Re: [RFC/PATCH 7/7] iommu-api: Add domain attribute to enable coherent HTW

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

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-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org [mailto:iommu-
> >> bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org] On Behalf Of Olav Haugan
> >> Sent: Monday, June 30, 2014 10:22 PM
> >> To: linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org; iommu-cunTk1MwBs/ROKNJybVBZg@public.gmane.org
> >> foundation.org
> >> Cc: linux-arm-msm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; will.deacon-5wv7dgnIgG8@public.gmane.org;
> >> thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org; vgandhi-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org
> >> 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.

Will