From mboxrd@z Thu Jan 1 00:00:00 1970 From: Logan Gunthorpe Date: Thu, 27 Feb 2020 17:21:50 +0000 Subject: Re: [PATCH v3 0/7] Allow setting caching mode in arch_add_memory() for P2PDMA Message-Id: List-Id: References: <20200221182503.28317-1-logang@deltatee.com> <20200227171704.GK31668@ziepe.ca> In-Reply-To: <20200227171704.GK31668@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Jason Gunthorpe Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Peter Zijlstra , Catalin Marinas , Dave Hansen , platform-driver-x86@vger.kernel.org, linux-mm@kvack.org, Will Deacon , Christoph Hellwig , linux-s390@vger.kernel.org, David Hildenbrand , Ingo Molnar , Benjamin Herrenschmidt , Dan Williams , Borislav Petkov , Andy Lutomirski , Thomas Gleixner , Michal Hocko , linux-arm-kernel@lists.infradead.org, Eric Badger , linux-kernel@vger.kernel.org, Andrew Morton , linuxppc-dev@lists.ozlabs.org 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