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

[Hans de Goede]

Hi,

On 3/16/21 6:04 PM, Alan Stern wrote:
> 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.

Ah, I did not know that the default polling interval was that low(ish),
given that the default indeed is already that low, then changing it to
0.8 seconds would not be a big change.  And we probably have a lot of
lower hanging fruit for unnecessary wakeups then that.

Regards,

Hans