Message-ID: <20201110193309.GB2303484@ulmo>
Date:   Tue, 10 Nov 2020 20:33:09 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Robin Murphy <robin.murphy@....com>, devicetree@...r.kernel.org,
        Frank Rowand <frowand.list@...il.com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 1/4] dt-bindings: reserved-memory: Document "active"
 property

On Fri, Nov 06, 2020 at 04:25:48PM +0100, Thierry Reding wrote:
> On Thu, Nov 05, 2020 at 05:47:21PM +0000, Robin Murphy wrote:
> > On 2020-11-05 16:43, Thierry Reding wrote:
> > > 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@...dia.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.
> > > 
> > > 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.
> > 
> > Hmm, "active" is not a property of the memory itself, though, it's really a
> > property of the device accessing it. If several distinct devices share a
> > carveout region, and for simplicity the bootloader marks it as active
> > because one of those devices happens to be using some part of it at boot, we
> > don't really want to have to do all the reserved region setup for all the
> > other devices unnecessarily, when all that matters is not disrupting one of
> > them when resetting the IOMMU.
> > 
> > That leads to another possible hiccup - some bindings already have a defined
> > meaning for a "memory-region" property. If we use that to point to some
> > small region for a temporary low-resolution bootsplash screen for visibility
> > to an IOMMU driver, the device's own driver might also interpret it as a
> > private carveout from which it is expected to allocate everything, and thus
> > could end up failing to work well or at all.
> > 
> > I agree that we should only need a relatively simple binding, and that
> > piggybacking off reserved-memory nodes seems like an ideal way of getting
> > address range descriptions without too much extra complexity; the tricky
> > part is how best to associate those with the other information needed, which
> > is really the "iommus" property of the relevant device, and how to make it
> > as generically discoverable as possible. Perhaps it might be workable to
> > follow almost the same approach but with a dedicated property (e.g.
> > "active-memory-region") that the IOMMU code can simply scan the DT for to
> > determine relevant device nodes. Otherwise properties on the IOMMU node
> > itself would seem the next most practical option.
> 
> We did recently introduce a "memory-region-names" property that's used
> to add context for cases where multiple memory regions are used. Perhaps
> the simplest to address the above would be to describe the region as
> active by naming it "active". That has the disadvantage of restricting
> the number of active regions to 1, though I suspect that may even be
> enough for the vast majority of cases where we need this. This would be
> similar to how we use the "dma-mem" string in the "interconnect-names"
> property to specify the "DMA parent" of a device node.
> 
> Alternatively, we could perhaps support multiple occurrences of "active"
> in the "memory-region-names" property. Or we could add a bit of
> flexibility by considering all memory regions whose names have an
> "active-" prefix as being active.
> 
> > We've also finally got things going on the IORT RMR side[1], which helps add
> > a bit more shape to things too; beyond the actual firmware parsing, DT and
> > ACPI systems should definitely be converging on the same internal
> > implementation in the IOMMU layer.
> 
> Yeah, from a quick look at that series, this actually sounds really
> close to what I'm trying to achieve here.
> 
> The patch set that I have would nicely complement the code added to
> iommu_dma_get_resv_regions() for RMR regions. It's not exactly the same
> code, but it's basically the DT equivalent of
> iort_dev_rmr_get_resv_regions().

Hi Rob,

what's your preference here for DT bindings? Do you want me to reuse the
existing memory-region/memory-region-names properties, or do you want
something completely separate?

Thierry

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