[ELDK] mmap: first byte always wrong

Alberto Caballero alberto.kavan at gmail.com
Fri Nov 26 09:14:12 CET 2010

Hi Stefano,

Yes, board flash is NOR-type. I will try to check the flash operation scheme
and also the use of the MTD partitions.



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

> On 11/26/2010 08:28 AM, Alberto Caballero wrote:
> > Hi,
> Hi,
> >
> > At my project, I will have a data configuration file stored in the board
> > Flash memory using an U-Boot filesystem image. At the initialization
> stage,
> > I read the image from the Flash and store it in the Linux filesystem.
> >
> > Of course at the U-Boot prompt I am able to check that image header at
> Flash
> > is correctly stored and all the bytes has the right values. I was
> wondering
> > if this has something to do with first access to mapped memory.
> You can try to read the flash at the required address twice and check if
> the first byte is always wrong. However, with mmap() you are bypassing
> the whole linux flash driver that it is responsible to provide the same
> interface independently from the underlying hardware.
> And again, I suppose you have NOR flash on board, because reading NAND
> flash with mmap simply does not work...
> >
> > When Linux initializes it creates a series of MTD partitions by its own.
> I do not think so. Linux creates partitions as you asked to do. Check
> your DTS file (I assume you are using a recent kernel) and modify the
> partition's size according to your requirement.
> > Now
> > the data in the Flash will be overlapped in two partitions so I will try
> to
> > use just one of them.
> Then it seems you have to adapt the configuration of the partitions.
> >
> > Regarding the mmap usage, I was able to check with another board with
> > another PowerPC processor using ELDK also, and in this case there is no
> > issue with Flash access.
> This does not depend on ELDK. Reading directly the flash chip can work
> on some hardware, but it could be unreliable with some other chips. For
> example, you do not know which was the last operation issued on the
> chip. If the flash driver has sent a CFI-Query, what you read is the
> internal mapping of the chip. Using the MTD subsystem you are sure that
> the flash driver will take care of all internal details.
> Best regards,
> Stefano Babic
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