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SubjectRe: [PATCH 08/18] dma-mapping: add a new dma_alloc_noncoherent API
On Fri, Sep 25, 2020 at 12:15:37PM +0100, Robin Murphy wrote:
> On 2020-09-15 16:51, Christoph Hellwig wrote:
> [...]
>> +These APIs allow to allocate pages in the kernel direct mapping that are
>> +guaranteed to be DMA addressable. This means that unlike dma_alloc_coherent,
>> +virt_to_page can be called on the resulting address, and the resulting
>
> Nit: if we explicitly describe this as if it's a guarantee that can be
> relied upon...
>
>> +struct page can be used for everything a struct page is suitable for.
>
> [...]
>> +This routine allocates a region of <size> bytes of consistent memory. It
>> +returns a pointer to the allocated region (in the processor's virtual address
>> +space) or NULL if the allocation failed. The returned memory may or may not
>> +be in the kernels direct mapping. Drivers must not call virt_to_page on
>> +the returned memory region.
>
> ...then forbid this document's target audience from relying on it,
> something seems off. At the very least it's unhelpfully unclear :/
>
> Given patch #17, I suspect that the first paragraph is the one that's no
> longer true.

Yes. dma_alloc_pages is the replacement for allocations that need the
direct mapping. I'll send a patch to document dma_alloc_pages and
fixes this up

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