linux-kernel - Re: [PATCH] usb: chipidea: Grab the (legacy) USB PHY by phandle first

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Message-ID: <20190116144429.7df1d0c5@windsurf>
Date:   Wed, 16 Jan 2019 14:44:29 +0100
From:   Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Chen <Peter.Chen@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] usb: chipidea: Grab the (legacy) USB PHY by phandle
 first

Hello,

On Wed, 16 Jan 2019 14:30:28 +0100, Paul Kocialkowski wrote:

> > Why is this conditional on the generic PHY lookup failing?
> > 
> > Don't we simply want:
> > 
> > 	ci->phy = devm_phy_get(dev->parent, "usb-phy");
> > 	ci->usb_phy = devm_usb_get_phy_by_phandle(dev->parent, "phys", 0);
> > 	if (IS_ERR(ci->usb_phy))
> > 		ci->usb_phy = devm_usb_get_phy(dev->parent, USB_PHY_TYPE_USB2);
> > 
> >  ?  
> 
> Well, the code dealing with the PHY later on will use ci->phy over ci-
> >usb_phy (so generic PHY API first). As a result, if the  
> devm_usb_get_phy_by_phandle lookup fails but we got a generic PHY, the
> latter will be used and there is no need for a fallback. That's why I
> put both conditions there. Maybe that's too much of an assumption?

Well prior to your code, there was already a possibility for both
ci->phy and ci->usb_phy to be valid. I don't think it's really useful
to avoid the fallback when a generic PHY has already been found, it's
confusing. If really you want to clarify that, it should be:

	/* Let's first try to find a generic PHY */
	ci->phy = devm_phy_get(dev->parent, "usb-phy");
	if (IS_ERR(ci->phy)) {
		/* Fall back to legacy USB PHY */
		ci->usb_phy = devm_usb_get_phy_by_phandle(dev->parent, "phys", 0);
		if (IS_ERR(ci->usb_phy))
			ci->usb_phy = devm_usb_get_phy(dev->parent, USB_PHY_TYPE_USB2);
	}

With that, you would only have either ci->phy or ci->usb_phy be valid,
and never both. With  your change, you can have ci->phy and ci->usb_phy
both be valid if the legacy USB PHY was found using
devm_usb_get_phy_by_phandle(), but not if we fell back to
devm_usb_get_phy().

> > Does this needs a "Fixes:" tag ? It's not fixing a regression because
> > nobody complained until now, but it's really fixing a behavior that
> > wasn't correct.  
> 
> Yes I it this makes sense to consider that this was incorrect behavior
> starting from the moment the dt bindings were formalized for the
> driver, which would be commit d7d30c911dd957e274c3da6910d4286862ab1d78.
> 
> Do you think that would nake sense?

Up to the maintainer I'd say. I don't have any preference here.

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com