iommu/amd: Fix the left value check of cmd buffer

commit 432abf68 upstream.

The generic command buffer entry is 128 bits (16 bytes), so the offset
of tail and head pointer should be 16 bytes aligned and increased with
0x10 per command.

When cmd buf is full, head = (tail + 0x10) % CMD_BUFFER_SIZE.

So when left space of cmd buf should be able to store only two
command, we should be issued one COMPLETE_WAIT additionally to wait
all older commands completed. Then the left space should be increased
after IOMMU fetching from cmd buf.

So left check value should be left <= 0x20 (two commands).
Signed-off-by: Huang Rui <ray.huang@amd.com>
Fixes: ac0ea6e9 ('x86/amd-iommu: Improve handling of full command buffer')
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/iommu/amd_iommu.c

@@ -1044,7 +1044,7 @@ static int iommu_queue_command_sync(struct amd_iommu *iommu,
 	next_tail = (tail + sizeof(*cmd)) % iommu->cmd_buf_size;
 	left      = (head - next_tail) % iommu->cmd_buf_size;
 
-	if (left <= 2) {
+	if (left <= 0x20) {
 		struct iommu_cmd sync_cmd;
 		volatile u64 sem = 0;
 		int ret;