linux-kernel - RE: [PATCH 3/3] dma-mapping: better document dma_addr_t and DMA_MAPPING

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Date:   Tue, 22 Sep 2020 13:56:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
CC:     Dominik Brodowski <linux@...inikbrodowski.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 3/3] dma-mapping: better document dma_addr_t and
 DMA_MAPPING_ERROR

From: Christoph Hellwig
> Sent: 22 September 2020 14:40
...
> @@ -131,6 +125,16 @@ struct dma_map_ops {
>  	unsigned long (*get_merge_boundary)(struct device *dev);
>  };
> 
> +/*
> + * A dma_addr_t can hold any valid DMA or bus address for the platform.  It can
> + * be given to a device to use as a DMA source or target.  A CPU cannot
> + * reference a dma_addr_t directly because there may be translation between its
> + * physical address space and the bus address space.

It can't access it 'directly' because it isn't a virtual address....

> + *
> + * DMA_MAPPING_ERROR is the magic error code if a mapping failed.  It should not
> + * be used directly in drivers, but checked for using dma_mapping_error()
> + * instead.
> + */

I think it might be worth adding:

A dma_addr_t value may be device dependant and differ from the
'physical address' of the memory.

	David

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