Raspberry Pi Forums

Hola fellow Rasperians

I'm gone through Lesson 1-7 in assembly (and it all works woo!), so I figured I'd try to
do the same now, but in C. So far I've successfully taken command of the LED, so I've got something working at least.
My problems starts with Lesson 6 Screen 01, I have with the usage of the LED managed to determine that
my mailboxRead() function gets stuck, however I can't for the life of me figure out why.

I am a little bit confused as to how the whole "storing the address of a c variable at the correct address (in the mailbox) works.
The code is pretty long and messy, so I'll try to just ask the things (I think) I'm struggling with.

To write to the mailbox, I do this:
I feel like I might've gone wrong here already, I know these addresses work in assembly
and as far as I can tell I'm using the same logic as well.

Might be wondering what my mailboxRead function looks like as I mentioned it gets stuck there: I'm not even sure if I can just access the mailbox like this.
Any pointers/ideas are greatly appreciated, thank you.
(Sorry for a long first post by the way, I tried to keep it short!)

// Simon

You need to declare the mailbox addresses (and, in fact, all peripheral addresses) as volatile, so that the optimizer don't turn this:into this: The volatile keyword tells the compiler that that address is out of its control and can change at any moment.
So change your declarations into this: (probably unneeded for mailboxWrite, but you never know)

Note it's the only thing I checked, there may be other issues I didn't look for.

I've never seen this done before, but I unfortunately have no way of testing it. Why not use a pointer like you did prior, with mailbox write?

Also, I don't think you need to typecast when assigning values to a (dereferenced?) Pointer.

Alas, not home ATM so I can't be much more help.

Pik wrote:You need to declare the mailbox addresses (and, in fact, all peripheral addresses) as volatile, so that the optimizer don't turn this:

Code: Select all

while(*mailboxStatus & MAILBOX_FULL);

into this:

Code: Select all

unsigned int val = *mailboxStatus & MAILBOX_FULL;
while(val);

The volatile keyword tells the compiler that that address is out of its control and can change at any moment.
So change your declarations into this:

Code: Select all

volatile unsigned int *mailboxStatus= (unsigned int *)(0x2000b898);
volatile unsigned int* mailboxWrite = (unsigned int *)0x2000b8a0;

(probably unneeded for mailboxWrite, but you never know)

Note it's the only thing I checked, there may be other issues I didn't look for.

First off, thanks for helping.
I don't have the code at hand right now, but I do believe it is marked as volatile, I didn't copy paste
straight off the code, but extracted the pieces I thought was relevant so might've made a mistake there, my apologies.

@ambient Same as above really, shamefully bad copy paste, it is in a (I believe) volatile variable.

Ah, I see it better now. When I looked at your code initially, I was on my cellular device, which was being fickle (and not wanting to show the entirety of your code box).
This, I believe, is your problem. Bear with me, I haven't used channel 1 for the framebuffer in a while and I don't remember it very well.

According to the tutorial,

So, now that we have our message, we can write code to send it. The communication will go as follows:
Write the address of FrameBufferInfo + 0x40000000 to mailbox 1.
Read the result from mailbox 1. If it is not zero, we didn't ask for a proper frame buffer.
Copy our images to the pointer, and they will appear on screen!

which would make your logic incorrect. You are writing the value of info (which appears to be a structure), when you should be writing the address to your structure. The correct code (should) be this:

Thanks ambivalent, I'll look into your suggestions first thing I do when I get home from work.
If it doesn't work I'll try to clean up my code, reduce the size of it to a minimum and post it so you can see
the entire thing.