From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from frasgout.his.huawei.com (frasgout.his.huawei.com [185.176.79.56]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7D8F72F2D; Tue, 19 Jul 2022 14:47:23 +0000 (UTC) Received: from fraeml738-chm.china.huawei.com (unknown [172.18.147.226]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4LnM822Ntyz67kvY; Tue, 19 Jul 2022 22:45:38 +0800 (CST) Received: from lhreml710-chm.china.huawei.com (10.201.108.61) by fraeml738-chm.china.huawei.com (10.206.15.219) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Tue, 19 Jul 2022 16:47:20 +0200 Received: from localhost (10.81.209.49) by lhreml710-chm.china.huawei.com (10.201.108.61) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Tue, 19 Jul 2022 15:47:20 +0100 Date: Tue, 19 Jul 2022 15:47:18 +0100 From: Jonathan Cameron To: Dan Williams CC: , , , , , "Ben Widawsky" Subject: Re: [PATCH 36/46] cxl/region: Add interleave ways attribute Message-ID: <20220719154718.000077ec@Huawei.com> In-Reply-To: <62cb6f9a74b33_3535162944e@dwillia2-xfh.notmuch> References: <165603869943.551046.3498980330327696732.stgit@dwillia2-xfh> <20220624041950.559155-11-dan.j.williams@intel.com> <20220630144420.000005b5@Huawei.com> <62cb6f9a74b33_3535162944e@dwillia2-xfh.notmuch> Organization: Huawei Technologies Research and Development (UK) Ltd. X-Mailer: Claws Mail 4.0.0 (GTK+ 3.24.29; i686-w64-mingw32) Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.81.209.49] X-ClientProxiedBy: lhreml749-chm.china.huawei.com (10.201.108.199) To lhreml710-chm.china.huawei.com (10.201.108.61) X-CFilter-Loop: Reflected On Sun, 10 Jul 2022 17:32:26 -0700 Dan Williams wrote: > Jonathan Cameron wrote: > > On Thu, 23 Jun 2022 21:19:40 -0700 > > Dan Williams wrote: > > > > > From: Ben Widawsky > > > > > > Add an ABI to allow the number of devices that comprise a region to be > > > set. > > > > Should at least mention interleave_granularity is being added as well! > > Added. > > > > > > > > > Signed-off-by: Ben Widawsky > > > [djbw: reword changelog] > > > Signed-off-by: Dan Williams > > > > Random diversion inline... > > > > > --- > > > Documentation/ABI/testing/sysfs-bus-cxl | 21 ++++ > > > drivers/cxl/core/region.c | 128 ++++++++++++++++++++++++ > > > drivers/cxl/cxl.h | 33 ++++++ > > > 3 files changed, 182 insertions(+) > > > > > diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c > > > index f75978f846b9..78af42454760 100644 > > > --- a/drivers/cxl/core/region.c > > > +++ b/drivers/cxl/core/region.c > > > @@ -7,6 +7,7 @@ > > > > > > > +static ssize_t interleave_granularity_store(struct device *dev, > > > + struct device_attribute *attr, > > > + const char *buf, size_t len) > > > +{ > > > + struct cxl_root_decoder *cxlrd = to_cxl_root_decoder(dev->parent); > > > + struct cxl_decoder *cxld = &cxlrd->cxlsd.cxld; > > > + struct cxl_region *cxlr = to_cxl_region(dev); > > > + struct cxl_region_params *p = &cxlr->params; > > > + int rc, val; > > > + u16 ig; > > > + > > > + rc = kstrtoint(buf, 0, &val); > > > + if (rc) > > > + return rc; > > > + > > > + rc = granularity_to_cxl(val, &ig); > > > + if (rc) > > > + return rc; > > > + > > > + /* region granularity must be >= root granularity */ > > > > In general I think that's an implementation choice. Sure today > > we only support it this way, but it's perfectly possible to build > > setups where that's not the case. > > If the region granularity is smaller than the host bridge interleave > granularity it means that multiple devices per host bridge are needed to > satsify a single "slot" in the interleave. Valid? Yes. Useful for Linux > to support, not clear. True. Wait and see on this one makes sense to me. I only noticed because my older test scripts (against hacks on top of Ben's code) were broken as I did it the silly way :) > > > Maybe the comment should say that this code goes with an > > implementation choice inline with the software guide (that argues you > > will always prefer small ig for interleaving at the host to make best > > use of bandwidth etc). > > No, I would prefer that as far as the Linux implementation is concerned > the software-guide does not exist. In the sense that the Linux > implementation choices supersede and are otherwise a superset of what > the guide recommends. ah. I phrased that badly. I just meant lift the argument as a comment rather than a cross reference. > > Also, for the same reason that the code does not anticipate future > implementation possibilities, neither should the comments. It is > sufficient to just change this comment when / if the implemetation stops > expecting region granularity >= root granularity. > > > Interestingly the code I was previously testing QEMU with > > allowed that option (might have been only option that worked). > > That code was a mixture of Ben's earlier version and my own hacks. > > It probably doesn't make sense to support other ways of picking > > the interleaving granularity until / if we ever get a request > > to do so. > > > > I think it results in a different device ordering. > > > > Ordering with this > > > > Host > > | 4k > > / \ > > / \ > > HB HB 8k > > | | > > / \ / \ > > 0 2 1 3 > > > > Ordering with Larger granularity CFMWS over finer granularity HB > > > > Host > > | 8k > > / \ > > / \ > > HB HB 4k > > | | > > / \ / \ > > 0 1 2 3 > > > > Not clear why you'd do the second one though :) So can ignore for now. > > All I can think of is "ZOMG! My platform failed and the only one I have > to recover my data has HB interleaves with larger granularity than my > failed system!". Otherwise, I expect cross-platform CXL persistent > memory recovery to be so rare as to not need to spend time too much time > worrying about it now. It seems a straightforward constraint to lift at > a later date without any risk to breaking the ABI. It was cross platform that I was thinking but you make a fair point that it is unlikely to occur that often. + If another OS want's to do it wrong that's their problem :) J