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Date:   Wed, 20 Apr 2022 09:12:17 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Serge Semin <fancer.lancer@gmail.com>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        Serge Semin <Sergey.Semin@baikalelectronics.ru>,
        Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
        Vinod Koul <vkoul@kernel.org>,
        Jingoo Han <jingoohan1@gmail.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Frank Li <Frank.Li@nxp.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Vladimir Murzin <vladimir.murzin@arm.com>,
        Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@baikalelectronics.ru>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Rob Herring <robh@kernel.org>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        linux-pci@vger.kernel.org, dmaengine@vger.kernel.org,
        linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org
Subject: Re: [PATCH 03/25] dma-direct: take dma-ranges/offsets into account
 in resource mapping
Message-ID: <20220420071217.GA5152@lst.de>
References: <20220324014836.19149-1-Sergey.Semin@baikalelectronics.ru> <20220324014836.19149-4-Sergey.Semin@baikalelectronics.ru> <0baff803-b0ea-529f-095a-897398b4f63f@arm.com> <20220417224427.drwy3rchwplthelh@mobilestation>
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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.