linux-kernel - Re: a saner API for allocating DMA addressable pages v2

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Date:   Mon, 14 Sep 2020 16:26:17 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Matt Porter <mporter@...nel.crashing.org>,
        iommu@...ts.linux-foundation.org,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        linux1394-devel@...ts.sourceforge.net, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        nouveau@...ts.freedesktop.org, netdev@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-mm@...ck.org,
        alsa-devel@...a-project.org
Subject: Re: a saner API for allocating DMA addressable pages v2

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?