[rom, rom_ext] Enable pull-up on UART_RX

Fixes: #24551

Signed-off-by: Chris Frantz <[email protected]>
(cherry picked from commit 6ca7587)

Author: cfrantz

sw/device/silicon_creator/lib/drivers/pinmux.c

@@ -187,6 +187,9 @@ void pinmux_init(void) {
     configure_input(kInputSwStrap2);
   }
 
+  // Pull the UART_RX line high (idle state for UART).  This prevents a
+  // floating UART_RX from incorrectly triggering serial break.
+  enable_pull(kInputUart0.pad, /*enable=*/true, /*up=*/true);
   configure_input(kInputUart0);
   configure_output(kOutputUart0);
 }

sw/device/silicon_creator/lib/drivers/pinmux_unittest.cc

@@ -100,10 +100,13 @@ TEST_F(InitTest, WithBootstrap) {
                      kTopEarlgreyPinmuxInselIoc1)
   EXPECT_ABS_WRITE32(RegInSel(kTopEarlgreyPinmuxPeripheralInGpioGpio24),
                      kTopEarlgreyPinmuxInselIoc2)
+
+  // UART configuration: RX line pulled up, RX is IOC3, TX is IOC4.
+  EXPECT_ABS_WRITE32(RegPadAttr(kTopEarlgreyMuxedPadsIoc3),
+                     {{PINMUX_MIO_PAD_ATTR_0_PULL_EN_0_BIT, 1},
+                      {PINMUX_MIO_PAD_ATTR_0_PULL_SELECT_0_BIT, 1}});
   EXPECT_ABS_WRITE32(RegInSel(kTopEarlgreyPinmuxPeripheralInUart0Rx),
                      kTopEarlgreyPinmuxInselIoc3);
-
-  // The outputs that will be configured.
   EXPECT_ABS_WRITE32(RegOutSel(kTopEarlgreyPinmuxMioOutIoc4),
                      kTopEarlgreyPinmuxOutselUart0Tx);
 
@@ -115,10 +118,13 @@ TEST_F(InitTest, WithoutBootstrap) {
   EXPECT_CALL(otp_,
               read32(OTP_CTRL_PARAM_OWNER_SW_CFG_ROM_BOOTSTRAP_DIS_OFFSET))
       .WillOnce(Return(kHardenedBoolTrue));
+
+  // UART configuration: RX line pulled up, RX is IOC3, TX is IOC4.
+  EXPECT_ABS_WRITE32(RegPadAttr(kTopEarlgreyMuxedPadsIoc3),
+                     {{PINMUX_MIO_PAD_ATTR_0_PULL_EN_0_BIT, 1},
+                      {PINMUX_MIO_PAD_ATTR_0_PULL_SELECT_0_BIT, 1}});
   EXPECT_ABS_WRITE32(RegInSel(kTopEarlgreyPinmuxPeripheralInUart0Rx),
                      kTopEarlgreyPinmuxInselIoc3);
-
-  // The outputs that will be configured.
   EXPECT_ABS_WRITE32(RegOutSel(kTopEarlgreyPinmuxMioOutIoc4),
                      kTopEarlgreyPinmuxOutselUart0Tx);