netdev - Re: [RFC 13/13] USB: Disable hub-initiated LPM for comms devices.

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Message-ID: <4FB6CF71.7080307@imap.cc>
Date:	Sat, 19 May 2012 00:38:41 +0200
From:	Tilman Schmidt <tilman@...p.cc>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, Alan Stern <stern@...land.harvard.edu>,
	Hansjoerg Lipp <hjlipp@....de>,
	linux-bluetooth@...r.kernel.org,
	gigaset307x-common@...ts.sourceforge.net, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org, ath9k-devel@...ts.ath9k.org,
	libertas-dev@...ts.infradead.org, users@...x00.serialmonkey.com
Subject: Re: [RFC 13/13] USB: Disable hub-initiated LPM for comms devices.

Am 17.05.2012 19:31, schrieb Sarah Sharp:
> On Thu, May 17, 2012 at 07:07:32PM +0200, Tilman Schmidt wrote:
>>
>> I follow the argument for class drivers. But this patch also
>> modifies drivers for specific existing USB 2.0 only devices
>> which are unlikely to ever grow USB 3.0 support, such as the
>> Gigaset ISDN driver:
>>
>>>  drivers/isdn/gigaset/bas-gigaset.c            |    1 +
>>>  drivers/isdn/gigaset/usb-gigaset.c            |    1 +
> 
> Is there a particular reason why you think that driver is unlikely to
> ever get USB 3.0 support?

Actually, there is. :-)
- The USB devices driven by this driver aren't built anymore.
- Their USB interface design is quite, um, idiosyncratic, and it's
  pretty unlikely that anyone will reuse it. (At least I truly hope
  no one will.)
- Their successor models have completely different and incompatible
  USB interfaces which this driver is unable to handle.

>> What is the interest of setting the disable_hub_initiated_lpm
>> flag for these?
> 
> It's partially to lay the foundation for anyone who wants to make a USB
> 3.0 communications driver in the future.  They're likely to start from
> some USB 2.0 class driver, and copy a lot of code.  If they notice that
> flag is set in all the USB communications class drivers, they're likely
> to set it as well.

You've got a point there.

> I'm not quite sure where the best place to provide documentation on the
> flag is.  I've added the kernel doc comments to the structure, but maybe
> it needs to be documented somewhere in Documentation/usb/?

Documentation/usb/power-management.txt would seem like a natural
place. Although it appears to limit itself to "suspending" in its
first paragraph, it does have a section "xHCI hardware link PM"
at the end already, added by Andiry Xu on 2011-09-23.

Hmmm, that section seems to suggest that LPM exists for USB2, too.
Perhaps I should reconsider my attitude towards your patch.

Thanks,
Tilman

-- 
Tilman Schmidt                    E-Mail: tilman@...p.cc
Bonn, Germany
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