sg: Improve zbc_sg_get_max_cmd_blocks()

Some HBAs report a maximum SG count equal to the the device reported
count + 1 for handling buffer alignment internally. This leads to errors
if the maximum command size is calculated using that incremented value.
So also get the maximum reported command size and preferably use this
value rather than the SG count times system page size as the maximum
command size.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>

lib/zbc_sg.c

@@ -19,6 +19,7 @@
 #include <libgen.h>
 #include <string.h>
 #include <linux/limits.h>
+#include <linux/fs.h>
 #include <sys/types.h>
 #include <sys/ioctl.h>
 #include <sys/stat.h>
@@ -543,7 +544,8 @@ static unsigned long zbc_sg_get_max_bytes(struct zbc_device *dev)
  */
 void zbc_sg_get_max_cmd_blocks(struct zbc_device *dev)
 {
-	unsigned long max_bytes = 0, max_segs = ZBC_SG_MAX_SEGMENTS;
+	unsigned int max_bytes = 0, max_segs = ZBC_SG_MAX_SEGMENTS;
+	size_t pagesize = sysconf_pagesize();
 	struct stat st;
 	int ret;
 
@@ -564,15 +566,26 @@ void zbc_sg_get_max_cmd_blocks(struct zbc_device *dev)
 				  strerror(errno));
 			max_segs = ZBC_SG_MAX_SEGMENTS;
 		}
-		max_bytes = 0;
+
+		ret = ioctl(dev->zbd_sg_fd, BLKSECTGET, &max_bytes);
+		if (ret != 0) {
+			zbc_debug("%s: BLKSECTGET ioctl failed %d (%s)\n",
+				  dev->zbd_filename,
+				  errno,
+				  strerror(errno));
+			max_bytes = 0;
+		}
 	} else if (S_ISBLK(st.st_mode)) {
 		max_segs = zbc_sg_get_max_segments(dev);
 		max_bytes = zbc_sg_get_max_bytes(dev);
+	} else {
+		/* Use default */
+		max_segs = ZBC_SG_MAX_SEGMENTS;
 	}
 
 out:
-	if (!max_bytes || max_segs * sysconf_pagesize() < max_bytes)
-		max_bytes = max_segs * sysconf_pagesize();
+	if (!max_bytes || max_bytes > max_segs * pagesize)
+		max_bytes = max_segs * pagesize;
 	dev->zbd_info.zbd_max_rw_sectors = max_bytes >> 9;
 
 	zbc_debug("%s: Maximum command data transfer size is %llu sectors\n",