My in-house writeup on how I got this to work.
This gets confusing. These (115 & 117) are the pins we use as COM4 on Windows CE. In the T30 pin function table, the Nvidia name is UART3. Toradex uses letter designations instead of numbers, but their UART_C is NOT Nvidia UART3. Yet in kernel code they also use letter designations, and there UARTC = UART3. Focus on the pins and GPIO designations to keep this straight.
I found a table which has possible functions for each of the pins. Following are the definitions for pins 115/117.
arch/arm/mach-tegra/pinmux-t3-tables.c
NAME GPIO VDD f0 f1 f2 f3 fSafe io reg
PINGROUP(UART3_TXD, PW6, UART, UARTC, RSVD1, GMI, RSVD2, UARTC, INPUT, 0x3174),\
PINGROUP(UART3_RXD, PW7, UART, UARTC, RSVD1, GMI, RSVD2, UARTC, INPUT, 0x3178),\
In the kernel code, these pins are configured to work as GPIOs. There is no simple kernel .config entry to get them to switch to the UART function we use in CE. You have to modify the source code. There are two different files which need to be modified, following are context diffs of the necessary changes. Note after these changes the kernel name will have a “-dirty” appended to it. You’ll see this in the modules path name and from getting the kernel name via “uname -r”. This is OK, from Yocto Project “this simply means that modifications in the source directory have not been committed.”
*** arch/arm/mach-tegra/board-colibri_t30_ORIGINAL.c 2021-02-25 10:38:03.911409336 -0800
— arch/arm/mach-tegra/board-colibri_t30.c 2021-03-01 11:56:05.149381453 -0800
— 1313,1319 ----
&tegra_uarta_device, /* Colibri UART_A (formerly FFUART) */
&tegra_uartd_device, /* Colibri UART_B (formerly BTUART) */
&tegra_uartb_device, /* Colibri UART_C (formerly STDUART) */
+ &tegra_uartc_device, /* Tegra UART 3 */
};
static struct uart_clk_parent uart_parent_clk[] = {
— 1393,1399 ----
colibri_t30_uart_pdata.parent_clk_count = ARRAY_SIZE(uart_parent_clk);
tegra_uarta_device.dev.platform_data = &colibri_t30_uart_pdata;
tegra_uartb_device.dev.platform_data = &colibri_t30_uart_pdata;
+ tegra_uartc_device.dev.platform_data = &colibri_t30_uart_pdata;
tegra_uartd_device.dev.platform_data = &colibri_t30_uart_pdata;
/* Register low speed only if it is selected */
*** arch/arm/mach-tegra/board-colibri_t30-pinmux_ORIGINAL.c 2021-02-25 14:57:27.703779342 -0800
— arch/arm/mach-tegra/board-colibri_t30-pinmux.c 2021-03-01 11:58:17.013387635 -0800
— 577,586 —
DEFAULT_PINMUX(UART2_RXD, IRDA, NORMAL, NORMAL, INPUT),
DEFAULT_PINMUX(UART2_TXD, IRDA, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_CTS_N, GMI, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_RTS_N, GMI, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_RXD, GMI, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_TXD, GMI, NORMAL, NORMAL, INPUT),
//multiplexed SDMMC3_DAT7
DEFAULT_PINMUX(ULPI_CLK, SPI1, NORMAL, NORMAL, INPUT),
— 577,586 ----
DEFAULT_PINMUX(UART2_RXD, IRDA, NORMAL, NORMAL, INPUT),
DEFAULT_PINMUX(UART2_TXD, IRDA, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_CTS_N, UARTC, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_RTS_N, UARTC, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_RXD, UARTC, NORMAL, NORMAL, INPUT),
! DEFAULT_PINMUX(UART3_TXD, UARTC, NORMAL, NORMAL, INPUT),
//multiplexed SDMMC3_DAT7
DEFAULT_PINMUX(ULPI_CLK, SPI1, NORMAL, NORMAL, INPUT),
