Re: [PATCH v2 1/4] dt-bindings: reserved-memory: Document "active" property

From: Thierry Reding <thierry.reding@gmail.com>
To: Rob Herring <robh@kernel.org>
Cc: Joerg Roedel <joro@8bytes.org>,
	Frank Rowand <frowand.list@gmail.com>,
	Will Deacon <will@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	iommu@lists.linux-foundation.org, devicetree@vger.kernel.org,
	linux-kernel@vger.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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