[coreboot] Discussion about dynamic PCI MMIO size on x86

[Patrick Rudolph]

On 2016-06-06 09:58 PM, ron minnich wrote:
> On Mon, Jun 6, 2016 at 12:52 PM Patrick Rudolph <siro at das-labor.org>
> wrote:
> 
>> To summarize:
>> The easy way is to use 2G.
>> The preferred way would be to mimic mrc behaviour and reboot after
>> finding the correct size.
> 
> I'm not sure it's "easy vs. preferred" so much as 
> - simple that has no known failure cases (yet).
> - best that requires that we have another variable we have to store in
> flash
> 
> At least to me, it's not as cut and dried as easy vs. preferred. The
> preferred way adds a lot of moving runtime parts, and we'll need to
> cover for the case that the stored variable gets damaged somehow. The
> easy way is a runtime constant, which I kind of like. 
> 
> If the "easy" way always works, i.e. we never find a case where it
> causes a problem, then it seems superior to me.
> 
> ron

There's a known failure case.
If someone puts in multiple PCI cards that uses more than 2GB of mmio
it'll break again.
Most users won't reach this limit, but I'm wondering if I'm reaching it
anytime soon.
I want to use 2 PEG cards and the onboard intel card, which will be a
good test-system.

I've read this serveral times and wonder how a stored variable could get
damaged ?
There is a checksum to verify the mrc cache. How should anything go
wrong ?
In case something writes to the flash you might get unexpected behavior
anyway, won't you ?

Regards,
Patrick