Flash filesystem questions

[Ricard Wanderlof]

On Fri, 16 Jan 2015, Erik Ekman wrote:

> It does not use the nand layer at all.
> ...
> The device does not give access to the raw nand. The driver creates transfer
> commands by writing address, number of sectors and a dma bus address
> of the source/destination data to a buffer. The bus address of that command
> buffer is then written to a register on the PCI device and an interrupt is
> received when the transfer is done. Had it not been for the erase command
> and some restrictions on sector count and alignment would it have been a
> perfect block device interface.
> 
> The OOB data/ECC handling is hidden, and the only operations
> possible are read, write and erase of sectors. This fits well with the
> MTD interface (struct mtd_info) so I was planning to add my driver to
> /drivers/mtd/devices instead of under the /drivers/mtd/nand directory more
> commonly used for nand controllers.

I see.

> I have looked at nand.h and its structs but since I cannot get access to the
> raw NAND chips I didn't feel those interfaces helped me in any way.

Well, one advantage to using the NAND layer is that you'd get access to 
the NAND BBT management. However, to scan the device it needs to have 
access to the OOB to read the bad block markers. If OOB data is hidden, 
how do you determine if a block is bad? In a previous post you said that 
read returns no error when reading a bad block, but that write and erase 
do. Does that mean that the only way to determine if a block is bad is by 
attempting to write or erase it?

I suppose one way to handle this would be to implement a fake oob read 
function which would always return an all-erased (i.e. good block) state. 
As time goes and writes and erase operations fail, the block can be marked 
bad on the fly, and the information kept in a BBT.

Your other suggestion of implementing a general BBT functionality which is 
not NAND-dependent would work too, but it seems like a lot of work to 
acheive what can be done in other ways, especially if no other mtd related 
technology needs a BBT so there would basically only be your use case.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30