linux-kernel - Re: [PATCH RESEND V4 5/9] of: Add NVIDIA Tegra xHCI controller binding

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Date:	Fri, 31 Oct 2014 12:32:51 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Andrew Bresticker <abrestic@...omium.org>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Jassi Brar <jassisinghbrar@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mathias Nyman <mathias.nyman@...el.com>,
	Grant Likely <grant.likely@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnd Bergmann <arnd@...db.de>,
	Kishon Vijay Abraham I <kishon@...com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH RESEND V4 5/9] of: Add NVIDIA Tegra xHCI controller
 binding

On Thu, Oct 30, 2014 at 10:26:47AM -0700, Andrew Bresticker wrote:
> On Thu, Oct 30, 2014 at 10:24 AM, Thierry Reding
> <thierry.reding@...il.com> wrote:
> > On Thu, Oct 30, 2014 at 10:19:21AM -0700, Andrew Bresticker wrote:
> >> On Thu, Oct 30, 2014 at 6:55 AM, Thierry Reding
> >> <thierry.reding@...il.com> wrote:
> >> > On Wed, Oct 29, 2014 at 09:37:14AM -0700, Andrew Bresticker wrote:
> >> >> On Wed, Oct 29, 2014 at 2:43 AM, Thierry Reding
> >> >> <thierry.reding@...il.com> wrote:
> >> >> > On Tue, Oct 28, 2014 at 03:27:50PM -0700, Andrew Bresticker wrote:
> >> >> > [...]
> >> >> >> diff --git a/Documentation/devicetree/bindings/pinctrl/nvidia,tegra124-xusb-padctl.txt b/Documentation/devicetree/bindings/pinctrl/nvidia,tegra124-xusb-padctl.txt
> >> >> > [...]
> >> >> >> +Optional properties:
> >> >> >> +-------------------
> >> >> >> +- vbus-{0,1,2}-supply: VBUS regulator for the corresponding UTMI pad.
> >> >> >> +- vddio-hsic-supply: VDDIO regulator for the HSIC pads.
> >> >> >> +- nvidia,usb3-port-{0,1}-lane: PCIe/SATA lane to which the corresponding USB3
> >> >> >> +  port is mapped.  See <dt-bindings/pinctrl/pinctrl-tegra-xusb.h> for the list
> >> >> >> +  of valid values.
> >> >> >
> >> >> > I dislike how we now need to provide a list of all pins in the header
> >> >> > file, where previously we used strings for this. This could become very
> >> >> > ugly if the set of pins changes in future generations of this IP block.
> >> >> >
> >> >> > Could we instead derive this from the pinmux nodes? For example you have
> >> >> > this in the example below:
> >> >> >
> >> >> >         usb3p0 {
> >> >> >                 nvidia,lanes = "pcie-0";
> >> >> >                 ...
> >> >> >         };
> >> >> >
> >> >> > Perhaps what we need is to either key off the node name or add another
> >> >> > property, such as:
> >> >> >
> >> >> >         nvidia,usb3-port = <0>;
> >> >> >
> >> >> > This would match the nvidia,usb2-port property that you've added below.
> >> >>
> >> >> That is actually how I described the USB3 port to SS lane mapping
> >> >> originally, but in review of an earlier version of this series,
> >> >> Stephen suggested that I make it a separate, not pinconfig property
> >> >> since it wasn't a value written directly to the hardware.  I'm fine
> >> >> with changing it back as the pinconfig property makes more sense to me
> >> >> as well.
> >> >
> >> > Hmm... I had considered it a mux option of the specific lane. If the
> >> > function is usb3, it'd still need to be muxed to one of the ports. So
> >> > it's additional information associated with the usb3 function.
> >> >
> >> > I did look through the driver changes and can't really make out which
> >> > part of the code actually performs this assignment. Can you point me to
> >> > it?
> >>
> >> There's not really an assignment.  The property is used to map between
> >> a lane (e.g. PCIe-0 or SATA) and the USB3.0 port it's mapped to.  For
> >> an example of where it's used, take a look at usb3_phy_power_on().
> >> There are certain per-lane registers which need to be programmed in
> >> addition to the per-USB3.0 port pad registers.  This mapping is used
> >> to determine which lane needs to be programmed.
> >
> > Are you saying the mapping of lane to USB port is fixed? That is, PCIe-0
> > lane is always used for USB port X and SATA always for USB port Y?
> 
> No, sorry if that was unclear, it's not fixed - it's a board specific
> property.

Okay, but there's no register that contains the mapping of the port to a
lane, similar to what's done for the functions, right? I mean the driver
only uses the lane to find out which register to write. Doesn't that
imply that two lanes (or more) could be mapped to the same USB 3.0 port?

I'm not sure I'm being clear here, so let me try another way. In order
to establish a mapping between USB port and lane, I would've expected
one of the following to happen:

	- A value derived from the lane number is written to a register
	  belonging to a given port.

	- A value derived from the port number is written to a register
	  belonging to a given lane.

I can't see the code do either of the above, which to me implies that
there's a fixed mapping between lanes and ports. What am I missing?

Thierry

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