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

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Date:	Mon, 4 Mar 2013 13:25:15 +0530
From:	Venu Byravarasu <vbyravarasu@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"stern@...land.harvard.edu" <stern@...land.harvard.edu>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb: host: tegra: Reset Tegra USB controller before init

> -----Original Message-----
> From: Stephen Warren [mailto:swarren@...dotorg.org]
> Sent: Thursday, February 28, 2013 11:47 PM
> To: Venu Byravarasu
> Cc: gregkh@...uxfoundation.org; stern@...land.harvard.edu; linux-
> usb@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] usb: host: tegra: Reset Tegra USB controller before init
> 
> On 02/27/2013 11:36 PM, Venu Byravarasu wrote:
> > To clear any configurations made by U-Boot on Tegra USB controller,
> > reset it before init in probe.
> 
> > diff --git a/drivers/usb/host/ehci-tegra.c b/drivers/usb/host/ehci-tegra.c
> 
> > @@ -691,6 +692,10 @@ static int tegra_ehci_probe(struct platform_device
> *pdev)
> >  	if (err)
> >  		goto fail_clk;
> >
> > +	tegra_periph_reset_assert(tegra->clk);
> > +	udelay(1);
> > +	tegra_periph_reset_deassert(tegra->clk);
> 
> 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?
 
Yes, PHY driver probe does not touch any registers. It just sets up the PHY API hooks.
These APIs will be called from ehci-tegra.c as part of ehci tegra probe function, after
getting  PHY handle, which in turn happens after issuing above reset.

Thanks to Stephen & Alan, for the review comments.
 
> 
> 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. 

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