[PATCH 2/2] env: add CONFIG_ENV_SECT_SIZE_AUTO

[Mike Looijmans]

Met vriendelijke groet / kind regards,

Mike Looijmans
System Expert


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Please consider the environment before printing this e-mail
On 24-09-2020 13:08, Rasmus Villemoes wrote:
> On 24/09/2020 12.54, Wolfgang Denk wrote:
>> Dear Rasmus,
>>
>> In message <125344f4-2b69-1c7a-0f5a-6d27ab312487 at prevas.dk> you wrote:
>>>>
>>>> On our newer boards with the 4K erase size, saving the environment now
>>>> takes 0.080 seconds instead of 0.53 seconds, which directly translates
>>>> to that much faster boot time since our logic always causes the
>>>> environment to be written during boot.
>>
>> Is this your way of implementing scheduled obsolescence?
> 
> I'll ignore that.
> 
>> Automatically writing the environment on every boot is always a
>> really bad idea. Never ever do that.
> 
> Well, there's really no other place on this piece of hardware to
> implement a boot counter/slot selection. Sure, we could use another part
> of the spi flash and define our own storage format etc., but then we
> might as well use the u-boot environment, which allows us to use
> existing tools/APIs both on the U-Boot and linux side.
> 
> And yes, the number of erase cycles compared to how often this device is
> booted was taken into account when this was decided.

If it's SPI NOR flash, you can make use of the fact that you can change any 1 
into a 0 without erasing the flash sector.

So if you use a unary counter (the number of consecutive zero bits), 
incrementing the counter doesn't require a sector flash.

I once used this for a medical device. After an upgade, the flash would 
boot-count and after 3 failed boots fall back to the previous image. The 
software would mark the image as "good" once booted okay by clearing the 
special "image is okay" bit.

This way there's only one erase cycle for every software upgrade, even with 
the use of a bootcounter.