linux-kernel - Re: [PATCH] usb: dwc3: xilinx: Prevent spike in reset signal

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Message-ID: <20250321212419.fhyj4rcx4emte6qm@synopsys.com>
Date: Fri, 21 Mar 2025 21:24:20 +0000
From: Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To: Mike Looijmans <mike.looijmans@...ic.nl>
CC: Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michal Simek <michal.simek@....com>,
        "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: dwc3: xilinx: Prevent spike in reset signal

On Tue, Mar 18, 2025, Mike Looijmans wrote:
> On 18-03-2025 01:12, Thinh Nguyen wrote:
> > On Mon, Mar 17, 2025, Mike Looijmans wrote:
> > > On 14-03-2025 22:14, Thinh Nguyen wrote:
> > > > On Fri, Mar 14, 2025, Mike Looijmans wrote:
> > > > > Set the gpio to "high" on acquisition, instead of acquiring it in low
> > > > > state and then immediately setting it high again. This prevents a
> > > > > short "spike" on the reset signal.
> > > > How does this affect the current programming flow beside preventing a
> > > > spike to the reset signal?
> > > I don't understand your question. What "programming flow" are you referring
> > > to?
> > It's not obvious to me if this is an error in Xilinx documentation, the
> > driver issue, or whether this is found through experiment. Since I don't
> > have the info of this platform, it would help to know where the source
> > of error is so we can document this in the code or change-log.
> 
> It's a bug in the driver, found through code inspection.
> 
> The reset GPIO here is to control the reset signal to an external, usually
> ULPI PHY, chip. This external chip is not part of the Xilinx hardware.
> 
> > > The reset sequence was just plain wrong, the issue is almost the same as the
> > Do we need a fix tag and add to stable then?
> 
> That would be appropriate I think.
> 
> 
> > 
> > > one described in this commit:
> > > e0183b974d30 "net: mdiobus: Prevent spike on MDIO bus reset signal"
> > > 
> > > Note that I see this high-low-high-low double reset toggle in many Xilinx
> > > software drivers, they seem to teach that at the Xilinx academy or so.
> > > 
> > > 
> > > > > Signed-off-by: Mike Looijmans <mike.looijmans@...ic.nl>
> > > > > ---
> > > > > 
> > > > >    drivers/usb/dwc3/dwc3-xilinx.c | 3 +--
> > > > >    1 file changed, 1 insertion(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/usb/dwc3/dwc3-xilinx.c b/drivers/usb/dwc3/dwc3-xilinx.c
> > > > > index a33a42ba0249..a159a511483b 100644
> > > > > --- a/drivers/usb/dwc3/dwc3-xilinx.c
> > > > > +++ b/drivers/usb/dwc3/dwc3-xilinx.c
> > > > > @@ -207,7 +207,7 @@ static int dwc3_xlnx_init_zynqmp(struct dwc3_xlnx *priv_data)
> > > > >    skip_usb3_phy:
> > > > >    	/* ulpi reset via gpio-modepin or gpio-framework driver */
> > > > > -	reset_gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_LOW);
> > > > > +	reset_gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
> > > > >    	if (IS_ERR(reset_gpio)) {
> > > > >    		return dev_err_probe(dev, PTR_ERR(reset_gpio),
> > > > >    				     "Failed to request reset GPIO\n");
> > > > > @@ -215,7 +215,6 @@ static int dwc3_xlnx_init_zynqmp(struct dwc3_xlnx *priv_data)
> > > > >    	if (reset_gpio) {
> > > > >    		/* Toggle ulpi to reset the phy. */
> > > > Does the comment above still apply?
> > > Now you mention it, the comment never made any sense anyway.
> > > 
> > Then can we remove it?
> 
> Removing would be better, yes. I'll make a v2 patch.
> 
> 
> > > > > -		gpiod_set_value_cansleep(reset_gpio, 1);
> > > > >    		usleep_range(5000, 10000);
> > > > Do we still need this usleep_range here?
> > > Yes, this is the "reset active" time.
> > > 
> > But why do we need 2 calls to usleep_range? From just looking at this
> > here, it appears that the first was intended for the removed
> > gpiod_set_value_cansleep(reset_gpio, 1). If this "reset active" time is
> > needed irrespective of the existent reset_gpio, then shouldn't it be set
> > outside of this if statement?
> 
> It helps to see the whole thing instead of just the patch.
> 
> If I omit error handling and comments then the original code reads:
> 
>         reset_gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_LOW);
>         if (reset_gpio) {
>                 gpiod_set_value_cansleep(reset_gpio, 1);
>                 usleep_range(5000, 10000);
>                 gpiod_set_value_cansleep(reset_gpio, 0);
>                 usleep_range(5000, 10000);
>         }
> 
> So the gpio is acquired in a LOW state and then, without delay, is set to a
> high state again. This causes the "spike" I'm mentioning here. The correct
> procedure is to acquire it in the HIGH state immediately, so the sequence
> becomes:
> 
>         reset_gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
>         if (reset_gpio) {
>                 usleep_range(5000, 10000);
>                 gpiod_set_value_cansleep(reset_gpio, 0);
>                 usleep_range(5000, 10000);
>         }
> 
> This patch does exactly that.
> 

Ah, that makes sense. Thanks for the explaination.

BR,
Thinh