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

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Message-ID: <CANc+2y50jirP=_V0qBkDuVwkhcmJWo3tQ-rwnxOtw3GQ5gP3tQ@mail.gmail.com>
Date:	Tue, 22 May 2012 11:42:28 +0530
From:	prasannatsmkumar <prasannatsmkumar@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Why USB Link Power Management only for devices connected to root hub

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?

Thanks,
Prasanna Kumar T S M
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