linux-kernel - Re: Why USB Link Power Management only for devices connected to root hub

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Message-ID: <20120524044537.GB14325@kroah.com>
Date:	Wed, 23 May 2012 21:45:37 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	prasannatsmkumar <prasannatsmkumar@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Why USB Link Power Management only for devices connected to root
 hub

On Tue, May 22, 2012 at 11:42:28AM +0530, prasannatsmkumar wrote:
> I have started looking into USB stuff. In the EHCI driver code I could
> see the comment that "we only support lpm device connected to root hub
> yet" in drivers/usb/host/ehci-pci.c line 466 (3.4 kernel -
> http://lxr.free-electrons.com/source/drivers/usb/host/ehci-pci.c#L466).
> Is there any reason to not support lpm in devices connected to hub
> (other than root hub)? Also inside ehci_lpm_check function in
> drivers/usb/host/ehci-lpm.c (3.4 kernel -
> http://lxr.free-electrons.com/source/drivers/usb/host/ehci-lpm.c) the
> USB device is asked to go to LPM mode and there is a wait for 10ms. I
> guess the LPM transition (L0 to L1 state transition) should take place
> within some micro seconds then why should we wait for 10ms for ACK?

Please as USB-specific questions like this on the linux-usb mailing list
if you wish to get answers.

greg k-h
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