linux-kernel - Re: [PATCH v3 2/2] usb: host: ehci-tegra: Avoid getting the same reset twice

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Message-ID: <20160504202544.GB21480@mithrandir.ba.sec>
Date:	Wed, 4 May 2016 22:25:46 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Jon Hunter <jonathanh@...dia.com>, linux-usb@...r.kernel.org,
	linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Hans de Goede <hdegoede@...hat.com>
Subject: Re: [PATCH v3 2/2] usb: host: ehci-tegra: Avoid getting the same
 reset twice

On Wed, May 04, 2016 at 11:23:20AM -0600, Stephen Warren wrote:
> On 05/04/2016 08:40 AM, Thierry Reding wrote:
> > From: Thierry Reding <treding@...dia.com>
> > 
> > Starting with commit 0b52297f2288 ("reset: Add support for shared reset
> > controls") there is a reference count for reset control assertions. The
> > goal is to allow resets to be shared by multiple devices and an assert
> > will take effect only when all instances have asserted the reset.
> > 
> > In order to preserve backwards-compatibility, all reset controls become
> > exclusive by default. This is to ensure that reset_control_assert() can
> > immediately assert in hardware.
> > 
> > However, this new behaviour triggers the following warning in the EHCI
> > driver for Tegra:
> ...
> > The reason is that Tegra SoCs have three EHCI controllers, each with a
> > separate reset line. However the first controller contains UTMI pads
> > configuration registers that are shared with its siblings and that are
> > reset as part of the first controller's reset. There is special code in
> > the driver to assert and deassert this shared reset at probe time, and
> > it does so irrespective of which controller is probed first to ensure
> > that these shared registers are reset before any of the controllers are
> > initialized. Unfortunately this means that if the first controller gets
> > probed first, it will request its own reset line and will subsequently
> > request the same reset line again (temporarily) to perform the reset.
> > This used to work fine before the above-mentioned commit, but now
> > triggers the new WARN.
> > 
> > Work around this by making sure we reuse the controller's reset if the
> > controller happens to be the first controller.
> 
> > diff --git a/drivers/usb/host/ehci-tegra.c b/drivers/usb/host/ehci-tegra.c
> 
> > @@ -81,15 +81,23 @@ static int tegra_reset_usb_controller(struct platform_device *pdev)
> 
> > +	bool has_utmi_pad_registers = false;
> > 
> >   	phy_np = of_parse_phandle(pdev->dev.of_node, "nvidia,phy", 0);
> >   	if (!phy_np)
> >   		return -ENOENT;
> > 
> > +	if (of_property_read_bool(phy_np, "nvidia,has-utmi-pad-registers"))
> > +		has_utmi_pad_registers = true;
> 
> Isn't that just:
> 
> has_utmi_pad_registers = of_property_read_bool(phy_np,
>     "nvidia,has-utmi-pad-registers");
> 
> ... and then you can remove " = false" from the declaration too?

Yes. This is really only for aesthetics. The direct assignment doesn't
fit within 80 columns, and wrapping it looks ugly whichever way you do
it.

> >   	if (!usb1_reset_attempted) {
> >   		struct reset_control *usb1_reset;
> > 
> > -		usb1_reset = of_reset_control_get(phy_np, "utmi-pads");
> > +		if (!has_utmi_pad_registers)
> > +			usb1_reset = of_reset_control_get(phy_np, "utmi-pads");
> > +		else
> > +			usb1_reset = tegra->rst;
> ...
> >   		usb1_reset_attempted = true;
> >   	}
> 
> This is a pre-existing issue, but what happens if the probes for two USB
> controllers run in parallel; there seems to be missing locking related to
> testing/setting usb1_reset_attempted, which could cause multiple parallel
> attempts to get the "utmi-pads" reset object, which would presumably cause
> essentially the same issue this patch is solving in other cases?

Hah! Interestingly my initial attempt at fixing this was to introduce a
lock to serialize these, because I thought that was what was going on. I
don't think this function can ever run concurrently for different
devices because the driver core already serializes probes (unless a
driver specifically requests asynchronous probing, which this one
doesn't).

Thierry

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