| Message ID | 20250318064518.9320-1-mike.looijmans@topic.nl (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] usb: dwc3: xilinx: Prevent spike in reset signal | expand |
+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@topic.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);
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);
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@topic.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(-)