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SubjectRe: [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.

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