[coreboot] Flash ROM access during boot

[Andrey Petrov]

Hi,

On 01/21/2017 02:30 PM, Paul Menzel via coreboot wrote:
> Dear coreboot folks,
>
>
> Playing around with the trace feature of the Dediprog EM100Pro, I
> noticed several flash ROM accesses until the payload is loaded.
>
> Are there ways or strategies to preload the whole flash ROM chip
> content into memory for faster access right after RAM is set up for
> example? What does that depend on? Does that make any sense at all?

preloading whole flash is a bad idea, because you have to pay upfront 
IO cost for whole-flash read. And then most of that is going to be 
wasted anyways, because you likely need only parts of the flash at that 
point.

On Apollolake at least, SPI hardware sequencer has some internal cache 
and when combined with regular CPU cache (just set MTRRs to cover memory 
mapped SPI flash) seems to work effectively. There was an issue we found 
recently where ramstage never cached mmaped BIOS area but that was 
addressed swiftly.

What you could do is to pre-populate cache with flash data right before 
it is going to be used. So you could read just a byte from each page of 
memory-mapped payload, and cause spi hardware to read the whole page in 
'background' due to work of prefetchers. This may be useful if you are 
at the last stages of ramstage and doing PCI device IO and 
waiting/spinning. So by the time you want to load payload it is already 
"preloaded" in the cache.

However on apollolake grand total for IO is less than 100ms and even 
less for payload so I suspect benefits from such hack are going to be 
pretty small.

Andrey