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

[Thierry Reding]

On Thu, Sep 24, 2020 at 01:27:25PM +0200, Thierry Reding wrote:
> 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@xxxxxxxxxx>
> > > > 
> > > > 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.

Hi Rob,

do you have any suggestions on how to proceed with this? I'd like to get
this moving again because it's something that's been nagging me for some
months now. It also requires changes across two levels in the bootloader
stack as well as Linux and it takes quite a bit of work to make all the
changes, so before I go and rewrite everything I'd like to get the DT
bindings sorted out first.

So just to summarize why I think this simple solution is good enough: it
tries to solve a very narrow and simple problem. This is not an attempt
at describing the firmware's full IOMMU setup to the kernel. In fact, it
is primarily targetted at cases where the firmware hasn't setup an IOMMU
at all, and we just want to make sure that when the kernel takes over
and does want to enable the IOMMU, that all the regions that are
actively being accessed by non-quiesced hardware (the most typical
example would be a framebuffer scanning out a splat screen or animation,
but it could equally well be some sort of welcoming tone or music being
played back) are described in device tree.

In other words, and this is perhaps better answering your second
question: in addition to describing reserved memory regions, we want to
add a bit of information here about the usage of these memory regions.
Some memory regions may contain information that the kernel may want to
use (such an external memory frequency scaling tables) and those I would
describe as "inactive" memory because it isn't being accessed by
hardware. The framebuffer in this case is the opposite and it is being
actively accessed (hence it is marked "active") by hardware while the
kernel is busy setting everything up so that it can reconfigure that
hardware and take over with its own framebuffer (for the console, for
example). It's also not so much that we know enough about the region to
assume it needs identity mapping. We don't really care about that from
the DT point of view. In fact, depending on the rest of the system
configuration, we may not need identity mapping (i.e. if none of the
users of the reserved memory region are behind an IOMMU). But the point
here is that the IOMMU drivers can use this "active" property to
determine that if a device is using an "active" region and it is behind
an IOMMU, then it must identity map that region in order for the
hardware, which is not under the kernel's control yet, to be able to
continue to access that memory through an IOMMU mapping.

Thanks,
Thierry

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