scsi: core: Clean up SG_NONE

Remove SG_NONE and a related misleading comment. Update documentation.

This patch does not affect behaviour as zero initialization is redundant.

Cc: Jonathan Corbet <[e-mail]>
Cc: Bartlomiej Zolnierkiewicz <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Viresh Kumar <[e-mail]>
Cc: Oliver Neukum <[e-mail]>
Cc: Alan Stern <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Finn Thain <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

Documentation/scsi/scsi_mid_low_api.txt

···
                    commands to the adapter.
     this_id      - scsi id of host (scsi initiator) or -1 if not known
     sg_tablesize - maximum scatter gather elements allowed by host.
-                   0 implies scatter gather not supported by host
+                   Set this to SG_ALL or less to avoid chained SG lists.
+                   Must be at least 1.
     max_sectors  - maximum number of sectors (usually 512 bytes) allowed
                    in a single SCSI command. The default value of 0 leads
                    to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in

drivers/ata/pata_arasan_cf.c

···
 
 static struct scsi_host_template arasan_cf_sht = {
 	ATA_BASE_SHT(DRIVER_NAME),
-	.sg_tablesize = SG_NONE,
 	.dma_boundary = 0xFFFFFFFFUL,
 };
 

drivers/scsi/atp870u.c

···
      .bios_param        	= atp870u_biosparam	/* biosparm */,
      .can_queue         	= qcnt			/* can_queue */,
      .this_id           	= 7			/* SCSI ID */,
-     .sg_tablesize      	= ATP870U_SCATTER	/*SG_ALL*/ /*SG_NONE*/,
+     .sg_tablesize      	= ATP870U_SCATTER	/*SG_ALL*/,
      .max_sectors		= ATP870U_MAX_SECTORS,
 };
 

drivers/usb/storage/uas.c

···
 	.eh_abort_handler = uas_eh_abort_handler,
 	.eh_device_reset_handler = uas_eh_device_reset_handler,
 	.this_id = -1,
-	.sg_tablesize = SG_NONE,
 	.skip_settle_delay = 1,
 	.dma_boundary = PAGE_SIZE - 1,
 };

include/scsi/scsi_host.h

···
 struct scsi_transport_template;
 
 
-/*
- * The various choices mean:
- * NONE: Self evident.	Host adapter is not capable of scatter-gather.
- * ALL:	 Means that the host adapter module can do scatter-gather,
- *	 and that there is no limit to the size of the table to which
- *	 we scatter/gather data.  The value we set here is the maximum
- *	 single element sglist.  To use chained sglists, the adapter
- *	 has to set a value beyond ALL (and correctly use the chain
- *	 handling API.
- * Anything else:  Indicates the maximum number of chains that can be
- *	 used in one scatter-gather request.
- */
-#define SG_NONE 0
 #define SG_ALL	SG_CHUNK_SIZE
 
 #define MODE_UNKNOWN 0x00