Re: [PATCH v5 04/12] PCI: brcmstb: add dma-range mapping for inbound traffic

[Jim Quinlan <jim2101024@gmail.com>]

On Mon, Sep 24, 2018 at 4:25 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> On Fri, 21 Sep 2018 at 19:41, Jim Quinlan <jim2101024@gmail.com> wrote:
> >
> > On Thu, Sep 20, 2018 at 5:39 PM Florian Fainelli <f.fainelli@gmail.com> wrote:
> > >
> > > On 09/20/2018 02:33 PM, Ard Biesheuvel wrote:
> > > > On 20 September 2018 at 14:31, Florian Fainelli <f.fainelli@gmail.com> wrote:
> > > >> On 09/20/2018 02:04 PM, Ard Biesheuvel wrote:
> > > >>> On 20 September 2018 at 13:55, Florian Fainelli <f.fainelli@gmail.com> wrote:
> > > >>>> On 09/19/2018 07:19 PM, Ard Biesheuvel wrote:
> > > >>>>> On 19 September 2018 at 07:31, Jim Quinlan <jim2101024@gmail.com> wrote:
> > > >>>>>> The Broadcom STB PCIe host controller is intimately related to the
> > > >>>>>> memory subsystem.  This close relationship adds complexity to how cpu
> > > >>>>>> system memory is mapped to PCIe memory.  Ideally, this mapping is an
> > > >>>>>> identity mapping, or an identity mapping off by a constant.  Not so in
> > > >>>>>> this case.
> > > >>>>>>
> > > >>>>>> Consider the Broadcom reference board BCM97445LCC_4X8 which has 6 GB
> > > >>>>>> of system memory.  Here is how the PCIe controller maps the
> > > >>>>>> system memory to PCIe memory:
> > > >>>>>>
> > > >>>>>>   memc0-a@[        0....3fffffff] <=> pci@[        0....3fffffff]
> > > >>>>>>   memc0-b@[100000000...13fffffff] <=> pci@[ 40000000....7fffffff]
> > > >>>>>>   memc1-a@[ 40000000....7fffffff] <=> pci@[ 80000000....bfffffff]
> > > >>>>>>   memc1-b@[300000000...33fffffff] <=> pci@[ c0000000....ffffffff]
> > > >>>>>>   memc2-a@[ 80000000....bfffffff] <=> pci@[100000000...13fffffff]
> > > >>>>>>   memc2-b@[c00000000...c3fffffff] <=> pci@[140000000...17fffffff]
> > > >>>>>>
> > > >>>>>
> > > >>>>> So is describing this as
> > > >>>>>
> > > >>>>> dma-ranges = <0x0 0x0 0x0 0x0 0x0 0x40000000>,
> > > >>>>>              <0x0 0x40000000 0x1 0x0 0x0 0x40000000>,
> > > >>>>>              <0x0 0x80000000 0x0 0x40000000 0x0 0x40000000>,
> > > >>>>>              <0x0 0xc0000000 0x3 0x0 0x0 0x40000000>,
> > > >>>>>              <0x1 0x0 0x0 0x80000000 0x0 0x40000000>,
> > > >>>>>              <0x1 0x40000000 0x0 0xc0000000 0x0 0x40000000>;
> > > >>>>>
> > > >>>>> not working for you? I haven't tried this myself, but since DT permits
> > > >>>>> describing the inbound mappings this way, we should fix the code if it
> > > >>>>> doesn't work at the moment.
> > > >>>>
> > > >>>> You mean encoding the memory controller index in the first cell? If that
> > > >>>> works, that's indeed a much cleaner solution, though is it standard
> > > >>>> compliant in any form?
> > > >>>
> > > >>> No those are just memory addresses (although I may have screwed up the
> > > >>> order). From Documentation/devicetree/booting-without-of.txt:
> > > >>>
> > > >>> """
> > > >>> Optional property:
> > > >>> - dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
> > > >>>         (child-bus-address, parent-bus-address, length). Each triplet specified
> > > >>>         describes a contiguous DMA address range.
> > > >>> """
> > > >>>
> > > >>
> > > >> Then I am confused by your comment, that's what this patch does, it adds
> > > >> support for reading "dma-ranges" from Device Tree and setting up inbound
> > > >> windows using that. The only caveat is that because the PCIe root
> > > >> complex has some ties with the memory bus architecture it is connected
> > > >> to (SCB in our case) there is still a requirement to know the
> > > >> translation between a given physical address and its backing memory
> > > >> controller/aperture.
> > > >>
> > > >
> > > > Ah ok, apologies for the noise then.
> > > >
> > > > I was hoping that having working support for dma-ranges would remove
> > > > the need for the special phys<->dma conversion routines.
> > >
> > > What you describe definitively works with platform devices, but I am not
> > > sure this is working for PCIe devices, although, conceptually it should,
> > > yes.
> > Sorry for my delay in responding.  One problem is that
> > of_dma_configure() only looks at the first dma-range given and then
> > converts it to dev->dma_pfn_offset which is respected by the DMA API.
> > However, we often have multiple dma-ranges, not just one.  This is the
> > big issue.
> >
>
> Given the recent attention to getting these APIs in shape, this may be
> something Robin or Christoph may care to look into?

It looks like this has been brought up before in the "[RFC PATCH] of:
Fix DMA configuration for non-DT masters" thread aka

https://lists.linuxfoundation.org/pipermail/iommu/2017-April/021325.html

In the thread "Oza Oza", a Broadcom coworker probably dealing with the
same exact problem as I,  enumerates three problems.   #1 and #2 are
the exact same ones I've just given: the "dma-ranges" prop of the RC
DT node is "skipped", and of_dma_get_range() only considers the first
entry in any "dma-ranges".

Thanks, Jim

>
> In any case, the description of dma-ranges should be in sync with the
> way Linux interprets it, so this is either a documentation bug or a
> DMA layer bug.
>
> > There is another issue with of_dma_configure() being invoked by the EP
> > driver on "bridge->parent->of_node", which is our RC device,
> > Of_dma_configure() calls of_dma_range() on the of_get_next_parent() of
> > our RC's device node and this misses the dma-ranges property which is
> > contained within the RC.  I think I could workaround this but there is
> > no getting around the first problem.
> >
>
> IIUC dma-ranges should be added to the parent bus of a device, which I
> guess is slightly ambiguous for a root complex that incorporates a
> host bridge.