Re: [PATCH] mtd: spi-nor: read 6 bytes for the ID

[Huang Shijie <shijie8@gmail.com>]

On Tue, Apr 15, 2014 at 03:35:05PM +0200, Marek Vasut wrote:
> On Tuesday, April 15, 2014 at 07:22:39 AM, Huang Shijie wrote:
> > On Mon, Apr 14, 2014 at 08:23:47PM +0200, Marek Vasut wrote:
> 
> [...]
> 
> > > > > I wonder if the ID-bytes wraparound cannot cause us trouble here. For
> > > > > example if we try to detect a SPI NOR which has 5-byte ID code, but
> > > > > in the table, we'd also have a SPI NOR with has a 6-byte code where
> > > > > the last byte of ext-jedec matches the first byte of JEDEC ID , this
> > > > > would actually match on the later.
> > > > 
> > > > could you give me detail example?
> > > > 
> > > > I feel sorry that i do not quit understand your meaning.
> > > 
> > > Imagine two chips with two IDs:
> > > Chip 1 has IDs: 0xf00b42 0x4242f0 and readID[6] returns 0x420bf0f04242
> > 
> > It will not return 0x420bf0f04242.
> > 
> > The readID[6] should be: f0, 0b, 42, 42, 42, f0.
> > 
> > > Chip 2 has IDs: 0xf00b42   0x42f0 and readID[6] returns 0x420bf0f04242
> > 
> > the readID[6] should be: f0, 0b, 42, 42, f0, XX.
> > 
> >  "XX" stands for the sixth byte.
> > 
> > The current patch can distinguish these two chips.
> > 
> > > This is because in the second chips' case the ID wraps around at 5 bytes.
> > > But chip #1 matches the ID, so if chip #1 is earlier in the list of SPI
> > > NOR flashes, we will get an incorrect detection of that chip.
> > 
> > I guess your meaning is that the chip 2 has IDs: 0xf00b42   0x4242
> > and the sixth byte is 0xf0 which wraps the first byte.
> 
> Huang, what I meant is that if you read 6 bytes of ID from a chip which wraps 
> the READID command output on 5 bytes AND the first and last byte match in the 
> table for some 6-byte chip, then this 6-byte chip will be used as a 
> configuration for the different 5-byte chip.

Does the chip vendor so silly to produce such chips? :)

> 
> This code should be future-proof, but if we keep adding such special cases, we 
> will end up with false matches sooner or later anyway I'm afraid.
> 
> What do you say we add the READID length field into the table ?
If we add the length field into the table, we have to sort the table by
some kind of order. Btw: I do not object to add the length field. 

thanks
Huang Shijie