linux-kernel - Re: [PATCH] usb: xhci-plat: Enable USB 2.0 hardware LPM support for platform xHCs

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Date:	Wed, 28 Aug 2013 16:08:12 -0700
From:	Julius Werner <jwerner@...omium.org>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
Cc:	Julius Werner <jwerner@...omium.org>,
	Paul Zimmerman <Paul.Zimmerman@...opsys.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Vincent Palatin <vpalatin@...omium.org>,
	Benson Leung <bleung@...omium.org>,
	Felipe Balbi <balbi@...com>,
	"mathias.nyman@...ux.intel.com" <mathias.nyman@...ux.intel.com>
Subject: Re: [PATCH] usb: xhci-plat: Enable USB 2.0 hardware LPM support for
 platform xHCs

> So the 2.41a has BESL support, but may not set the BLC flag.  What
> happens if we use the HIRD encoding instead?  Will things break?  It
> seems like we would need to disable USB 2.0 LPM on that host all
> together, if it expects BESL encoding, but advertises HIRD encoding.

Wait a second, just for clarity: are you saying that BESL-capable
controllers do not support the old HIRD mechanism and thus just break
on non-BESL aware OSes? I would've assumed that they somehow notice if
software doesn't write to the new register and automatically fall back
to HIRD... it seems like a weird decision to break hardware backwards
compatibility like that (after all, it would mean that Linux 3.10 and
older would also break on LynxPoint systems right now).
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