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Date:   Mon, 14 Sep 2020 16:26:17 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
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        Ben Skeggs <bskeggs@redhat.com>,
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        Tomasz Figa <tfiga@chromium.org>,
        Matt Porter <mporter@kernel.crashing.org>,
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Subject: Re: a saner API for allocating DMA addressable pages v2
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References: <20200914144433.1622958-1-hch@lst.de>
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On Mon, Sep 14, 2020 at 04:44:16PM +0200, Christoph Hellwig wrote:
> I'm still a little unsure about the API naming, as alloc_pages sort of
> implies a struct page return value, but we return a kernel virtual
> address.

Erm ... dma_alloc_pages() returns a struct page, so is this sentence
stale?

From patch 14:

+struct page *dma_alloc_pages(struct device *dev, size_t size,
+               dma_addr_t *dma_handle, enum dma_data_direction dir, gfp_t gfp);

> The other alternative would be to name the API
> dma_alloc_noncoherent, but the whole non-coherent naming seems to put
> people off.

You say that like it's a bad thing.  I think the problem is more that
people don't understand what non-coherent means and think they're
supporting it when they're not.

dma_alloc_manual_flushing()?

> As a follow up I plan to move the implementation of the
> DMA_ATTR_NO_KERNEL_MAPPING flag over to this framework as well, given
> that is also is a fundamentally non coherent allocation.  The replacement
> for that flag would then return a struct page, as it is allowed to
> actually return pages without a kernel mapping as the name suggested
> (although most of the time they will actually have a kernel mapping..)

If the page doesn't have a kernel mapping, shouldn't it return a PFN
or a phys_addr?