linux-kernel - Re: [PATCH v2 4/4] usb: Add APIs to access host registers from Tegra PHY

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Date:	Thu, 17 Jan 2013 17:32:29 +0200
From:	Felipe Balbi <balbi@...com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Felipe Balbi <balbi@...com>,
	Venu Byravarasu <vbyravarasu@...dia.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] usb: Add APIs to access host registers from Tegra
 PHY

On Thu, Jan 17, 2013 at 10:21:53AM -0500, Alan Stern wrote:
> On Thu, 17 Jan 2013, Felipe Balbi wrote:
> 
> > > Bits 31 & 30 from PORTSC register were allocated by our SOC designers
> > > to inform the host controller about the PHY type to be used. 
> > 
> > Wow, that's something you should never do. PORTSC register belongs to
> > the EHCI controller and those bits are reserved for future use and they
> > *MUST* return zero. I wouldn't be surprised if current EHCI driver
> > assumes those bits will be zero and/or makes sure they're set to zero
> > when writing to PORTSC register.
> 
> In fact, those bits _have_ been assigned an official purpose in the 
> EHCI-1.1 addendum.  Presumably the Tegra hardware only supports 
> EHCI-1.0.

I see, they're used for device addresses now.

How can we make sure on Tegra systems we won't use those top two bits ?

-- 
balbi

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