linux-kernel - Re: [PATCH RESEND v4 15/15] PCI: dwc: Introduce dma-ranges property support for RC-host

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Date:   Thu, 28 Jul 2022 17:11:20 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Frank Li <Frank.Li@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, linux-pci@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v4 15/15] PCI: dwc: Introduce dma-ranges property
 support for RC-host

On Fri, Jun 24, 2022 at 05:39:47PM +0300, Serge Semin wrote:
> In accordance with the generic PCIe Root Port DT-bindings the "dma-ranges"
> property has the same format as the "ranges" property. The only difference
> is in their semantics. The "dma-ranges" property describes the PCIe-to-CPU
> memory mapping in opposite to the CPU-to-PCIe mapping of the "ranges"
> property. Even though the DW PCIe controllers are normally equipped with
> the internal Address Translation Unit which inbound and outbound tables
> can be used to implement both properties semantics, it was surprising for
> me to discover that the host-related part of the DW PCIe driver currently
> supports the "ranges" property only while the "dma-ranges" windows are
> just ignored. Having the "dma-ranges" supported in the driver would be
> very handy for the platforms, that don't tolerate the 1:1 CPU-PCIe memory
> mapping and require a customized PCIe memory layout. So let's fix that by
> introducing the "dma-ranges" property support.

Do we have a platform that requires this yet?  Or does this fix a bug?

I see that dw_pcie_host_init() calls devm_pci_alloc_host_bridge(),
which eventually parses "dma-ranges", but I don't see any DWC DT
bindings that use it yet.

I'm not clear on what value this adds today.

Bjorn