linux-kernel - Re: [PATCH] usb: host: tegra: Reset Tegra USB controller before init

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Date:	Thu, 28 Feb 2013 13:52:52 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Stephen Warren <swarren@...dotorg.org>
cc:	Venu Byravarasu <vbyravarasu@...dia.com>,
	<gregkh@...uxfoundation.org>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: host: tegra: Reset Tegra USB controller before init

On Thu, 28 Feb 2013, Stephen Warren wrote:

> I think this patch might cause unintended consequences.
> 
> When the Tegra PHY code is converted to a driver (i.e. has its own
> probe), the initial order of execution of the PHY and EHCI driver probes
> will not be guaranteed.
> 
> In particular, since the EHCI probe will attempt to "find" the PHY
> device, and defer the EHCI probe until it can do so, this guarantees
> that the PHY's probe() will have completed before EHCI's probe()
> completes (although EHCI's probe may start running first some number of
> times, and be retried with -EPROBE_DEFERRED for a variety of reasons).
> 
> Now, if the PHY driver's probe() actually touches HW and sets up some
> registers, isn't this reset call going to trash any of that register
> setup? Or, will PHY probe() not touch registers, but only do so during
> the standard PHY open/init "op"/API calls?
> 
> I think the way to solve this is to put the reset call into the PHY
> driver. I assume it has access to the appropriate clock object. This may
> also address Alan's query re: when the unexpected interrupt occurs; it's
> triggered by (or correlated with at least) the PHY (or USB port in
> general) being in device mode due to the boot ROM setting it up this
> way, then switching to host mode via the Linux driver. I /think/ that
> device/host mode switching is more related to the PHY than EHCI driver,
> although I could well be wrong here.

With the PCI platform driver, the handoff from the firmware (we can
categorize U-Boot as firmware for this discussion) is handled as soon
as the controller is discovered by the platform-specific code.  
There's a special pci-quirks.c file to take care of it.  It is not
handled by the driver or the glue layer.

In general I think that's what needs to be done.  Errant interrupt 
sources should be disabled as quickly as possible.

In this case I don't know exactly when the earliest opportunity is.  I
assume that the EHCI driver and/or the PHY driver gets probed because
some platform-layer code has registered the device.  If this is so then
that platform-layer code is the right place to do the reset.

Alan Stern

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