Re: [PATCH for-4.4 1/2] mtd: spi-nor: fix Spansion regressions (aliased with Winbond)

[Brian Norris]

On Wed, Mar 30, 2016 at 02:47:48PM +0200, Cyrille Pitchen wrote:
> Hi all,
> 
> [...] 
> > But this is interesting: I see the latest datasheet for Spansion
> > s25fl064k says it supports the Block Protect bits in the Status
> > Register, so presumably *some* version of s25fl064k should support
> > write_sr(nor, 0) to unlock it at boot...
> > 
> > If Felix's initial report is indeed correct, then I think we have:
> > (1) Spansion s25fl064k without Block Protect support (that breaks if you
> >     try to write SR=0)
> > (2) Spansion s25fl064k with Block Protect support (that requires you to
> >     unlock at boot by writing SR=0 (?))
> > (3) Winbond w25q64 with Block Protect support (that requires you to
> >     unlock at boot by writing SR=0)
> > 
> > And (1)-(3) all report the same ID, and (1) is incompatible with (2) and
> > (3). Am I right? Are flash vendors really this insane? Should we all
> > just give up and go home?
> > 
> 
> Just a general remark: maybe reading the JEDEC ID is not a so reliable mean to
> discover SPI flash hardware capabilities at runtime.
> 
> Two weeks ago some Macronix people came to Atmel to present us next evolutions
> of SPI flashes. We took this opportunity to ask them some questions and one of
> them was about memories with different hardware capabilities sharing the very
> same JEDEC ID. One example is Macronix MX25L25635E vs MX25L25673G.
> 
> They explained to us that for Macronix memories, the 2byte product ID is to be
> split into a 1byte code for the memory type and a second byte for the memory
> denstity. For instance:
> C2: Manufacturer ID, Macronix
> 20: Memory Type, SPI NOR flash
> 19: Memory density, 256Mib
> 
> Hence the JEDEC ID only provides information about the memory size and all
> SPI NOR memories of a given size actually share the same JEDEC ID.

That doesn't seem to be true though. There are flash entries for
Macronix with different ID's, but the same density. So maybe they mean
that *some* SPI NOR memories of a given size share the same JEDEC ID?

Anyway, even if they keep this pattern for the first 3 bytes, isn't it
possible for them to include extra bytes to differentiate? Or, you know,
support a standard like SFPD properly. But there are understood (but not
insurmountable) problems with that. (And anyway, SFDP doesn't tell us
all the things we need.)

> Similar cases can also be found with other manufacturers: Micron, Winbond,
> Spansion... 
> 
> Also the Macronix engineers asked us how software applications drive the (Q)SPI
> memories. I answered them that Linux or u-boot use a static table indexed by
> the JEDEC ID, which provides the hardware capabilities. I guess they didn't
> expect software developers to use the JEDEC ID for this purpose.
> Well, it's just a feeling.
> 
> Then the Macronix engineers proposed to use the Serial Flash Discoverable
> Parameter (SFDP) tables to make the difference between memories sharing the
> same JEDEC ID. This might help us in some cases.
> However we should be cautious when using this standard: last year, I've tried
> to discover hardware parameters through these tables when I was working with
> Spansion and Micron memories. I found out the Parameter Table Pointers inside
> the SFDP Header were expressed as byte offset with one memory and as dword
> offset with the other.

Yeah, I noticed this. And I think one or more of them noticed their
error and fixed it in later revs, so you can't depend on a manufacturer
always having the same broken interpretation consistently.

> So I gave up using these tables since some memories diverged from the standard,
> which was "work in progress" at that time.
> 
> Anyway if we cannot completely rely on the SFDP tables we could still use
> DT properties but we should no longer expect to guess all hardware parameters
> from the JEDEC ID alone.

In your conversations, did the vendors actually suggest a practical
method to differentiate flash? Since they've all screwed up SFDP, that's
not going to fly, unless we (e.g.) blacklist certain flash. Anyway, I'd
love to have some basic support for SFDP, even if we have to be
conservative at first. For one, I think it'd be fair to add another
compatible property "jedec,sfdp-vXXX", and then we only use that on
flash that support the actual spec.

Also rather than adding a ton of new DT properties, I think it might
make more sense to just add more DT compatible matches for the actual
part number (they're not going to start re-using part numbers are
they??). That way we don't rely on the DT writer to get every little
detail right, when the driver *should* be able to hold this info.

Brian