From: Stephen Warren <swarren@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/1] arm/dts: Tegra30: Add device tree support for SMMU
Date: Fri, 13 Apr 2012 21:25:20 +0200
Message-ID: <4F887DA0.8030103@xxxxxxxxxxxxx>
> On 04/13/2012 06:15 AM, Thierry Reding wrote:
> > * Hiroshi Doyu wrote:
> >> Thierry Reding wrote:
> >>> * Hiroshi Doyu wrote:
> >>>> + smmu: smmu@7000f000 {
> >>>> + compatible = "nvidia,tegra30-smmu";
> >>>> + reg = < 0x7000f000 0x400 /* controller registers */
> >>>> + 0x6000c000 0x150 /* AHB Arbitration registers */
> >>>> + 0x00001000 0x3ffff000 >;/* Virtual address space range
> >>>> + * Exclude the 1st & last page
> >>>> + */
> >>>> + interrupts = < 0 13 0x40 >;
> >>>> + };
> >>>> };
> >>>
> >>> Why is the virtual address space range limited to 1 GiB? What is the reason
> >>> for the exclusion of the first and last pages?
> >>
> >> It's because physical RAM is located 2-4GB, and we may want to use
> >> those area 1-1(V==P) mapping in some cases. This could be extended
> >> with larger RAM without 1-1 mapping theoretically. So far 1GB seems to
> >> be enough.
> >
> > I'm thinking that this would be better off in a separate property so that
> > it's easier for boards to override it.
>
> Yes, and for another reason: The entries in the reg property are
> supposed to be within the bus address space that contains the device,
> whereas here this 3rd reg entry is being used to define something about
> an entirely unrelated address space - the virtual address space seen by
> SMMU clients.
What about using "dma-window" property to specify IOVA range in dtsi as below?
arch/powerpc/platforms/cell/iommu.c:
698 static int __init cell_iommu_get_window(struct device_node *np,
699 unsigned long *base,
700 unsigned long *size)
701 {
702 const void *dma_window;
703 unsigned long index;
704
705 /* Use ibm,dma-window if available, else, hard code ! */
706 dma_window = of_get_property(np, "ibm,dma-window", NULL);
707 if (dma_window == NULL) { ^^^^^^^^^^^^^^
708 *base = 0;
709 *size = 0x80000000u;
710 return -ENODEV;
711 }
712
713 of_parse_dma_window(np, dma_window, &index, base, size);
714 return 0;
715 }
> >> The 1st page for AVP vector, and the last one is required by another
> >> H/W entity.
> >
> > I would expect such peculiarities to be handled by the driver internally.
> > That way users wouldn't have to know or care about these kind of details.
> > If it can't be handled by the driver then at least it should be mentioned
> > explicitly in the binding documentation.
>
> I agree, but:
>
> Why do those pages even need special handling? Doesn't the AVP get its
> own address space ID, and then can set it up however it wants?
This sounds better than dealing with prefixed mappings at booting. At
least we need to pass # of ASIDs, and client should claim one of them.
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