[PATCH] arm64: Document requirements for fine grained traps at boot

[Mark Brown]

On Mon, Mar 29, 2021 at 11:31:27AM +0100, Will Deacon wrote:

> One thing I don't understand about the registers listed here is that we're
> requiring firmware to initialise them when the kernel is entered at EL2. But
> they're *_EL2 registers, so why can't the kernel initialise them itself? The
> fewer dependencies on firmware, the better.

Well, there's nothing stopping us doing both - that's the most robust
thing.  Relying on the kernel alone to do this has the old kernels on
new systems issue, and there's always going to be some section of code
running before we manage to go through and enumerate features then reset
the system registers even if we run that code very early.  With
something like traps it seems especially relevant, it's more of a "could
affect anything" kind of thing than something where we can initialize
the feature before we start using it.
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