Re: [PATCH 28/74] Incrementing the ecc_pos array to contain 128 char

[Vipin Kumar <vipin.kumar@st.com>]

On 9/1/2010 4:15 PM, Artem Bityutskiy wrote:
> On Wed, 2010-09-01 at 09:43 +0530, Vipin Kumar wrote:
>> On 9/1/2010 5:06 AM, Artem Bityutskiy wrote:
>>> Hi,
>>>
>>> On Tue, 2010-08-31 at 12:04 +0530, Vipin Kumar wrote:
>>>>> Nack, breaking ABI Is not allowed in Linux.
>>>> I could not understand your point. Can you please elaborate. How does this patch 
>>>> break ABI
>>>
>>> You are changing data structure (struct nand_ecclayout) used for in MTD
>>> ioctl. Tha ioctl is part of the Linux ABI. By changing the data
>>> structure, you are breaking the ABI. This means that current binaries
>>> would stop working with newer versions of the Linux kernel if we'd
>>> accept your patch.
>>>
>> Hello,
>>
>> The only change that I have made is increasing the number of bytes to keep ecc. 
> 
> Right, but this break ABI.
> 
>> Since the ecc is generally kept in spare area, it makes sense to have the ecc 
>> locations to be equal to the maximum spare area possible.
> 
> May be.
> 
>> A NAND page with a page size of 4096 would contain a spare area of 128 bytes. 
>> Now, ecc for the page can be less/more than 64 bytes(currently allocated for 
>> ecc positions) depending on the algorithm used to generate ecc. 
>> Incidently, in our case the ecc can fit in 104 bytes and this is still quite 
>> logical to place it in spare area since the linux image supports 4096 page but 
>> the problem is that the ecc locations supported by linux are less than the 
>> practically possible scenario so in effect this change is an improvement in linux
> 

Hello David/Artem,

I got the point, but this change is essential (at least for me). It may be 
essential to others as well in near future. Please let me know how to handle more 
than 64 bytes of ecc

> Yes, this is historical and a bit unfortunate, but you cannot break ABI
> even if you have reasons like that
> 
>> Please let me know if you disagree
> 
> Please, create an app which uses 'struct nand_ecclayout' and compile it
> against the old headers. Check that it works. Then do you kernel
> modification and run the same program (without re-compiling) and my
> prediction is that it won't work. This is what I call ABI breaking which
> is disallowed.
> 

Regards
Vipin