Re: [Xenomai] ARM bus error due to alignment

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On Fri, Nov 06, 2015 at 02:25:57PM -0500, Lennart Sorensen wrote:
> On Thu, Nov 05, 2015 at 05:08:51PM -0500, Lennart Sorensen wrote:
> > On Thu, Nov 05, 2015 at 10:40:42PM +0100, Gilles Chanteperdrix wrote:
> > > On Thu, Nov 05, 2015 at 12:52:25PM -0500, Lennart Sorensen wrote:
> > > > I wish I could figure out why the backtraces aren't working, since they
> > > > are such a nice debug feature to have.
> > > 
> > > You can try disabling unwinding, this should enable frame pointers
> > > and maybe make the backtraces more reliable.
> > 
> > I can try that.  I always thought unwinding was part of making backtraces
> > work.  Not sure why I thought so.
> 
> So I removed H_DMA from the alloc cal and the alignment problem went away.
> Never actually did need the DMA flag in the case anyhow so it shouldn't
> have been there in the first place.
>  Now while poking at it, I did find
> that using H_NONCACHED instead causes a NULL pointer crash in the kernel,
> which seems like a bug (how can user space making a system call cause
> a NULL pointer dereference in the kernel unless there is a bug?).
> With 0 for the flags, it just works.


Well, mapping the same piece of RAM with cacheable and non-cacheable
mappings on ARM is "undefined behaviour" territory. If you want to
map some RAM with H_NONCACHED, you should reserve it with something
like the CMA allocator so that the kernel does not map it in any
other way. That is one possible reason.

H_NONCACHED is routinely used on armv4/armv5 (actually, I mixed
H_DMA with H_NONCACHED, Xenomai core never sets H_DMA, it sets
H_NONCACHED for shared user/kernel mappings for armv4/armv5, because
this is required with these architectures). So, it works. However,
that maybe an issue with LPAE, that would be a second possible reason.

-- 
					    Gilles.
https://click-hack.org