linux-kernel - Re: [PATCH 0/6] iommu: Enable devices to request non-strict DMA, starting with QCom SD/MMC

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Date:   Thu, 24 Jun 2021 10:23:05 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Clark <robdclark@...omium.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        linux-pci@...r.kernel.org, quic_c_gdjako@...cinc.com,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Sonny Rao <sonnyrao@...omium.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Linux MMC List <linux-mmc@...r.kernel.org>,
        Veerabhadrarao Badiganti <vbadigan@...eaurora.org>,
        Rajat Jain <rajatja@...gle.com>,
        Saravana Kannan <saravanak@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andy Gross <agross@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] iommu: Enable devices to request non-strict DMA,
 starting with QCom SD/MMC

Hi,

On Wed, Jun 23, 2021 at 10:29 AM Doug Anderson <dianders@...omium.org> wrote:
>
> * Instead of putting the details in per-device nodes, maybe it makes
> sense to accept that we should be specifying these things at the IOMMU
> level? If specifying things at the device tree level then the
> device-tree node of the IOMMU itself would just have a list of things
> that should be strict/non-strict. ...this could potentially be merged
> with a hardcoded list of things in the IOMMU driver based on the IOMMU
> compatible string.
>
> Do those sound right?
>
> I still haven't totally grokked the ideal way to identify devices. I
> guess on Qualcomm systems each device is in its own group and so could
> have its own strictness levels? ...or would it be better to list by
> "stream ID" or something like that?
>
> If we do something like this then maybe that's a solution that could
> land short-ish term? We would know right at init time whether a given
> domain should be strict or non-strict and there'd be no requirements
> to transition it.

OK, so I have attempted to implement this in the Qualcomm IOMMU driver
in v2 of this series:

https://lore.kernel.org/r/20210624171759.4125094-1-dianders@chromium.org/

Hopefully that doesn't fragment the discussion too much, but it seemed
like it might help move us forward to see what this would look like in
code.

I'll also note that I removed a few people from the CC list on v2 of
the series because I'm no longer touching any code outside of the
IOMMU subsystem and I thought folks would appreciate less noise in
their inboxes. I've CCed a boatload of mailing lists though so it
should be easy to find. If I dropped you from the CC list of v2 and
you really want back on then I'm more than happy to re-add you.

-Doug