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

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Date:   Wed, 20 Apr 2022 10:47:46 +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

I can't really comment on the dma-ranges exlcusion for P2P mappings,
as that predates my involvedment, however:

On Wed, Apr 20, 2022 at 11:32:07AM +0300, Serge Semin wrote:
> See, if I get to map a virtual memory address to be accessible by any
> PCIe peripheral device, then the dma_map_sg/dma_map_page/etc
> procedures will take the PCIe host controller dma-ranges into account.
> It will work as expected and the PCIe devices will see the memory what
> I specified. But if I get to pass the physical address of the same
> page or a physical address of some device of the DEVs space to the
> dma_map_resource(), then the PCIe dma-ranges won't be taken into
> account, and the result mapping will be incorrect. That's why the
> current dma_map_resource() implementation seems very confusing to me.
> As I see it phys_addr_t is the type of the Interconnect address space,
> meanwhile dma_addr_t describes the PCIe, DEVs address spaces.
> 
> Based on what I said here and in my previous email could you explain
> what do I get wrong?

You simply must not use dma_map_resource for normal kernel memory.
So while the exclusion might be somewhat confusing, that confusion
really should not matter for any proper use of the API.