Message Structure inconsistencies

[yanniknelson]

Your message buffer structure is inconsistent with the specification in https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface AND in your readme https://github.com/bztsrc/raspi3-tutorial/tree/master/04_mailboxes however you're consistent:

your description of the tags is:

0 size of the message in bytes, (x+1)*4
1 MBOX_REQUEST magic value, indicates request message
2-x tags
x+1 MBOX_TAG_LAST magic value, indicates no more tags

Where each tag looks like:

n+0. tag identifier
n+1. value buffer size in bytes
n+2. must be zero
n+3. optional value buffer

and then your code message looks like:

    mbox[0] = 8*4;                  // length of the message
    mbox[1] = MBOX_REQUEST;         // this is a request message
    
    mbox[2] = MBOX_TAG_GETSERIAL;   // get serial number command
    mbox[3] = 8;                    // buffer size
    mbox[4] = 8;
    mbox[5] = 0;                    // clear output buffer
    mbox[6] = 0;

    mbox[7] = MBOX_TAG_LAST;

The specification for get serial is:

Get board serial

• Tag: 0x00010004
• Request:
  • Length: 0
• Response:
  • Length: 8
  • Value:
    • u64: board serial

shouldn't your message look like:

[0] -> size
[1] -> MBOX_REQUEST;
[2] -> MOBOX_TAG_GETSERIAL;
[3] -> 0; // there is no request data
[4] -> MBOX_TAG_LAST

if you need to allocate the space for the response as well (which seems to be required after looking examples?) then it would be:
[0] -> size
[1] -> MBOX_REQUEST;
[2] -> MOBOX_TAG_GETSERIAL;
[3] -> 0; // there is no request data
[4] -> 8; // response length is 8 bytes
[5] -> 0; // first 4 bytes
[6] -> 0; // second 4 bytes
[7] -> MBOX_TAG_LAST;

This confusion is strengthened in https://github.com/bztsrc/raspi3-tutorial/blob/master/05_uart0/uart.c
when you set the clock rate:
spec:

Set clock rate

• Tag: 0x00038002
• Request:
  • Length: 12
  • Value:
    • u32: clock id
    • u32: rate (in Hz)
    • u32: skip setting turbo
  • Response:
    • Length: 8
  • Value:
  • u32: clock id
  • u32: rate (in Hz)

your code:

mbox[0] = 9*4;
mbox[1] = MBOX_REQUEST;
mbox[2] = MBOX_TAG_SETCLKRATE; // set clock rate
mbox[3] = 12;
mbox[4] = 8;
mbox[5] = 2;           // UART clock
mbox[6] = 4000000;     // 4Mhz
mbox[7] = 0;           // clear turbo
mbox[8] = MBOX_TAG_LAST;

why are you putting 8 in mbox[4]?
in this case mbox[4] is n+2 and your read me says:

n+2. must be zero