Bare metal progress and an oddity.

Bare metal progress and an oddity.

[Richard Pennington]

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As part of my quixotic quest to run musl on ARM bare metal I've started 
to handle interrupts and an ARM timer to implement the clock_* functions 
to prepare for pre-emptive scheduling of pthread_create'd threads. The 
current sources are here:
http://ellcc.org/viewvc/svn/ellcc/trunk/baremetal/arm/

An oddity: If I use printf() to print out values in an interrupt handler 
it sort of works. The format string prints perfectly, but it seems that 
the va_arg values are getting messed up. The stack is large enough, fp 
and sp are sane. Am I missing something in pop_arg() that is causing 
this? Come to think of it, do I have to insure a certain stack 
alignment, for example? Most of the messed up values are "long long".

Any thoughts appreciated.

-Rich

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Re: Bare metal progress and an oddity.

[Rich Felker]

On Sun, Apr 27, 2014 at 12:37:13PM -0500, Richard Pennington wrote:
> As part of my quixotic quest to run musl on ARM bare metal I've
> started to handle interrupts and an ARM timer to implement the
> clock_* functions to prepare for pre-emptive scheduling of
> pthread_create'd threads. The current sources are here:
> http://ellcc.org/viewvc/svn/ellcc/trunk/baremetal/arm/
> 
> An oddity: If I use printf() to print out values in an interrupt
> handler it sort of works. The format string prints perfectly, but it
> seems that the va_arg values are getting messed up. The stack is
> large enough, fp and sp are sane. Am I missing something in
> pop_arg() that is causing this? Come to think of it, do I have to
> insure a certain stack alignment, for example? Most of the messed up
> values are "long long".
> 
> Any thoughts appreciated.

Yes, ARM EABI requires the stack to be aligned to at least an 8-byte
boundary (maybe 16 but I think 8 is all that's required). If your
stack is only aligned to 4 bytes, this will mess things up badly.

Rich

Re: Bare metal progress and an oddity.

[Richard Pennington]

On 04/27/2014 01:00 PM, Rich Felker wrote:
> On Sun, Apr 27, 2014 at 12:37:13PM -0500, Richard Pennington wrote:
>> As part of my quixotic quest to run musl on ARM bare metal I've
>> started to handle interrupts and an ARM timer to implement the
>> clock_* functions to prepare for pre-emptive scheduling of
>> pthread_create'd threads. The current sources are here:
>> http://ellcc.org/viewvc/svn/ellcc/trunk/baremetal/arm/
>>
>> An oddity: If I use printf() to print out values in an interrupt
>> handler it sort of works. The format string prints perfectly, but it
>> seems that the va_arg values are getting messed up. The stack is
>> large enough, fp and sp are sane. Am I missing something in
>> pop_arg() that is causing this? Come to think of it, do I have to
>> insure a certain stack alignment, for example? Most of the messed up
>> values are "long long".
>>
>> Any thoughts appreciated.
> Yes, ARM EABI requires the stack to be aligned to at least an 8-byte
> boundary (maybe 16 but I think 8 is all that's required). If your
> stack is only aligned to 4 bytes, this will mess things up badly.
>
> Rich

Rich,

You're absolutely right. As I was writing my initial email I thought of 
the stack alignment. It turns out that my idle thread pushed 12 bytes on 
the stack making it unaligned. The idle thread is a leaf function (of 
course) so I guess clang didn't feel the need to keep the stack aligned.
After fixing the alignment, interrupt printf() works fine.

-Rich