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

[Alan Stern <stern@rowland.harvard.edu>]

On Tue, Mar 16, 2021 at 05:43:34PM +0100, Hans de Goede wrote:
> Hi,
> 
> On 3/16/21 5:26 PM, Alan Stern wrote:
> > 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.
> > 
> > Alan Stern
> 
> Thank you for this patch, yes if this works it would IMHO be
> a much better solution then the udev rule.

I think it would be mildly better, but not a whole lot.  Since the 
Kindle describes itself as having removable media, the kernel normally 
probes it periodically to make sure the media remains present.  The 
default probing interval is 2 seconds.  Reducing it to 0.9 seconds 
doesn't represent an exorbitant additional load IMO -- especially since 
Kindles don't tend to spend huge amounts of time connected to computers.

If it's merely a question of where to change the polling interval from 
the default (automatically in the kernel or by userspace), that also 
doesn't seem like a terribly important choice.  Certainly a udev rule or 
hwdb entry is a lot easier to maintain than special-case code in the 
kernel.

> One question though, if this works to fix the undesired ejects,
> will an actual eject (using e.g. the eject utility as say
> "sudo eject /dev/sda") still be seen as an eject by the kindle
> after this ?

It should be.  Maybe Matthias will try it and let us know.

> Because that is actually kind of important for everyone using their
> Kindle with Calibre, breaking that would not be good.

Of course.

Alan Stern