[flashrom] [PATCH] Check if an erase is needed to write

[Stefan Reinauer]

Carl-Daniel Hailfinger wrote:
> On 16.09.2009 19:07, Stefan Reinauer wrote:
>   
>> Carl-Daniel Hailfinger wrote:
>>   
>>     
>>> For optimal partial reflashing, we have to find out which parts of the
>>> chip can be written without erase. For that, the only criterion (except
>>> a limit on the number of writes for very old chips) is whether the write
>>> will only clear bits (set them to 0).
>>> If (current&new==new) we can skip the erase.
>>> If any bit would have to be set to 1, we need to erase.
>>>
>>>   
>>>     
>>>       
>> Is that sufficient? Ie is it always ok to skip an erase if we're only
>> clearing bits?
>>   
>>     
>
> It depends on the chip, but given that the following appears in the ICH7
> and all later datasheets as absolute flash requirement, I'm positive
> most current flash chips support it.
>
> -----------------
> The system BIOS and Intel® Active Management Technology firmware usage
> models
> require that the serial flash device support multiple writes (minimum of
> 512 writes) to
> a page (256 bytes) without requiring a preceding erase command.
> -----------------
>   
That means you can theoretically write it twice (2*256 byte writes on an
x8 device) without erasing it.
I don't think this is something we should rely on.

It also says serial flash device, which implies that LPC/FWH might
behave differently.

Stefan