[RFC/PATCH 1/5] mtd: ubi: Read disturb infrastructure

[Tanya Brokhman]

Hi Richard,

Sorry it took me some time to answer, got per-occupied with some urgent 
staff.

On 9/28/2014 1:54 PM, Richard Weinberger wrote:
> Am 28.09.2014 12:46, schrieb Tanya Brokhman:
>> On 9/28/2014 11:54 AM, Richard Weinberger wrote:
>>> Am 28.09.2014 10:48, schrieb Tanya Brokhman:
>>>>>> @@ -424,6 +440,8 @@ struct ubi_fm_sb {
>>>>>>         __be32 used_blocks;
>>>>>>         __be32 block_loc[UBI_FM_MAX_BLOCKS];
>>>>>>         __be32 block_ec[UBI_FM_MAX_BLOCKS];
>>>>>> +    __be32 block_rc[UBI_FM_MAX_BLOCKS];
>>>>>> +    __be64 block_let[UBI_FM_MAX_BLOCKS];
>>>>>
>>>>> Doesn't this break the fastmap on-disk layout?
>>>>
>>>> What do you mean "break"? I verified fastmap feature is working. the whole read-disturb depends on it so I tested this thoroughly.
>>>
>>> Did you write a fastmap with your changes applied and then an attach using a fastmap implementation *without*
>>> you changes?
>>> I bet it will not work because the disk layout is now different.
>>
>> you're right, it wont work. I did a set of attach/detach tests to verify fastmap, but of course with my changes.
>>
>>> Linux is not the only user of fastmap. We need to be very careful here.
>>
>> Could you please elaborate here? I'm not sure I understand the use case you're referring to.
>
> Consider the case where you have a board with a fastmap enabled bootloader and a Linux OS.
> The bootloader does a fastmap attach and boots the kernel from UBI and the kernel it self has the rootfs
> on UBI too. If you install a new kernel with your changes applied it will write the fastmap in a different
> format and the bootloader will fail badly. In worst case the board bricks, in best case the bootloader can fall back
> to scanning mode but it will be slow and the customer unhappy.
>

Ok, I understand the problem now. I wanted to discuss a possible 
solution before implementing it:
We have a "fastmap version" in fm_sb. At the moment UBI_FM_FMT_VERSION = 
1 and any other is not supported. We can use that; Add another fm 
version (UBI_FM_FMT_VERSION_RD = 2) and then decide according to it. 
Meaning, if during attach process we find fm superblock we check it's 
version, if it's != UBI_FM_FMT_VERSION_RD, we fall back to full scan. 
The next fastmap will be written with the new layout (and new version 
number) so second boot will attach from fastmap without any issues.

>
> Thanks,
> //richard
>
>
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Thanks,
Tanya Brokhman
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