize function succeeded.

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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.

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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(fp, dat, btw, bw) calls, make sure that (((UINT)dat & 3) == (f_tell(fp) & 3)) is true. - Word alignment of buff is guaranteed no matter dat is not word aligned.

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Generally, a multiple sector read request must not be split into single sector transactions to the storage device, or read throughput gets worse.

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