On Fri, Mar 05, 2021 at 05:54:43PM +0100, Matthias Schwarzott wrote:

Hi folks,

I have an issue with my Amazon Kindle. Since some time the device
disconnects 2 seconds after a sync command sent via USB.

See also this matching bug report:
https://bugzilla.kernel.org/show_bug.cgi?id=203973

My current workaround is this udev-rule:
	SUBSYSTEM=="block", ACTION=="add", ENV{DEVTYPE}=="disk",
ENV{ID_VENDOR}=="Kindle", RUN+="/bin/bash -c 'echo write\ through >
/sys/block/%k/queue/write_cache'"

But I like to find a proper solution.

I did various recordings of usb-traffic with wireshark on linux and windows.

On windows, the device does not disconnect after the "Synchronize Cache"
command.

One major difference I noticed looking at service answer time statistics:
Windows sends a lot more requests of type "Test Unit Ready".
* Windows: 6385 calls
* linux: 71 calls

It's generally well known that Windows issues lots and lots of redundant 
commands to USB storage drives.

After most of the "Synchronize Cache" commands on windows there was directly
a following "WRITE" command. It seems WRITE commands avoid the disconnect.

But sending a plain "Synchronize Cache" under windows (8.1 and 10) does not
trigger the disconnect.

Windows:
1583	14.891478	host	1.6.1	USBMS	58	SCSI: Synchronize Cache(10) LUN: 0x00
(LBA: 0x00000000, Len: 0)
1584	14.891595	1.6.1	host	USB	27	URB_BULK out
1585	14.891613	host	1.6.1	USB	27	URB_BULK in
1586	14.896866	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Synchronize
Cache(10)) (Good)
1589	15.687209	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1590	15.687353	1.6.1	host	USB	27	URB_BULK out
1591	15.687358	host	1.6.1	USB	27	URB_BULK in
1592	15.687405	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)
1713	16.699689	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1714	16.699820	1.6.1	host	USB	27	URB_BULK out
1715	16.699825	host	1.6.1	USB	27	URB_BULK in
1716	16.699915	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)
1717	17.709334	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1718	17.709547	1.6.1	host	USB	27	URB_BULK out
1719	17.709552	host	1.6.1	USB	27	URB_BULK in
1720	17.709586	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)
1721	18.712864	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1722	18.713081	1.6.1	host	USB	27	URB_BULK out
1723	18.713086	host	1.6.1	USB	27	URB_BULK in
1724	18.713148	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)
1741	19.735245	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1742	19.735410	1.6.1	host	USB	27	URB_BULK out
1743	19.735415	host	1.6.1	USB	27	URB_BULK in
1744	19.735474	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)
1811	20.747477	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1812	20.747699	1.6.1	host	USB	27	URB_BULK out
1813	20.747704	host	1.6.1	USB	27	URB_BULK in
1814	20.747766	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)
1905	21.755419	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1906	21.755579	1.6.1	host	USB	27	URB_BULK out
1907	21.755584	host	1.6.1	USB	27	URB_BULK in
1908	21.755674	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)
1911	22.769205	host	1.6.1	USBMS	58	SCSI: Test Unit Ready LUN: 0x00
1912	22.769355	1.6.1	host	USB	27	URB_BULK out
1913	22.769360	host	1.6.1	USB	27	URB_BULK in
1914	22.769415	1.6.1	host	USBMS	40	SCSI: Response LUN: 0x00 (Test Unit
Ready) (Good)

Unless the Kindle advertises removable media, there doesn't seem to be 
any real point to all those TEST UNIT READY commands.  Unless they are 
what prevents the disconnections...

How can I get further on this topic?

Is runtime power management enabled?  Maybe the Kindle disconnects 
whenever the computer tries to suspend it.  This typically happens 2 
seconds after the last command was issued, which matches your 
observations.  If runtime PM is enabled, you can try disabling it.

Alternatively, a number of Linux kernel developers work for Amazon (or 
at least, use email addresses ending in "@amazon.com"), as shown by the 
MAINTAINERS file.  Maybe one of them can get in touch with the Kindle 
software development people and find the actual answer.

Alan Stern