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Date: Tue, 28 May 2024 08:33:32 +0200
From: Christoph Hellwig <hch@lst.de>
To: Dave Stevenson <dave.stevenson@raspberrypi.com>
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Subject: Re: [PATCH 01/18] dma-direct: take dma-ranges/offsets into account
 in resource mapping
Message-ID: <20240528063332.GA30051@lst.de>
References: <20240524182702.1317935-1-dave.stevenson@raspberrypi.com> <20240524182702.1317935-2-dave.stevenson@raspberrypi.com>
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On Fri, May 24, 2024 at 07:26:45PM +0100, Dave Stevenson wrote:
> From: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> 
> A basic device-specific linear memory mapping was introduced back in
> commit ("dma: Take into account dma_pfn_offset") as a single-valued offset
> preserved in the device.dma_pfn_offset field, which was initialized for
> instance by means of the "dma-ranges" DT property. Afterwards the
> functionality was extended to support more than one device-specific region
> defined in the device.dma_range_map list of maps. But all of these
> improvements concerned a single pointer, page or sg DMA-mapping methods,
> while the system resource mapping function turned to miss the
> corresponding modification. Thus the dma_direct_map_resource() method now
> just casts the CPU physical address to the device DMA address with no
> dma-ranges-based mapping taking into account, which is obviously wrong.
> Let's fix it by using the phys_to_dma_direct() method to get the
> device-specific bus address from the passed memory resource for the case
> of the directly mapped DMA.

My memory is getting a little bad, but as dma_direct_map_resource is
mostly used for (non-PCIe) peer to peer transfers, any kind of mapping
from the host address should be excluded.

(dma_direct_map_resource in general is a horrible interface and I'd
prefer everyone to switch to the map_sg based P2P support, but we
have plenty of users for it unfortunately)