linux-kernel - Re: [PATCH 1/2] usb: dwc3: Add avoiding vbus glitch happen during xhci reset

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Date:   Mon, 21 Jan 2019 19:03:10 -0600
From:   Rob Herring <robh@...nel.org>
To:     Ran Wang <ran.wang_1@....com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Felipe Balbi <balbi@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] usb: dwc3: Add avoiding vbus glitch happen during
 xhci reset

On Wed, Jan 16, 2019 at 06:48:06AM +0000, Ran Wang wrote:
> When DWC3 is set to host mode by programming register DWC3_GCTL, VUBS

s/VUBS/VBUS/

> (or its control signal) will on immediately on related Root Hub ports.

/will on/will turn on/

> Then the VUBS 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@....com>
> ---
>  Documentation/devicetree/bindings/usb/dwc3.txt |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/usb/dwc3.txt b/Documentation/devicetree/bindings/usb/dwc3.txt
> index 8e5265e..dadb530 100644
> --- a/Documentation/devicetree/bindings/usb/dwc3.txt
> +++ b/Documentation/devicetree/bindings/usb/dwc3.txt
> @@ -106,6 +106,9 @@ Optional properties:
>  			When just one value, which means INCRX burst mode enabled. When
>  			more than one value, which means undefined length INCR burst type
>  			enabled. The values can be 1, 4, 8, 16, 32, 64, 128 and 256.
> + - snps,avoid-vbus-glitch-when-set-host: 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.

Can't you imply this from the compatible string. You should have an SoC 
specific compatible.

Does this even need to be conditional? What would be the harm in doing 
this unconditionally for all DWC3 hosts? 

Rob