linux-kernel - Re: [RESEND PATCH v2 7/7] usb: xhci: plat: add vbus regulator control

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Date:	Wed, 27 Apr 2016 14:24:03 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Jisheng Zhang <jszhang@...vell.com>
Cc:	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	mathias.nyman@...ux.intel.com, gregkh@...uxfoundation.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, yendapally.reddy@...adcom.com
Subject: Re: [RESEND PATCH v2 7/7] usb: xhci: plat: add vbus regulator control

On Wed, Apr 27, 2016 at 06:25:16PM +0800, Jisheng Zhang wrote:
> On Wed, 27 Apr 2016 10:57:39 +0100 Mark Brown wrote:
> > On Wed, Apr 27, 2016 at 08:37:20AM +0300, Felipe Balbi wrote:

> > > > +	vbus = devm_regulator_get(&pdev->dev, "vbus");  

> > > devm_regulator_get_optional() ??  

> > Does USB work without a VBUS?  Unless the answer is yes then I'd expect
> > this to be just a normal regulator_get().

> Per spec no. But the vbus may be transparent to SW on some platforms, so I
> think devm_regulator_get_optional() is better.

Really, no.  If it's physically there the software needs to be written
as such.  Please see the documentation and list archives for extensive
discussion on this topic.

> > This is all completely broken unless the supply is optional.

> The supply is optional.

To repeat a supply is only optional if it might be physically absent.

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