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

[Rasmus Villemoes]

On 23/06/2021 08.46, Tudor.Ambarus at microchip.com wrote:
> 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. 

It seems that we have established that by now, right?

Rasmus