[PATCH 3.16 021/202] scsi: sd: Fix cache_type_store()

[Ben Hutchings]

3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ivan Mironov <mironov.ivan@xxxxxxxxx>

commit 44759979a49bfd2d20d789add7fa81a21eb1a4ab upstream.

Changing of caching mode via /sys/devices/.../scsi_disk/.../cache_type may
fail if device responds to MODE SENSE command with DPOFUA flag set, and
then checks this flag to be not set on MODE SELECT command.

In this scenario, when trying to change cache_type, write always fails:

	# echo "none" >cache_type
	bash: echo: write error: Invalid argument

And following appears in dmesg:

	[13007.865745] sd 1:0:1:0: [sda] Sense Key : Illegal Request [current]
	[13007.865753] sd 1:0:1:0: [sda] Add. Sense: Invalid field in parameter list

=46romSBC-4 r15, 6.5.1 "Mode pages overview", description of DEVICE-SPECIFIC
PARAMETER field in the mode parameter header:
	...
	The write protect (WP) bit for mode data sent with a MODE SELECT
	command shall be ignored by the device server.
	...
	The DPOFUA bit is reserved for mode data sent with a MODE SELECT
	command.
	...

The remaining bits in the DEVICE-SPECIFIC PARAMETER byte are also reserved
and shall be set to zero.

[mkp: shuffled commentary to commit description]

Signed-off-by: Ivan Mironov <mironov.ivan@xxxxxxxxx>
Signed-off-by: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/scsi/sd.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -194,6 +194,12 @@ cache_type_store(struct device *dev, str
 	sp = buffer_data[0] & 0x80 ? 1 : 0;
 	buffer_data[0] &= ~0x80;
 
+	/*
+	 * Ensure WP, DPOFUA, and RESERVED fields are cleared in
+	 * received mode parameter buffer before doing MODE SELECT.
+	 */
+	data.device_specific = 0;
+
 	if (scsi_mode_select(sdp, 1, sp, 8, buffer_data, len, SD_TIMEOUT,
 			     SD_MAX_RETRIES, &data, &sshdr)) {
 		if (scsi_sense_valid(&sshdr))