Re: [PATCH] mtd: mtdchar: Exit write loop when hitting end of OTP memory

[Christian Riesch <christian.riesch@omicron.at>]

[cc'ed the author of onenand otp support]

On 2013-05-29 15:56, Artem Bityutskiy wrote:
> On Wed, 2013-05-29 at 15:19 +0200, Christian Riesch wrote:
>> The OTP code for the AMD command set in my recent patchset is modeled
>> after the existing code in drivers/mtd/chips/cfi_cmdset_0001.c.
>> Therefore it has a ...walk() function that walks from chip to chip and
>> tries to write as much data as possible into the OTP memories of these
>> chips. Until the last iteration of this loop it does not know how much
>> OTP memory is available. Therefore, a check for insufficient OTP memory
>> and returning an error before writing any data is not possible.
>>
>> Of course I could change my code to obtain the available OTP memory
>> before writing any data. But then the code in cfi_cmdset_0001.c would
>> still suffer from this issue.
>
> Could you please check OneNAND and other drivers which implement OTP and
> see whether they check for space availability?

mtd->_write_user_prot_reg is currently implemented by 
drivers/mtd/chips/cfi_cmdset_0001.c, drivers/mtd/onenand/onenand_base.c, 
and drivers/mtd/devices/mtd_dataflash.c.

mtd_dataflash checks if the offset is larger than 64 and returns -EINVAL 
in this case. If offset <= 64, but offset + len > 64, len is decreased 
to fit into the OTP memory, the number of actually written bytes is 
returned. It therefore suffers from the same issue in mtdchar.c as the 
Intel command set.

onenand seems to do some kind of length check (although I do not fully 
understand why it does not include the offset in 'from' in this check if 
the factory OTP is addressed), but if the data does not fit into the 
memory it returns 0 instead of an error code, resulting in an infinite 
loop as well.

Regards, Christian

>
> On the first glance, I'd say that 0001 should be amended as well. But if
> all OTP writers behave this way, then may be we can document this
> clearly at least somewhere in a commentary.
>