Date:   Mon, 24 Sep 2018 10:25:10 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Jim Quinlan <jim2101024@...il.com>, Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        linux-pci <linux-pci@...r.kernel.org>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        Gregory Fong <gregory.0xf0@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Brian Norris <computersforpeace@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 04/12] PCI: brcmstb: add dma-range mapping for inbound traffic

On Fri, 21 Sep 2018 at 19:41, Jim Quinlan <jim2101024@...il.com> wrote:
>
> On Thu, Sep 20, 2018 at 5:39 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> >
> > On 09/20/2018 02:33 PM, Ard Biesheuvel wrote:
> > > On 20 September 2018 at 14:31, Florian Fainelli <f.fainelli@...il.com> wrote:
> > >> On 09/20/2018 02:04 PM, Ard Biesheuvel wrote:
> > >>> On 20 September 2018 at 13:55, Florian Fainelli <f.fainelli@...il.com> wrote:
> > >>>> On 09/19/2018 07:19 PM, Ard Biesheuvel wrote:
> > >>>>> On 19 September 2018 at 07:31, Jim Quinlan <jim2101024@...il.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?

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.

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