Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC

[Robin Murphy]

On 2021-07-08 09:08, Joerg Roedel wrote:
> On Wed, Jul 07, 2021 at 01:00:13PM -0700, Doug Anderson wrote:
> > a) Nothing is inherently broken with my current approach.
> >
> > b) My current approach doesn't make anybody terribly upset even if
> > nobody is totally in love with it.
>
> Well, no, sorry :)
>
> I don't think it is a good idea to allow drivers to opt-out of the
> strict-setting. This is a platform or user decision, and the driver
> should accept whatever it gets.
>
> So the real question is still why strict is the default setting and how
> to change that.

It's occurred to me whilst hacking on the relevant area that there's an 
important point I may have somewhat glossed over there: most of the 
IOMMU drivers that are used for arm64 do not take advantage of 
non-strict mode anyway. If anything it would be detrimental, since 
iommu-dma would waste a bunch of time and memory managing flush queues 
and firing off the batch invalidations while internally the drivers are 
still invalidating each unmap synchronously.

Those IOMMUs in mobile and embedded SoCs are also mostly used for media 
devices, where the buffers are relatively large and change relatively 
infrequently, so they are less likely to gain significantly from 
supporting non-strict mode. It's primarily the Arm SMMUs which get used 
in the more "x86-like" paradigm (especially in larger systems) of being 
stuck in front of everything including networking/storage/PCIe/etc. 
where the workloads are far more varied.

Robin.

> Or document for the users that want performance how to
> change the setting, so that they can decide.
>
> Regards,
>
> 	Joerg
>
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