Re: [PATCH] mtd: nandbiterrs: Support for NAND biterrors test on platforms without raw write

[Iwo Mergler <iwo.mergler@netcommwireless.com>]

On 05/10/2016 06:48 PM, Boris Brezillon wrote:
> Hi Iwo,
>
> On Mon, 9 May 2016 14:09:46 +1000
> Iwo Mergler <iwo.mergler@netcommwireless.com> wrote:
>
>> Hi Boris,
>>
>>
>> I have to admit that your NACK surprised me.
>>
>> My patch removes an unnecessary use of raw write
>> from the test. It was only there because of my
>> original implementation, which I now consider
>> mistaken.
> Sorry, I didn't look at the diff itself, and focused on the commit
> message :-/. Indeed, using normal write in the overwrite test should be
> harmless, but I still think that all controller should properly
> implement raw access functions, otherwise the "incremental errors"
> test is irrelevant (you'll overwrite ECC bytes along with in-band
> data, and will end up with more bitflips than you expected).
Indeed. Fully agree.


>> I fully agree with you that raw write should
>> be implemented, despite the impediments.
>> Although I have seen at least one NAND controller
>> that always computed and wrote ECC, with no way
>> for software to circumvent it.
> Hm, I was told that so many times and each time I had a closer look it
> appeared to be untrue, so I tend to be skeptical on these kind of
> statement now. Could you tell me more about this controller?
It's been a while. Probably Philips/NXP mobile phone chip
set, PNX8000 or similar. Don't think it ever made it into the
market as such, but the earliest ARM9 'microcontrollers' (LPC3xxx)
have a high likelihood of being the same silicon.

DMA with no provision of software I/O was fashionable in
the early '00s. Patent US8060688 is a reflection of that pain,
despite the attempts at obfuscation by the attorneys. ;-)

>> Could you please elaborate a little why you
>> don't want a test module to work with incomplete
>> MTD drivers?
> As I said, this test module will only work in overwrite mode when the
> controller does not support raw accesses.
>> Is that supposed to be motivating
>> driver writers for better implementations? ;-)
> Yes, partly, and also because it's really helpful when you need to debug
> NAND stuff.
>
> Honestly, I'd rather see NAND implementations return -ENOTSUPP when
> they do not support raw accesses than pretending they are.
I think the Qualcomm driver does that, the original form
of the test fails with an error code on the raw write.

>> Would you accept the patch if I remove the comment
>> about data reshuffling drivers? It's not required
>> for the patch and, as you correctly pointed out,
>> now inaccurate.
> At least rework it to mention that you're only modifying the overwrite
> test, and that writing in normal mode in this case is harmless.
I'll do that. Patch follows.

Best regards,

Iwo