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

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Date:	Wed, 16 Jan 2013 17:25:58 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Stephen Warren <swarren@...dotorg.org>
cc:	Venu Byravarasu <vbyravarasu@...dia.com>,
	<gregkh@...uxfoundation.org>, <balbi@...com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 4/4] usb: Add APIs to access host registers from Tegra
 PHY

On Wed, 16 Jan 2013, Stephen Warren wrote:

> On 01/16/2013 08:08 AM, Alan Stern wrote:
> > On Wed, 16 Jan 2013, Venu Byravarasu wrote:
> > 
> >> As Tegra PHY driver needs to access one of the Host registers,
> >> added few APIs.
> 
> >> --- a/drivers/usb/host/ehci-tegra.c
> >> +++ b/drivers/usb/host/ehci-tegra.c
> 
> >> +void tegra_ehci_set_wakeon_events(struct usb_phy *x, bool enable)
> >> +{
> >> +	unsigned long val;
> >> +	struct usb_hcd *hcd = bus_to_hcd(x->otg->host);
> >> +	void __iomem *base = hcd->regs;
> >> +	u32 wake = USB_PORTSC1_WKOC | USB_PORTSC1_WKDS | USB_PORTSC1_WKCN;
> >> +
> >> +	val = readl(base + USB_PORTSC1);
> >> +	if (enable)
> >> +		val |= wake;
> >> +	else
> >> +		val &= ~wake;
> >> +	writel(val, base + USB_PORTSC1);
> >> +}
> > 
> > Here and below, this sort of code is highly questionable.  You
> > evidently don't realize that some of the bits in the PORTSC registers
> > are R/WC.  This means writing a 1 to these bits will clear them.  
> > 
> > Consequently it is almost always wrong to read a PORTSC register and
> > then write back the same (or a slightly modified) value.
> 
> Sorry I'm not familiar with USB... Are the bits being manipulated here
> (i.e. USB_PORTSC1_WKOC | USB_PORTSC1_WKDS | USB_PORTSC1_WKCN)
> standardized USBisms, or some custom Tegra stuff?

The _symbols_ are not standard (i.e., not used in the regular ehci-hcd
driver) but the _values_ are: compare with PORT_WKOC_E etc. in
include/linux/usb/ehci_defs.h.  I'm not sure about the register 
addresses, however.

> Anyway, is the solution here to do:
> 
> val = readl(addr)
> 
> // i.e. add the following line:
> val &= ~(all write-to-clear bits);
> 
> if (enable) val |= wake; else val &= ~wake;
> 
> writel(val, addr)
> 
> ... or is there more broken than just that?

Nope, that's the right way to do it.  There are loads of examples in 
drivers/usb/host/ehci-hub.c; look at usages of PORT_RWC_BITS.

> Also note that the driver is already doing exactly what is in these new
> functions; the code is just being split out so that only the EHCI driver
> touches EHCI registers, and the PHY driver only touches PHY registers.
> Still, I'll admit it's a good time to fix any mistakes in this part of
> the code.

Agreed.

Alan Stern

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