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

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Message-ID: <20250318001211.tnjwfckt5j4fbvjf@synopsys.com>
Date: Tue, 18 Mar 2025 00:12:16 +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 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.

> 
> 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?

> 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?

> 
> > > -		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?

BR,
Thinh

> 
> 
> > 
> > BR,
> > Thinh
> > 
> > >   		gpiod_set_value_cansleep(reset_gpio, 0);
> > >   		usleep_range(5000, 10000);
> > > -- 
> > > 2.43.0
> > > 
> > > 
> > > Met vriendelijke groet / kind regards,
> > > 
> > > Mike Looijmans
> > > 
> > > 
> 
> -- 
> Mike Looijmans
> System Expert
> 
> TOPIC Embedded Products B.V.
> Materiaalweg 4, 5681 RJ Best
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> 
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> E: mike.looijmans@...ic.nl
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> 
> 
>