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 09:12:17 +0200
From:   Christoph Hellwig <hch@....de>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     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>,
        Christoph Hellwig <hch@....de>,
        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 Mon, Apr 18, 2022 at 01:44:27AM +0300, Serge Semin wrote:
> > but a DMA controller might also want to access something in the MMIO range
> > 0x0-0x7fffffff, of which it still has an identical non-offset view. If a
> > driver was previously using dma_map_resource() for that, it would now start
> > getting DMA_MAPPING_ERROR because the dma_range_map exists but doesn't
> > describe the MMIO region. I agree that in hindsight it's not an ideal
> > situation, but it's how things have ended up, so at this point I'm wary of
> > making potentially-breaking changes.
> 
> Hmm, what if the driver was previously using for instance the
> dma_direct_map_sg() method for it?

dma_map_resource is for mapping MMIO space, and must not be called on
memory in the kernel map.  For dma_map_sg (or all the other dma_map_*
interface except for dma_map_resource), the reverse is true.