[PATCH v4 02/10] scsi: scsi_debug: Make atomic writes and ZBC emulation mutually exclusive

[Niklas Cassel <cassel@kernel.org>]

There are no SMR drives that support atomic writes. Emulating a device
that supports both is emulating something that a user cannot encounter,
rather than the drives that exist, and it means maintaining the zone
handling of a command that no zoned device implements.

Nothing in the standards forbids the combination. For what would be by
far the most common case, a host managed SMR drive whose logical block
size equals its physical block size, there is no problem in the two
features being combined: the drive vendor only has to give atomic
writes an alignment that makes sense with regards to ZBC, for instance
equal to the physical block size. This is therefore a choice not to
complicate scsi_debug, rather than a restriction that the standards
impose.

Refuse the combination at initialisation. If SMR drives that support
atomic writes ever become a thing, this can be changed back.

Suggested-by: Damien Le Moal <dlemoal@kernel.org>
Assisted-by: LLM
Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
Tested by loading the module with each combination:

  zbc=managed atomic_wr=1    refused, -EINVAL
  zbc=aware   atomic_wr=1    refused, -EINVAL
  zbc=managed                loads
  atomic_wr=1                loads
---
 drivers/scsi/scsi_debug.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index c664966e95cb..8e45a57ae406 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -8739,6 +8739,11 @@ static int __init scsi_debug_init(void)
 		return -EINVAL;
 	}
 
+	if (sdeb_zbc_in_use && sdebug_atomic_wr) {
+		pr_err("atomic_wr is not supported by a zoned device\n");
+		return -EINVAL;
+	}
+
 	if (sdebug_dev_size_mb == DEF_DEV_SIZE_PRE_INIT)
 		sdebug_dev_size_mb = DEF_DEV_SIZE_MB;
 	if (sdebug_dev_size_mb < 1)
-- 
2.55.0