+back to the mid level. The SCSI mid level will ensure that no more SCSI
+commands become queued against the LLD than are indicated by
+scsi_adjust_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
+be at least one instance of struct scsi_cmnd available for each SCSI device.
+Members of interest:
+ cmnd - array containing SCSI command
+ cmnd_len - length (in bytes) of SCSI command
+ sc_data_direction - direction of data transfer in data phase. See
+ "enum dma_data_direction" in include/linux/dma-mapping.h
+ request_bufflen - number of data bytes to transfer (0 if no data phase)
+ use_sg - ==0 -> no scatter gather list, hence transfer data
+ to/from request_buffer
+ - >0 -> scatter gather list (actually an array) in
+ request_buffer with use_sg elements
+ request_buffer - either contains data buffer or scatter gather list
+ depending on the setting of use_sg. Scatter gather
+ elements are defined by 'struct scatterlist' found
+ in include/asm/scatterlist.h .
+ done - function pointer that should be invoked by LLD when the
+ SCSI command is completed (successfully or otherwise).
+ Should only be called by an LLD if the LLD has accepted
+ the command (i.e. queuecommand() returned or will return
+ 0). The LLD may invoke 'done' prior to queuecommand()
+ finishing.
+ result - should be set by LLD prior to calling 'done'. A value
+ of 0 implies a successfully completed command (and all
+ data (if any) has been transferred to or from the SCSI
+ target device). 'result' is a 32 bit unsigned integer that
+ can be viewed as 4 related bytes. The SCSI status value is
+ in the LSB. See include/scsi/scsi.h status_byte(),
+ msg_byte(), host_byte() and driver_byte() macros and
+ related constants.
+ sense_buffer - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
+ should be written when the SCSI status (LSB of 'result')
+ is set to CHECK_CONDITION (2). When CHECK_CONDITION is
+ set, if the top nibble of sense_buffer[0] has the value 7
+ then the mid level will assume the sense_buffer array
+ contains a valid SCSI sense buffer; otherwise the mid
+ level will issue a REQUEST_SENSE SCSI command to
+ retrieve the sense buffer. The latter strategy is error
+ prone in the presence of command queuing so the LLD should
+ always "auto-sense".
+ device - pointer to scsi_device object that this command is
+ associated with.
+ resid - an LLD should set this signed integer to the requested
+ transfer length (i.e. 'request_bufflen') less the number
+ of bytes that are actually transferred. 'resid' is
+ preset to 0 so an LLD can ignore it if it cannot detect
+ underruns (overruns should be rare). If possible an LLD
+ should set 'resid' prior to invoking 'done'. The most
+ interesting case is data transfers from a SCSI target
+ device device (i.e. READs) that underrun.
+ underflow - LLD should place (DID_ERROR << 16) in 'result' if
+ actual number of bytes transferred is less than this
+ figure. Not many LLDs implement this check and some that
+ do just output an error message to the log rather than
+ report a DID_ERROR. Better for an LLD to implement
+ 'resid'.
+
+The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h