[RFC 2/3] mtd: spi-nor: core: compare JEDEC bytes to already found flash_info

[Tudor.Ambarus at microchip.com]

Hi,

On 6/22/21 11:58 PM, Rasmus Villemoes wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On 22/06/2021 13.57, Michael Walle wrote:
>> [+ some people from MXIC as they are ones who posted to the ML
>> lately. Feel free to forward this mail to the corresponding people.]
>>
>> Am 2021-06-21 17:23, schrieb Rasmus Villemoes:
>>> Macronix engineers, in their infinite wisdom, have a habit of reusing
>>> JEDEC ids for different chips. There's already one
>>> workaround (MX25L25635F v MX25L25635E), but the same problem exists
>>> for MX25L3205D v MX25L3233F, the latter of which is not currently
>>> supported by linux.
>>>
>>> AFAICT, that case cannot really be handled with any of the ->fixup
>>> machinery: The correct entry for the MX25L3233F would read
>>>
>>>         { "mx25l3233f",  INFO(0xc22016, 0, 64 * 1024,  64, SECT_4K |
>>> SPI_NOR_DUAL_READ | SPI_NOR_QUAD_READ ) },
>>>
>>> while the existing one is
>>>
>>>     { "mx25l3205d",  INFO(0xc22016, 0, 64 * 1024,  64, SECT_4K) },
>>>
>>> So in spi_nor_init_params(), we won't even try reading the sfdp
>>> info (i.e. call spi_nor_sfdp_init_params), and hence
>>> spi_nor_post_sfdp_fixups() has no way of distinguishing the
>>> chips.
>>>
>>> Replacing the existing entry with the mx25l3233f one to coerce the
>>> core into issuing the SPINOR_OP_RDSFDP is also not really an option,
>>> because the data sheet for the mx25l3205d explicitly says not to issue
>>> any commands not listed ("It is not recommended to adopt any other
>>> code not in the command definition table, which will potentially enter
>>> the hidden mode.", whatever that means).
>>
>> Maybe we should ask Macronix if it is safe to send the RDSFDP command.
>> Can anyone from MXIC comment this?
> 
> Yeah, that would be useful to know, but I don't have any hopes
> whatsoever of Macronix engineers being able to help sort out the mess
> they've created by reusing IDs in the first place. They don't seem to
> understand how that can possibly be a problem.
> 
> I, and my client, have contacted them on several occasions to ask how
> we're supposed to deal with that. At one point, the answer was
> "MX25L3233F support Serial Flash Discoverable Parameters (SFDP) mode,
> MX25L3205D does not support.", but when I asked the obvious follow-up
> ("but the MX25L3205D datasheet warns against doing RDSFDP or any other
> not explicitly allowed command"), I got no response.
> 
> Another response was
> 
> "I can only comment on Linux 4.4, as that is the only version that I
> have supporting material for. Basically we have a patch for MTD/SPI-NOR
> (see attached). This is to allow allow the MTD subsystem to cope with
> devices that have the same ID (see below first paragraph of application
> note attached). Please note that the MX25L3205D had an EOL notification
> on 14th May 2010."
> 
> and that attached patch is a 173KB .patch file that made me taste my
> breakfast again.
> 
> And they keep repeating the argument that when a chip is EOL, it's OK to
> reuse its ID (because obviously nobody have used that chip in a product
> that would receive OS updates, so any OS released later than that EOL
> date can just include support for the newer chip and drop the old one...).
> 
>>> In order to support such cases, extend the logic in spi_nor_read_id()
>>> a little so that if we already have a struct flash_info* from the name
>>> in device tree, check the JEDEC bytes against that, and if it is a
>>> match, accept that (device tree compatible + matching JEDEC bytes) is
>>> stronger than merely matching JEDEC bytes.
>>
>> This won't help much without a proper dt schema. No in-tree devicetree
>> could use is because the DT validation would complain.
> 
> I can certainly extend the regexp in jedec,spi-nor.yaml to match this
> new one. DT is supposed to describe the hardware, so I can't see how
> that could possibly be controversial.

No, please don't go that path yet.

> 
>  So if this will
>> go in (and the maintainers are rather hesitant to add it, I tried
>> it myself [1]), you'd also need to add it to jedec,spi-nor.yaml and
>> get an ack from Rob.

I'm not hesitant, I'm keeping my NACK until we're sure there isn't any other way
to differentiate at run-time. I've contacted a macronix representative, asking
if RDSFDP is harmful for MX25L3205D, let's wait for a few days. Maybe Zhengxun or
ycllin in cc can help too. Is there anyone here having a MX25L3205D?

Cheers,
ta

> 
> Thanks for that link. So it seems this isn't the first time recycled IDs
> have come up, and not just for Macronix.
> 
> Yes, vendors shouldn't recycle IDs. They do. They should be punished by
> people not using those chips in new designs. Doesn't work, hardware
> designers do use them. Auto-detection is preferred over using hard-coded
> values from DT. Sure, absolutely, and when the ID is known to be
> ambiguous, by all means throw in all the heuristics and chip-specific
> quirks one can think of to disambiguate. But at the end of the day,
> there are chips out there which cannot be distinguished without help
> from DT, and as DT is supposed to describe the hardware, why is that
> such a big problem?
> 
> And I'm not suggesting any change whatsoever as to how a compatible
> string of merely "jedec,spi-nor" would be handled - it would just take
> the first chip with the read JEDEC id, then apply any appropriate quirks
> and fixups.
> 
> Rasmus
>