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02-platform_init-and-uart-driver.md

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Platform Initialization

Be sure to read 01-setting-up-new-project.md before continuing as this document will make references to it and assuming that the reading is aware of the background as to why platform initialization is required.

Platform initialization is a way for the user to do one time configuration of the hardware and is done by implementing platform_init() functions. The initialization could be for example - clock configurations, enabling or disabling certain blocks etc. If not anything, the user needs to provide a way for shell to put and get a character from some stream (e.g uart). This is Mandatory! And expected!

platform_init() can be implemented at any level but for the reasons of making the source code organization obvious, it's preferred to have a platform.c file which implements it. As discussed in Define platform_init() section we have the following directory structure:

new_project
|-- Makefile
└── platform.c

With platform.c having the following:

/**
 * Copyright <year> <author>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 **/

#include "shell.h"

void platform_init(void) {

}
  • Our interest is in populating the contents of pltform_init(). As hinted earlier the shell needs a way to put and get a character from somewhere so user interaction is possible.
  • Typically this would be a uart and in certain cases can be memory, mailbox or some other interface. We assume it is the uart.

The Uart interfaces

The user would need to provide a uart driver that provides the following interfaces:

  • void uart_init(void) uart initialization.
  • void uart_putc(char c) a way to send a character out.
  • int uart_getc(void) a way to read a character in.

More details on the uart driver are discussed in the 03-uart-driver.md documentation.

Assuming these implementations to be available through uart.h (that the user has separately implemented), the platform.c would then look like so -

/**
 * Copyright <year> <author>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 **/

#include "shell.h"
#include "uart.h"

void platform_init(void) {
  /**
   * User can do other configurations here.
   * E.g clocks, enable/disable blocks
   */

  /* Initialize the UART */
  uart_init();

  /* Register with shell, a function to get a character from the uart */
  set_read_char(uart_getc);

  /* Register with shell, a function to send a character to the uart */
  set_write_char(uart_putc);
}
  • Issuing make in new_project/ should result in the following:
$ make
Detected Configuration
*
|- PROJECT             : shell
|- BUILD USER          : itankar
|- BUILD HOST          : sandbox
|- SHELL VERSION       : a2dc94e
|- USER REPO VERSION   : a2dc94e
|- SHELL ROOT          : /home/itankar/early-bringup-tool/shell
|- PROJECT ROOT        : /home/itankar/early-bringup-tool/new_project
|- OPTIMIZATION        : 0
*
|- TOOLCHAIN           : riscv64-linux-gnu-
|- STARTUP             : /home/itankar/early-bringup-tool/shell/startup/riscv64.S
|- LAYOUT FILE         : /home/itankar/early-bringup-tool/shell/scatter/layout.ld
|- RAM BASE            : 0x80000000
|- RAM SIZE            : 0x4000000
`- UART BASE           : 0x10000000

>> If incorrect, please set these Variables in Makefile <<

compiling /home/itankar/early-bringup-tool/new_project/platform.c
/home/itankar/early-bringup-tool/new_project/platform.c:18:10: fatal error: uart.h: No such file or directory
   18 | #include "uart.h"
      |          ^~~~~~~~
compilation terminated.
make: *** [../shell/Makefile:148: /home/itankar/early-bringup-tool/new_project/platform.o] Error 1
  • we would need to fix this. Create a directory new_project/uart/ and add two files uart.c and uart.h in it. (if the user already has a uart driver, then the source should be simply copied to new_project/ and the expected interfaces as expected above should be provided in uart.h).
  • the directory structure should look as follows:
new_project
|-- Makefile
|-- platform.c
└──  uart
    |-- uart.c
    └── uart.h
  • put the following in uart.h
/**
 * Copyright <year> <author>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 **/

#ifndef __UART_H__
#define __UART_H__

void uart_init(void);
void uart_putc(char c);
int uart_getc(void);

#endif
  • put the following in uart.c
/**
 * Copyright <year> <author>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 **/

#include "uart.h"

void uart_init(void) {

}

void uart_putc(char c) {

}

int uart_getc(void) {
  return -1;
}
  • Issue make in new_project/ again. Prints similar to following should be seen:
Detected Configuration
*
|- PROJECT             : shell
|- BUILD USER          : itankar
|- BUILD HOST          : sandbox
|- SHELL VERSION       : a2dc94e
|- USER REPO VERSION   : a2dc94e
|- SHELL ROOT          : /home/itankar/early-bringup-tool/shell
|- PROJECT ROOT        : /home/itankar/early-bringup-tool/new_project
|- OPTIMIZATION        : 0
*
|- TOOLCHAIN           : riscv64-linux-gnu-
|- STARTUP             : /home/itankar/early-bringup-tool/shell/startup/riscv64.S
|- LAYOUT FILE         : /home/itankar/early-bringup-tool/shell/scatter/layout.ld
|- RAM BASE            : 0x80000000
|- RAM SIZE            : 0x4000000
`- UART BASE           : 0x10000000

>> If incorrect, please set these Variables in Makefile <<

compiling /home/itankar/early-bringup-tool/new_project/uart/uart.c
compiling /home/itankar/early-bringup-tool/new_project/platform.c
compiling /home/itankar/early-bringup-tool/shell/lib/utils/utils.c
compiling /home/itankar/early-bringup-tool/shell/lib/string/string.c
compiling /home/itankar/early-bringup-tool/shell/lib/a2i/a2i.c
compiling /home/itankar/early-bringup-tool/shell/lib/printf/printf.c
compiling /home/itankar/early-bringup-tool/shell/shell.c
assembling /home/itankar/early-bringup-tool/shell/startup/riscv64.S
generating /home/itankar/early-bringup-tool/shell/scatter/layout.ld
generating shell.elf
section size:
   text    data     bss     dec     hex filename
   5576    1112      88    6776    1a78 shell.elf
generating shell.bin
Done!

Next:

  • While the compilation is successful. Becuase the uart driver in this case was incomplete (empty functions), the shell would boot but now work as expected.
  • 03-uart-driver.md discusses more on the uart driver implementation which should be the last step in making the shell completely functional.