RE: Accessing flash directly from User Space [SOLVED]

[Kenneth Johansson <kenneth@southpole.se>]

On Thu, 2009-10-29 at 10:00 +0100, Joakim Tjernlund wrote:
> >
> > > > On Tue, Oct 27, 2009 at 04:24:53PM -0600, Jonathan Haws wrote:
> > > > >> >>> How can I get that pointer?  Unfortunately I cannot simply
> > > > use
> > > > >> the
> > > > >> >>>
> > > > >> >> address of the flash.  Is there some magical function call
> > > > that
> > > > >> >> gives me access to that portion of the memory space?
> > > > >> >>
> > > > >> >> $ man 2 mmap
> > > > >> >>
> > > > >> >> You want MAP_SHARED and O_SYNC.
> > > > >> >>
> > > > >> >
> > > > >> >
> > > > >> > To use that I need to have a file descriptor to a device, do
> > > I
> > > > >> not?  However, I do not have a base flash driver to give me
> > > that
> > > > >> file descriptor.  Am I missing something with that call?
> > > > >> >
> > > > >>
> > > > >> /dev/mem
> > > > >>
> > > > >Okay, I now have access to the flash memory, however when I write
> > > > to it the writes do not take.  I have tried calling msync() on the
> > > > mapping to no avail.  I have opened the fd with O_SYNC, but cannot
> > > > get things to work right.
> > > > >
> > > > >Here are the calls:
> > > > >
> > > > >   int fd = open("/dev/mem", O_SYNC | O_RDWR);
> > > > >   uint16_t * flash = (uint16_t *)mmap(NULL, NOR_FLASH_SIZE,
> > > > >         (PROT_READ | PROT_WRITE), MAP_PRIVATE, fd,
> > > > >         NOR_FLASH_BASE_ADRS);
> > > >
> > > > What board and CPU are you using?  Is your flash really at
> > > > 0xFC800000, or is
> > > > that the virtual address that VxWorks puts it at?
> > >
> > > I am using a custom board based on the AMCC Kilauea development
> > > board.  It uses a 405EX CPU.  Yes, the flash is really at
> > > 0xFC000000.
> >
> > I have found the problem.  It occurred to me in the shower (okay not really,
> > but most good ideas happen there).
> >
> > What was happening is that I was in fact able to write to the correct
> > registers.  However, I would try and write to them in a batch.  But the way
> > mmap works (at least according to the man page) with MAP_SHARED is that the
> > file may not be updated until msync() is called.  Now, I thought that O_SYNC
> > would take care of that when I open /dev/mem, but that was not the case.
> >
> > Anyway, to make a long story short, I inserted an msync() after each
> > assignment to the flash.  This resolved my problem and I can now program my flash.
> 
> Ouch, this was news to me too. Calling msync() after every write kills performance.
> We use mmap(/dev/mem) to access HW and havn't seen any issues yet. Is this
> perhaps a new behaviour for mmap(/dev/mem) and is there a way
> to avoid calling msync()?

The address range should be outside the dram and thus uncached. Any
write to any address in the range mmaped should go directly to the NOR
flash. Any other behavior is a bug. It's not mapping an actual file
here.