RFC ARM core support for hardware I/O coherency in non-SMP platforms
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Cover letter [RFC PATCHv1 0/7] ARM core support for hardware I/O coherency in non-SMP platforms
Quoted 7 lines · Thomas Petazzoni
Quoted 5 lines · Rob Herring
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This hardware I/O coherency mechanism needs a set of ARM core requirements to operate properly:
* On Armada 370 (a single core processor)
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- The cache policy of pages must be set to "write allocate".
Why do we want !SMP to be no write allocate in the first place? Seems like we should always enable write-allocate at least for v7.
Ok, seems like it matches the suggestion from Catalin. I've sent a quick patch in my reply to Catalin, if you could have a look and tell if it's OK, I can use that in my next version.
It's probably also worth mentioning in your commit log that modern ARM CPUs can change the allocation policy dynamically based on the access pattern (e.g. switch to no-allocate when there is a series of streaming stores), so historical reasons for forcing write-no-allocate aren't applicable on these cores.
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I really don't see any justification for why it some situations the bootloader would be responsible for it, and why in some other situations the kernel would be responsible for it.
Well, for better or worse, we are moving in the direction of Linux running non-secure with a non-trivial amount of higher privileged software running in the system. We can use this as a basis to decide whether or not Linux should be setting configuration bits by looking at whether or not these bits are accessible from non-secure svc mode.
I'm not denying that we've not been following this rule in the past, but given where we're going with ARMv8 and arm64, the sooner people realise that the firmware and bootloader have some duties in the way of system configuration, then the less pain they will have later on when they try to boot Linux.
Will