The structure that we are passed should look like:

   struct sdata {
    unsigned int inlen;     [i] Length of data written to device
    unsigned int outlen;    [i] Length of data read from device
    unsigned char cmd[x];   [i] SCSI command (6 <= x <= 16)
                            [o] Data read from device starts here
                            [o] On error, sense buffer starts here
    unsigned char wdata[y]; [i] Data written to device starts here

  • The SCSI command length is determined by examining the 1st byte of the given command [1] . There is no way to override this.

  • Data transfers are limited to PAGE_SIZE (4K on i386, 8K on alpha).

  • The length (x + y) must be at least OMAX_SB_LEN bytes long to accommodate the sense buffer when an error occurs. The sense buffer is truncated to OMAX_SB_LEN (16) bytes so that old code will not be surprised.

  • If a Unix error occurs (e.g. ENOMEM) then the user will receive a negative return and the Unix error code in 'errno'. If the SCSI command succeeds then 0 is returned. Positive numbers returned are the compacted SCSI error codes (4 bytes in one int) where the lowest byte is the SCSI status. See the drivers/scsi/scsi.h file for more information on this.



Here is the mapping from the SCSI opcode "group" (top 3 bits of opcode) to the assumed length (in lk 2.4.15):

unsigned char scsi_command_size[8] =
        6, 10, 10, 12,
        16, 12, 10, 10
The assumed length of group 4 commands changed from 12 to 16 in lk 2.4.15 reflecting support for 16 byte SCSI commands being added to the kernel.

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