linux-kernel - Re: [PATCH v4 4/8] PCI: brcmstb: Add dma-range mapping for inbound traffic

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Date:   Mon, 12 Feb 2018 08:39:43 -0500
From:   Jim Quinlan <jim2101024@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Brian Norris <computersforpeace@...il.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Robin Murphy <robin.murphy@....com>,
        Jonas Gorski <jonas.gorski@...il.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-MIPS <linux-mips@...ux-mips.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Kevin Cernekee <cernekee@...il.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        Gregory Fong <gregory.0xf0@...il.com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 4/8] PCI: brcmstb: Add dma-range mapping for inbound traffic

On Fri, Jan 26, 2018 at 12:46 PM, Jim Quinlan <jim2101024@...il.com> wrote:
> On Fri, Jan 26, 2018 at 2:53 AM, Christoph Hellwig <hch@....de> wrote:
>> On Wed, Jan 24, 2018 at 12:04:58PM -0800, Florian Fainelli wrote:
>>> This looks nicer than the current shape, but this still requires to
>>> register a PCI fixup to override phys_to_dma() and dma_to_phys(), and it
>>> would appear that you have dodged my question about how this is supposed
>>> to fit with an entirely modular PCIe root complex driver? Are you
>>> suggesting that we split the module into a built-in part and a modular part?
>>
>> I don't think entirely modular PCI root bridges should be a focal point
>> for the design.  If we happen to support them by other design choices:
>> fine, but they should not be a priority.
>
> I disagree.  If there is one common thing our customers request  it is
> the ability to remove (or control the insmod of after boot)  the pcie
> RC driver.  I didn't add this in as a "nice-to-have".
>
>>
>> That being said if we have core dma mapping or PCIe code that has
>> a list of offsets and the root complex only populates them it should
>> work just fine.
>
> I'm looking at arch/arm/include/asm/dma-mapping.h.  In addition to
> overriding dma_to_phsy() and phys_to_dma(), it looks like I may have
> to define __arch_pfn_to_dma(), __arch_dma_to_pfn(),
> __arch_dma_to_virt(), __arch_virt_to_dma().  Do  you agree or is this
> not necessary?  If it is, this seems more intrusive than our
> pcie-brcmstb-dma.c solution which  doesn't require tentacles into
> major include files and Kconfigs.
>
> Another issue is that our function wrappers -- depending upon whether
> we are dealing with a pci device or not -- will have to possibly call
> the actual ARM and ARM64 definitions of these functions, which have
> been of course #ifdef'd out.  This means that our code must contain
> identical copies of these functions' code and that the code must
> somehow be kept in sync.  Do you see a solution to this?
>
> Jim

Cristoph,
Could you please respond to my comments?  Even a negative response is
better than none.  The problem with doing what you suggested is with
ARM -- ARM64 and MIPS relatively uncomplicated .  With ARM, I have to
define the aforementioned functions -- the only way of doing this is
to define arch/arm/mach-bcm/include/mach/memory.h, and for that to be
picked up we no longer can have CONFIG_ARCH_MULTIPLATFORM=y, which is
an unacceptable cost to pay for just an unusual PCIe RC controller.

Regarding my current submission -- you are right, SWIOTLB will not
work for EPs that require it.  However, we don't care about these
devices, and can just bailout with EPs when the dma_mask is <=
0xffff_ffff or if swiotlb_force ==  SWIOTLB_FORCE.  Note that this
would only affect PCIe DMA.  We also have no plan of using MIPS64.

-- Jim