[systemd-devel] [usb-storage] Re: Amazon Kindle disconnect after Synchronize Cache

[Matthias Schwarzott]

Am 16.03.21 um 17:26 schrieb Alan Stern:
> On Tue, Mar 16, 2021 at 06:26:30AM +0100, Matthias Schwarzott wrote:
>> I implemented solution 3b. This is the pullrequest for udev (systemd
>> repository):
>>
>> 	https://github.com/systemd/systemd/pull/19002
>>
>> Now Lennart asks if udev is the best place for such hacks/work-arounds?
>>
>> Well, I implemented it as suggested by Alan (see above). This was the
>> simplest of the considered alternatives. Different quirks in kernel has been
>> considered, but are more effort to be implemented.
> 
> Lennart probably isn't aware how the usb-storage driver works.  It does
> not create commands on its own; it merely sends the commands that it
> gets from higher SCSI layers.
> 
> It may be possible to modify the SCSI core, to make it send a TEST UNIT
> READY command immediately following any SYNCHRONIZE CACHE to a Kindle.
> 
> However, there may be an easier solution.  usb-storage does indeed send
> a command of its own, REQUEST SENSE, to get error data when a command
> fails.  The patch below will make it do the same thing whenever it sends
> a SYNCHRONIZE CACHE to a Kindle, failure or not.
> 
> The only question is whether the Kindle will regard REQUEST SENSE as a
> sufficient indication that it shouldn't do an eject.  The only way to
> find out is by testing the patch.
> 

The patch is a lot shorter than I expected it to be.

I tried it. The new udev-rule is commented, so polling interval is back 
at 2000 ms.

Testing it:
# cat /sys/block/sde/events_poll_msecs
-1
# cat /sys/module/block/parameters/events_dfl_poll_msecs
2000
# python -c "import time; import os; f = open('/dev/sde1', 'r+b'); 
os.fsync(f); time.sleep(4)"

This is wireshark output of the test:

85	4.701949	host	3.8.1	USBMS	95	SCSI: Test Unit Ready LUN: 0x00
86	4.701972	3.8.1	host	USB	64	URB_BULK out
87	4.701975	host	3.8.1	USB	64	URB_BULK in
88	4.702030	3.8.1	host	USBMS	77	SCSI: Response LUN: 0x00 (Test Unit 
Ready) (Good)
89	4.702043	host	3.8.1	USBMS	95	SCSI: Prevent/Allow Medium Removal LUN: 
0x00  PREVENT
90	4.702069	3.8.1	host	USB	64	URB_BULK out
91	4.702072	host	3.8.1	USB	64	URB_BULK in
92	4.703006	3.8.1	host	USBMS	77	SCSI: Response LUN: 0x00 (Prevent/Allow 
Medium Removal) (Good)

93	4.703052	host	3.8.1	USBMS	95	SCSI: Synchronize Cache(10) LUN: 0x00 
(LBA: 0x00000000, Len: 0)
94	4.703066	3.8.1	host	USB	64	URB_BULK out
95	4.703067	host	3.8.1	USB	64	URB_BULK in
96	4.704146	3.8.1	host	USBMS	77	SCSI: Response LUN: 0x00 (Synchronize 
Cache(10)) (Good)
97	4.704149	host	3.8.1	USBMS	95	SCSI: Request Sense LUN: 0x00
98	4.704177	3.8.1	host	USB	64	URB_BULK out
99	4.704179	host	3.8.1	USB	64	URB_BULK in
100	4.705032	3.8.1	host	USBMS	82	SCSI: Data In LUN: 0x00 (Request Sense 
Response Data)
101	4.705035	host	3.8.1	USB	64	URB_BULK in
102	4.705053	3.8.1	host	USBMS	77	SCSI: Response LUN: 0x00 (Request 
Sense) (Good)
105	6.740272	host	3.8.1	USBMS	95	SCSI: Test Unit Ready LUN: 0x00
106	6.740323	3.8.1	host	USB	64	URB_BULK out
107	6.740326	host	3.8.1	USB	64	URB_BULK in
108	6.740410	3.8.1	host	USBMS	77	SCSI: Response LUN: 0x00 (Test Unit 
Ready) (Good)
195	8.709417	host	3.8.1	USBMS	95	SCSI: Prevent/Allow Medium Removal LUN: 
0x00  ALLOW
196	8.709441	3.8.1	host	USB	64	URB_BULK out
197	8.709445	host	3.8.1	USB	64	URB_BULK in
198	8.709645	3.8.1	host	USBMS	77	SCSI: Response LUN: 0x00 (Prevent/Allow 
Medium Removal) (Good)

The patch indeed works. The kindle does not disconnect.
I also tried with a sync on a mounted filesystem. The effect is the 
same: No disconnect.

Calling "eject /dev/sde" still works as it should.

Tested-by: Matthias Schwarzott <zzam at gentoo.org>

Regards
Matthias