Re: [PATCH v3 0/7] Allow setting caching mode in arch_add_memory() for P2PDMA

Logan Gunthorpe
2020-02-27T17:23:03Z

On 2020-02-27 10:17 a.m., Jason Gunthorpe wrote:
>> Instead of this, this series proposes a change to arch_add_memory()
>> to take the pgprot required by the mapping which allows us to
>> explicitly set pagetable entries for P2PDMA memory to WC.
>
> Is there a particular reason why WC was selected here? I thought for
> the p2pdma cases there was no kernel user that touched the memory?

Yes, that's correct. I choose WC here because the existing users are
registering memory blocks without side effects which fit the WC
semantics well.

> I definitely forsee devices where we want UC instead.

Yes. My expectation is that once we have a kernel user that needs this,
we'd wire the option through struct dev_pagemap so the caller can choose
the mapping that makes sense.

Logan