On 7/3/2014 10:43 AM, Will Deacon wrote: > On Wed, Jul 02, 2014 at 11:11:13PM +0100, Olav Haugan wrote: >> On 7/1/2014 1:49 AM, Varun Sethi wrote: >>> >>> >>>> -----Original Message----- >>>> From: iommu-bounces@xxxxxxxxxxxxxxxxxxxxxxxxxx [mailto:iommu- >>>> bounces@xxxxxxxxxxxxxxxxxxxxxxxxxx] On Behalf Of Olav Haugan >>>> Sent: Monday, June 30, 2014 10:22 PM >>>> To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; iommu@lists.linux- >>>> foundation.org >>>> Cc: linux-arm-msm@xxxxxxxxxxxxxxx; will.deacon@xxxxxxx; >>>> thierry.reding@xxxxxxxxx; vgandhi@xxxxxxxxxxxxxx >>>> Subject: [RFC/PATCH 7/7] iommu-api: Add domain attribute to enable >>>> coherent HTW >>>> >>>> Add a new iommu domain attribute that can be used to enable cache >>>> coherent hardware table walks (HTW) by the SMMU. HTW might be supported >>>> by the SMMU HW but depending on the use case and the usage of the SMMU in >>>> the SoC it might not be always beneficial to always turn on coherent HTW >>>> for all domains/iommu's. >>>> >>> [Sethi Varun-B16395] Why won't you want to use the coherent table walk feature? >> >> Very good question. We have found that turning on IOMMU coherent HTW is >> not always beneficial to performance (performance either the same or >> slightly worse in some cases). Even if the perf. is the same we would >> like to avoid using precious L2 cache for no benefit to the IOMMU. >> Although our HW supports this feature we don't always want to turn this >> on for a specific use case/domain (bus master). > > Could we at least invert the feature flag, please? i.e. you set an attribute > to *disable* coherent walks? I'd also be interested to see some performance > numbers, as the added cacheflushing overhead from non-coherent walks is > going to be non-trivial. > Yes, agree that we can do the inverse. On one SoC I saw about 5% degradation in performance with coherent table walk enabled for a specific bus master. However, we have seen improved performance also with other SMMUs/bus masters. It just depends on the SMMU/bus master and how it is being used. Hence the need to be able to disable this on a per-domain basis. Thanks, Olav Haugan -- The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
