From: Thierry Reding <thierry.reding@gmail.com>
To: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org, Frank Rowand <frowand.list@gmail.com>,
Robin Murphy <robin.murphy@arm.com>,
linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
Will Deacon <will@kernel.org>
Subject: Re: [PATCH v2 1/4] dt-bindings: reserved-memory: Document "active" property
Date: Thu, 24 Sep 2020 13:27:25 +0200 [thread overview]
Message-ID: <20200924112725.GA2486709@ulmo> (raw)
In-Reply-To: <20200915123648.GA3496938@ulmo>
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On Tue, Sep 15, 2020 at 02:36:48PM +0200, Thierry Reding wrote:
> On Mon, Sep 14, 2020 at 04:08:29PM -0600, Rob Herring wrote:
> > On Fri, Sep 04, 2020 at 02:59:57PM +0200, Thierry Reding wrote:
> > > From: Thierry Reding <treding@nvidia.com>
> > >
> > > Reserved memory regions can be marked as "active" if hardware is
> > > expected to access the regions during boot and before the operating
> > > system can take control. One example where this is useful is for the
> > > operating system to infer whether the region needs to be identity-
> > > mapped through an IOMMU.
> >
> > I like simple solutions, but this hardly seems adequate to solve the
> > problem of passing IOMMU setup from bootloader/firmware to the OS. Like
> > what is the IOVA that's supposed to be used if identity mapping is not
> > used?
>
> The assumption here is that if the region is not active there is no need
> for the IOVA to be specified because the kernel will allocate memory and
> assign any IOVA of its choosing.
>
> Also, note that this is not meant as a way of passing IOMMU setup from
> the bootloader or firmware to the OS. The purpose of this is to specify
> that some region of memory is actively being accessed during boot. The
> particular case that I'm looking at is where the bootloader set up a
> splash screen and keeps it on during boot. The bootloader has not set up
> an IOMMU mapping and the identity mapping serves as a way of keeping the
> accesses by the display hardware working during the transitional period
> after the IOMMU translations have been enabled by the kernel but before
> the kernel display driver has had a chance to set up its own IOMMU
> mappings.
>
> > If you know enough about the regions to assume identity mapping, then
> > can't you know if active or not?
>
> We could alternatively add some property that describes the region as
> requiring an identity mapping. But note that we can't make any
> assumptions here about the usage of these regions because the IOMMU
> driver simply has no way of knowing what they are being used for.
>
> Some additional information is required in device tree for the IOMMU
> driver to be able to make that decision.
Rob, can you provide any hints on exactly how you want to move this
forward? I don't know in what direction you'd like to proceed.
Thierry
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next prev parent reply other threads:[~2020-09-24 11:27 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-04 12:59 [PATCH v2 1/4] dt-bindings: reserved-memory: Document "active" property Thierry Reding
2020-09-04 12:59 ` [PATCH v2 2/4] iommu: Implement of_iommu_get_resv_regions() Thierry Reding
2020-09-04 12:59 ` [PATCH v2 3/4] iommu: dma: Use of_iommu_get_resv_regions() Thierry Reding
2020-09-04 13:00 ` [RFC 4/4] iommu/tegra-smmu: Add support for reserved regions Thierry Reding
2020-09-14 22:08 ` [PATCH v2 1/4] dt-bindings: reserved-memory: Document "active" property Rob Herring
2020-09-15 12:36 ` Thierry Reding
2020-09-24 11:27 ` Thierry Reding [this message]
2020-11-05 16:43 ` Thierry Reding
2020-11-05 17:47 ` Robin Murphy
2020-11-06 15:25 ` Thierry Reding
2020-11-10 19:33 ` Thierry Reding
2020-12-17 15:00 ` Thierry Reding
2020-12-18 22:15 ` Rob Herring
2020-12-19 0:49 ` Thierry Reding
2020-09-24 13:23 ` Dmitry Osipenko
2020-09-24 14:01 ` Thierry Reding
2020-09-24 16:23 ` Dmitry Osipenko
2020-09-25 12:39 ` Robin Murphy
2020-09-25 13:21 ` Dmitry Osipenko
2020-11-05 16:23 ` Thierry Reding
2020-09-25 13:52 ` Dmitry Osipenko
2020-11-05 15:57 ` Thierry Reding
2020-11-05 15:49 ` Thierry Reding
2021-01-12 19:00 ` Dmitry Osipenko
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