[U-Boot] [PATCH v2] ARM926ejs: Add routines to invalidate D-Cache

Aneesh V
2011-08-05T09:56:47Z

Hi Albert,

On Friday 05 August 2011 02:50 PM, Albert ARIBAUD wrote:
> Hi Aneesh,
>
> On 05/08/2011 09:10, Aneesh V wrote:
>> Hi Hong, Albert,
>>
>> On Friday 05 August 2011 12:16 PM, Albert ARIBAUD wrote:
>>> Le 05/08/2011 08:38, Hong Xu a ?crit :
>>>> Hi Albert,
>>>>
>>>> I've tried to deal with the case that the (start, stop) is not aligned.
>>>> If mis-align happens, the adjacent lines will be cleaned before
>>>> invalidating. And from my view it's impossible for a driver to always
>>>> pass aligned address to invalidate_dcache_range.
>>>
>>> Why would it be impossible? If it is statically allocated it can use
>>> __attribute__(aligned)) and if it is dynamically aligned, it can be
>>> over-allocated to make sure it starts and ends at cache boundaries.
>>>
>>>> To answer your question in another email
>>>>
>>>> > How do you know the dirty data should be flushed rather than
>>>> invalidated?
>>>>
>>>> "Dirty" means this cache is changed by CPU but not has not been written
>>>> into memory or WB. If we invalidate it, data will lost. In most
>>>> cases, I
>>>> do not see a situation why the dirty data shall not be written into
>>>> memory.
>>>
>>> The problem is the cases that fall outside of 'most'. This kind of issue
>>> tends to have effects that, when unwanted, are extremely difficult to
>>> link to their cause and makes for long and painful debugging.
>>>
>>
>> IMHO, Hong's approach is correct. If the buffer that is invalidated is
>> not aligned to cache-line, one cache-line at the respective boundary
>> may have to be flushed to make sure the invalidation doesn't affect
>> somebody else's memory.
>>
>> The solution is for drivers to ensure that any buffer that needs to be
>> invalidated is aligned to cache-line boundary at both ends. The above
>> approach puts this onus on the driver. I have documented the alignment
>> requirement in my recent patch series for fixing arm cache problems.
>
> AIUI, you describe what I requested, so I agree with it. :)
>
> And if there is an alignment requirement, then there is no need any more
> to flush lines when unaligned start/stop is passed, because it would
> only have a use for call cases that do not conform to the requirement.

Yes, but when the alignment requirement is not met by a driver that
does invalidation:

1. Flushing will affect the driver itself.

2. Not flushing, may affect some other code for no fault of theirs.

(1) is clearly better in my opinion as it puts the penalty back on
the offending driver and not on some other code.

So, the flushing solution is a good way to make the drivers behave!

best regards,
Aneesh