linux-kernel - Re: [PATCH v2] usb: dwc3: xilinx: Prevent spike in reset signal

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Message-ID: <b106b46b-f00f-40bc-aafc-f57a879fe090@amd.com>
Date: Tue, 18 Mar 2025 08:36:49 +0100
From: Michal Simek <michal.simek@....com>
To: Mike Looijmans <mike.looijmans@...ic.nl>, linux-usb@...r.kernel.org,
 Radhey Shyam Pandey <radhey.shyam.pandey@....com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] usb: dwc3: xilinx: Prevent spike in reset signal

+Radhey

On 3/18/25 07:44, Mike Looijmans wrote:
> The "reset" GPIO controls the RESET signal to an external, usually
> ULPI PHY, chip. The original code path acquires the signal in LOW
> state, and then immediately asserts it HIGH again, if the reset
> signal defaulted to asserted, there'd be a short "spike" before the
> reset.
> 
> Here is what happens depending on the pre-existing state of the reset
> signal:
> Reset (previously asserted):   ~~~|_|~~~~|_______
> Reset (previously deasserted): _____|~~~~|_______
>                                    ^ ^    ^
>                                    A B    C
> 
> At point A, the low going transition is because the reset line is
> requested using GPIOD_OUT_LOW. If the line is successfully requested,
> the first thing we do is set it high _without_ any delay. This is
> point B. So, a glitch occurs between A and B.
> 
> Requesting the line using GPIOD_OUT_HIGH eliminates the A and B
> transitions. Instead we get:
> 
> Reset (previously asserted)  : ~~~~~~~~~~|______
> Reset (previously deasserted): ____|~~~~~|______
>                                     ^     ^
>                                     A     C
> 
> Where A and C are the points described above in the code. Point B
> has been eliminated.
> 
> The issue was found during code inspection.
> 
> Also remove the cryptic "toggle ulpi .." comment.
> 
> Fixes: ca05b38252d7 ("usb: dwc3: xilinx: Add gpio-reset support")
> Signed-off-by: Mike Looijmans <mike.looijmans@...ic.nl>
> ---
> 
> Changes in v2:
> Add "Fixes" tag
> Remove "toggle ulpi" comment
> Extend comment to explain what is happening in detail
> 
>   drivers/usb/dwc3/dwc3-xilinx.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/usb/dwc3/dwc3-xilinx.c b/drivers/usb/dwc3/dwc3-xilinx.c
> index a33a42ba0249..4ca7f6240d07 100644
> --- a/drivers/usb/dwc3/dwc3-xilinx.c
> +++ b/drivers/usb/dwc3/dwc3-xilinx.c
> @@ -207,15 +207,13 @@ 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");
>   	}
>   
>   	if (reset_gpio) {
> -		/* Toggle ulpi to reset the phy. */
> -		gpiod_set_value_cansleep(reset_gpio, 1);
>   		usleep_range(5000, 10000);
>   		gpiod_set_value_cansleep(reset_gpio, 0);
>   		usleep_range(5000, 10000);