Re: [PATCH] arm64: tegra: Set dma-ranges for memory subsystem

[Maxime Ripard <mripard@kernel.org>]

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On Wed, Oct 02, 2019 at 05:49:46PM +0200, Thierry Reding wrote:
> On Wed, Oct 02, 2019 at 05:46:54PM +0200, Thierry Reding wrote:
> > From: Thierry Reding <treding@nvidia.com>
> >
> > On Tegra194, all clients of the memory subsystem can generally address
> > 40 bits of system memory. However, bit 39 has special meaning and will
> > cause the memory controller to reorder sectors for block-linear buffer
> > formats. This is primarily useful for graphics-related devices.
> >
> > Use of bit 39 must be controlled on a case-by-case basis. Buffers that
> > are used with bit 39 set by one device may be used with bit 39 cleared
> > by other devices.
> >
> > Care must be taken to allocate buffers at addresses that do not require
> > bit 39 to be set. This is normally not an issue for system memory since
> > there are no Tegra-based systems with enough RAM to exhaust the 39-bit
> > physical address space. However, when a device is behind an IOMMU, such
> > as the ARM SMMU on Tegra194, the IOMMUs input address space can cause
> > IOVA allocations to happen in this region. This is for example the case
> > when an operating system implements a top-down allocation policy for IO
> > virtual addresses.
> >
> > To account for this, describe the path that memory accesses take through
> > the system. Memory clients will send requests to the memory controller,
> > which forwards bits [38:0] of the address either to the external memory
> > controller or the SMMU, depending on the stream ID of the access. A good
> > way to describe this is using the interconnects bindings, see:
> >
> > 	Documentation/devicetree/bindings/interconnect/interconnect.txt
> >
> > The standard "dma-mem" path is used to describe the path towards system
> > memory via the memory controller. A dma-ranges property in the memory
> > controller's device tree node limits the range of DMA addresses that the
> > memory clients can use to bits [38:0], ensuring that bit 39 is not used.
> >
> > Signed-off-by: Thierry Reding <treding@nvidia.com>
> > ---
> > Arnd, Rob, Robin,
> >
> > This is what I came up with after our discussion on this thread:
> >
> > 	[PATCH 00/11] of: dma-ranges fixes and improvements
> >
> > Please take a look and see if that sounds reasonable. I'm slightly
> > unsure about the interconnects bindings as I used them here. According
> > to the bindings there's always supposed to be a pair of interconnect
> > paths, so this patch is not exactly compliant. It does work fine with
> > the __of_get_dma_parent() code that Maxime introduced a couple of months
> > ago and really very neatly describes the hardware. Interestingly this
> > will come in handy very soon now since we're starting work on a proper
> > interconnect provider (the memory controller driver is the natural fit
> > for this because it has additional knobs to configure latency and
> > priorities, etc.) to implement external memory frequency scaling based
> > on bandwidth requests from memory clients. So this all fits together
> > very nicely. But as I said, I'm not exactly sure what to add as a second
> > entry in "interconnects" to make this compliant with the bindings.

It definitely sounds reasonable to me :)

Maxime

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