Re: [Yaffs] Power down during erase

On 09/08/2011 11:42 PM, Charles Manning wrote:
[...]
> YAFFS is designed for NAND flash. NAND typically erases in a few milliseconds.
> There is typically sufficient power rail capacitance to sustain the flash
> chip during the erasure.

Thus if I use YAFFS with a NAND flash, then I have to ensure that an erase
operation completes even in case of a power down or fatal software errors.

>
> Where YAFFS is used on NOR then an erasure marker strategy is suggested. There
> is an implementation of this in yaffs_norif1.c

Thanks for the hint. This is a good example. What is the reason for the

#define YNOR_PREMARKER          (0xF6)
#define YNOR_POSTMARKER         (0xF0)

values? Are they somewhat random or do they exploit some physical properties
of the NOR flash?

Lets have a look at the following check:

                 /* If the page status is YNOR_POSTMARKER then it was written 
properly
                  * so change that to 0xFF so that the rest of yaffs is happy.
                  */
                 if(spare->page_status == YNOR_POSTMARKER)
                         spare->page_status = 0xFF;
        else if(spare->page_status != 0xff &&
            (spare->page_status | YNOR_PREMARKER) != 0xff)
            spare->page_status = YNOR_PREMARKER;

The first part is clear to me, but I don't understand the second part. It was
added later with the following commit message:

"Extra page status checking to combat power loss causing only one bit to change"

Since the one bit count is used to determine if a chunk was deleted, I suppose
that this was added to cope with 0xf7 or 0xfe instead of 0xf6.

What is the reason for the "(spare->page_status | YNOR_PREMARKER) != 0xff"? Is
this wrong:

                 if(spare->page_status == YNOR_POSTMARKER)
                         spare->page_status = 0xFF;
        else if(spare->page_status != 0xff)
            spare->page_status = YNOR_PREMARKER;

?

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Sebastian Huber, embedded brains GmbH

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