RFC: MPC52xx serial port configuration from DT blob

We have a MPC5200B-based board running an arch/powerpc kernel and we need the ability to configure a non-console serial port for a particular baud rate during system start-up. It seems that the UART driver in drivers/serial/mpc52xx_uart.c does not support this. It only allows to set parameters for a port that is used as a console, and for which those parameters are passed in the kernel command line. We would like to extend the mpc52xx_uart.c driver to be able to retrieve port options from the DT blob and configure a given port accordingly. A new port-specific property called "options" would be used for this. It would have syntax following its namesake in "console" kernel parameter, as described in Documentation/kernel-parameters.txt.

For example, the following settings in the .dts file would make UART5 to be configured at 115200 baud, no parity, 8 bits.

serial@2800 { // PSC5

         device_type = "serial";
         compatible = "mpc5200b-psc-uart\0mpc5200-psc-uart";
         port-number = <4>;  // Logical port assignment
         options = "115200n8"
         cell-index = <4>;
         reg = <2800 100>;
         interrupts = <2 c 0>;
         interrupt-parent = <500>;
};

In case a console port has conflicting options given in the kernel command line and in the DT blob, the command line values would be used.

Any comments on the above will be appreciated.

Quoted 30 lines · Bartlomiej

We have a MPC5200B-based board running an arch/powerpc kernel and we need the ability to configure a non-console serial port for a particular baud rate during system start-up. It seems that the UART driver in drivers/serial/mpc52xx_uart.c does not support this. It only allows to set parameters for a port that is used as a console, and for which those parameters are passed in the kernel command line. We would like to extend the mpc52xx_uart.c driver to be able to retrieve port options from the DT blob and configure a given port accordingly. A new port-specific property called "options" would be used for this. It would have syntax following its namesake in "console" kernel parameter, as described in Documentation/kernel-parameters.txt.

For example, the following settings in the .dts file would make UART5 to be configured at 115200 baud, no parity, 8 bits.

serial@2800 { // PSC5

         device_type = "serial";
         compatible = "mpc5200b-psc-uart\0mpc5200-psc-uart";
         port-number = <4>;  // Logical port assignment
         options = "115200n8"
         cell-index = <4>;
         reg = <2800 100>;
         interrupts = <2 c 0>;
         interrupt-parent = <500>;
};

In case a console port has conflicting options given in the kernel command line and in the DT blob, the command line values would be used.

Quoted 1 line

Any comments on the above will be appreciated.

The device tree is intended to be an OS-independent description of the state of the platform hardware as left by firmware (or bootwrapper). It is not intended to contain kernel configuration parameters or options, though there are a few exceptions.

So, this kind of change is unlikely to be accepted.

Hi All,

We have a MPC5200B-based board running an arch/powerpc kernel and we need the ability to configure a non-console serial port for a particular baud rate during system start-up. It seems that the UART driver in drivers/serial/mpc52xx_uart.c does not support this. It only allows to set parameters for a port that is used as a console, and for which those parameters are passed in the kernel command line. We would like to extend the mpc52xx_uart.c driver to be able to retrieve port options from the DT blob and configure a given port accordingly. A new port-specific property called "options" would be used for this. It would have syntax following its namesake in "console" kernel parameter, as described in Documentation/kernel-parameters.txt.

For example, the following settings in the .dts file would make UART5 to be configured at 115200 baud, no parity, 8 bits.

serial@2800 { // PSC5

         device_type = "serial";
         compatible = "mpc5200b-psc-uart\0mpc5200-psc-uart";
         port-number = <4>;  // Logical port assignment
         options = "115200n8"
         cell-index = <4>;
         reg = <2800 100>;
         interrupts = <2 c 0>;
         interrupt-parent = <500>;
};

In case a console port has conflicting options given in the kernel command line and in the DT blob, the command line values would be used.

Any comments on the above will be appreciated.