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 10:08:35 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Venu Byravarasu <vbyravarasu@...dia.com>
cc:	gregkh@...uxfoundation.org, <balbi@...com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<swarren@...dotorg.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, Venu Byravarasu wrote:

> As Tegra PHY driver needs to access one of the Host registers,
> added few APIs.
> 
> Signed-off-by: Venu Byravarasu <vbyravarasu@...dia.com>

> --- a/drivers/usb/host/ehci-tegra.c
> +++ b/drivers/usb/host/ehci-tegra.c

> @@ -605,6 +619,50 @@ static const struct dev_pm_ops tegra_ehci_pm_ops = {
>  
>  #endif
>  
> +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.

Alan Stern


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