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

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Message-ID: <20200925041425.GA9419@lst.de>
Date:   Fri, 25 Sep 2020 06:14:25 +0200
From:   'Christoph Hellwig' <hch@....de>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Christoph Hellwig' <hch@....de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        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

On Tue, Sep 22, 2020 at 01:56:46PM +0000, David Laight wrote:
> > +/*
> > + * 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.

This is what I've committed:

 * 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.  It is specific to a
 * given device and there may be a translation between the CPU physical address
 * space and the bus address space.