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

[Conor Dooley <conor@kernel.org>]

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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.
> +    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?

Thanks,
Conor.

> +
>    snps,is-utmi-l1-suspend:
>      description:
>        True when DWC3 asserts output signal utmi_l1_suspend_n, false when
> -- 
> 2.34.1
> 

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