linux-kernel - Re: [PATCH 03/25] dma-direct: take dma-ranges/offsets into account in resource mapping

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Date:   Thu, 21 Apr 2022 16:45:36 +0200
From:   Christoph Hellwig <hch@....de>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Vinod Koul <vkoul@...nel.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Frank Li <Frank.Li@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        linux-pci@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 03/25] dma-direct: take dma-ranges/offsets into account
 in resource mapping

On Wed, Apr 20, 2022 at 11:55:38AM +0300, Serge Semin wrote:
> On Wed, Apr 20, 2022 at 10:47:46AM +0200, Christoph Hellwig wrote:
> > I can't really comment on the dma-ranges exlcusion for P2P mappings,
> > as that predates my involvedment, however:
> 
> My example wasn't specific to the PCIe P2P transfers, but about PCIe
> devices reaching some platform devices over the system interconnect
> bus.

So strike PCIe, but this our definition of Peer to Peer accesses.

> What if I get to have a physical address of a platform device and want
> have that device being accessed by a PCIe peripheral device? The
> dma_map_resource() seemed very much suitable for that. But considering
> what you say it isn't.

dma_map_resource is the right thing for that.  But the physical address
of MMIO ranges in the platform device should not have struct pages
allocated for it, and thus the other dma_map_* APIs should not work on
it to start with.