[ELDK] mmap: first byte always wrong

Alberto Caballero alberto.kavan at gmail.com
Sat Nov 27 17:33:08 CET 2010

Hi Stefano,

I am trying to read the Flash using the predefined MTD partitions. Although
my board it is not the Sequoia one, the Memory Map for the NOR Flash memory
is the same.

I am using the partitions scheme defined at the drivers/mtd/maps/sequoia.c
file, although I will update it with my own scheme.

After booting the system, only "mtd" and "mtd1" are present at /dev folder.
As far as I have found at the Internet, I can use a filesystem over those
partitions or use them as a character device. But I have not found yet how
to create a device for the MTD partitions. Should I have to create a new
character device at /dev folder and then mount the MTD partitions?

If I try the provided command "dd if=/dev/mtd1 | hexdump -C"I get a kernel
panic error, so this mtd1 device is not correct.



2010/11/26 Stefano Babic <sbabic at denx.de>

> On 11/26/2010 09:16 AM, Alberto Caballero wrote:
> > Hi,
> >
> Hi,
> > Just a clarification: I used U-Boot to store the image at the Flash
> memory.
> > Then, using the U-Boot command line I am able to see the contents of the
> > memory.
> At this point we are sure that the flash has the correct contents and
> can be accessed. However, we cannot make any assumptions about what
> happens in linux.
> > So the U-Boot operation has nothing to do with any memory mapping.
> Right. The kernel driver is then responsible to work with the flash.
> Surely the flash is accessed by MTD when the system boots to recognize
> the chip type, and at least the CFI-Query command is sent.
> > This is just a way of checking that the bytes are there directly
> accessing
> > the physical memory.
> >
> > Although the EP440xS board is currently unsupported, I cannot understand
> why
> > the access to the mmap returned pointer is wrong. Is there a particular
> mmap
> > code for each board type? Would it have something to do with issuing
> > commands to the flash memory?
> You are thinking that your flash memory works exactly as RAM, and even
> with NOR flash this is not always the case. As example, I already told
> you that if the MTD driver let the flash in a different status as result
> of the CFI-Query command, you do not read the flash contents. And you
> are not taking care that the flash cannot be accessed if another
> operation (for example, erasing sectors: do you have jffs2 on another
> partition ?) is running. The MTD take care with a mutex on a whole chip
> if there are operations that do not allow to access the chip, and the
> calling process is blocked until the chip becomes available. Maybe there
> is a race condition, or maybe support for your board is broken. Reading
> the flash via MTD you can check that.
> Accessing the flash with mmap on /dev/men is a fast way to get into
> trouble, as you have two actors (your code and the MTD driver) working
> on the same physical resource, maybe at the same time.
> Best regards,
> Stefano Babic
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