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Date: Tue, 30 Jan 2024 12:13:22 -0600
From: Rob Herring <robh@kernel.org>
To: Conor Dooley <conor@kernel.org>, Frank Li <Frank.li@nxp.com>
Cc: ran.wang_1@nxp.com, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Conor Dooley <conor+dt@kernel.org>, Felipe Balbi <balbi@kernel.org>,
	"open list:USB SUBSYSTEM" <linux-usb@vger.kernel.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>, mark.rutland@arm.com,
	pku.leo@gmail.com, sergei.shtylyov@cogentembedded.com
Subject: Re: [PATCH 1/2] dt-bindings: usb: dwc3: Add snps,host-vbus-glitches
 avoiding vbus glitch
Message-ID: <20240130181322.GA2079185-robh@kernel.org>
References: <20240119213130.3147517-1-Frank.Li@nxp.com>
 <20240124-unclothed-dodgy-c78b1fffa752@spud>
 <ZbFNIvEaAJCxC2VB@lizhi-Precision-Tower-5810>
 <20240124-video-lumpiness-178c4e317f5a@spud>
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In-Reply-To: <20240124-video-lumpiness-178c4e317f5a@spud>

On Wed, Jan 24, 2024 at 05:59:00PM +0000, Conor Dooley wrote:
> On Wed, Jan 24, 2024 at 12:47:14PM -0500, Frank Li wrote:
> > On Wed, Jan 24, 2024 at 05:36:42PM +0000, Conor Dooley wrote:
> > > On Fri, Jan 19, 2024 at 04:31:28PM -0500, Frank Li wrote:
> > > > From: Ran Wang <ran.wang_1@nxp.com>
> > > > 
> > > > When DWC3 is set to host mode by programming register DWC3_GCTL, VBUS
> > > > (or its control signal) will turn on immediately on related Root Hub
> > > > ports. Then the VBUS will be de-asserted for a little while during xhci
> > > > reset (conducted by xhci driver) for a little while and back to normal.
> > > > 
> > > > This VBUS glitch might cause some USB devices emuration fail if kernel
> > > > boot with them connected. One SW workaround which can fix this is to
> > > > program all PORTSC[PP] to 0 to turn off VBUS immediately after setting
> > > > host mode in DWC3 driver(per signal measurement result, it will be too
> > > > late to do it in xhci-plat.c or xhci.c).
> > > > 
> > > > Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
> > > > Reviewed-by: Peter Chen <peter.chen@nxp.com>
> > > > Signed-off-by: Frank Li <Frank.Li@nxp.com>
> > > > ---
> > > >  Documentation/devicetree/bindings/usb/snps,dwc3.yaml | 7 +++++++
> > > >  1 file changed, 7 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/usb/snps,dwc3.yaml b/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
> > > > index 203a1eb66691f..dbf272b76e0b5 100644
> > > > --- a/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
> > > > +++ b/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
> > > > @@ -273,6 +273,13 @@ properties:
> > > >        with an external supply.
> > > >      type: boolean
> > > >  
> > > > +  snps,host-vbus-glitches:
> > > > +    description:
> > > > +      When set, power off all Root Hub ports immediately after
> > > > +      setting host mode to avoid vbus (negative) glitch happen in later
> > > > +      xhci reset. And the vbus will back to 5V automatically when reset done.
> 
> nit: "will return to"
> 
> > > > +    type: boolean
> > > 
> > > Why do we want to have a property for this at all? The commit message
> > > seems to describe a problem that's limited to specific configurations
> > > and appears to be somethng the driver should do unconditionally.
> > > 
> > > Could you explain why this cannot be done unconditionally please?
> > 
> > It depends on board design, not all system vbus can be controller by root
> > hub port. If it is always on, it will not trigger this issue.
> 
> Okay, that seems reasonable to have a property for. Can you add that
> info to the commit message please?

But if vbus is always on, then applying the work-around would be a NOP, 
right? So you could just apply this unconditionally.

Rob