Re: [PATCH 1/2] dt-bindings: usb: dwc3: Add snps,host-vbus-glitches avoiding vbus glitch

[Thinh Nguyen]

On Tue, Jan 30, 2024, Frank Li wrote:
> On Tue, Jan 30, 2024 at 12:13:22PM -0600, Rob Herring wrote:
> > 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@xxxxxxx>
> > > > > > 
> > > > > > 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@xxxxxxx>
> > > > > > Reviewed-by: Peter Chen <peter.chen@xxxxxxx>
> > > > > > Signed-off-by: Frank Li <Frank.Li@xxxxxxx>
> > > > > > ---
> > > > > >  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.
> 
> Supposed yes. But I have not confidence to apply this unconditionaly.
> There are too much difference SOC and dwc3 version. Not sure if it brokes
> something. I think it should apply workround as less as possible.
> 

I can't confirm if there will be negative effect to any platform in the
market. But from review, IMHO functionally it should be fine applying
this unconditionally.

BR,
Thinh