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

[Frank Li]

On Thu, Jan 25, 2024 at 05:43:22PM +0000, Conor Dooley wrote:
> On Wed, Jan 24, 2024 at 02:17:22PM -0500, Frank Li 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?
> > 
> > By the way, I sent v4 at
> > https://lore.kernel.org/imx/20240124152525.3910311-1-Frank.Li@xxxxxxx/T/#t
> 
> I see.
> 
> > How about add below sentence?
> > 
> > This was only happen when PORTSC[PP} can control vbus. Needn't set it if
> > vbus is always on.
> 
> "This can only happen when ... controls vbus, if vbus is always on, omit
> this property".
> 
> Just a wee grammatical nitpicking.
> 
> > > On another note, I like it when the property name explains why you would
> > > add it, rather than the thing it is trying to solve.
> > > Named after the disease, rather than the symptoms, if you get me. I
> > > tried to come up with a name here, but could not really suggest
> > > something good. If you can think of something, that'd be good, but don't
> > > stress it.
> > 
> > snps,host-vbus-glitches change to snps,host-vbus-glitches-quirk.
> 
> I don't think adding "quirk" moves the needle.

I think "quirk" is reasonable because it is workaround.

Frank

> 
> > How about use below description:
> > 
> > When set, power off all Root Hub ports immediately after
> > setting host mode to avoid vbus (negative) glitch happen in later
> > xhci reset. That may cause some USB devices emuration fail when kernel boot
> > with device connected and PORTSC[PP] control vbus in board desgin.
> 
> "When set, all root hub ports should be powered off immediately after
> enabling host mode, to avoid Vbus (negative) glitches that may happen
> during xHCI reset. These glitches can cause enumeration of some USB
> devices to fail when PORTSC[PP] controls Vbus. If Vbus is always on,
> omit this property."
> 
> How's that?