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

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Date:   Fri, 26 Jan 2018 12:46:44 -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 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