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Subject: Re: a saner API for allocating DMA addressable pages
To:     Christoph Hellwig <hch@lst.de>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
        Joonyoung Shim <jy0922.shim@samsung.com>,
        Seung-Woo Kim <sw0312.kim@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Ben Skeggs <bskeggs@redhat.com>,
        Pawel Osciak <pawel@osciak.com>,
        Matt Porter <mporter@kernel.crashing.org>,
        iommu@lists.linux-foundation.org
Cc:     Tom Lendacky <thomas.lendacky@amd.com>, linux-doc@vger.kernel.org,
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From:   Marek Szyprowski <m.szyprowski@samsung.com>
Message-ID: <8fa1ce36-c783-1a02-6890-211eb504a33b@samsung.com>
Date:   Tue, 25 Aug 2020 13:30:41 +0200
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Hi Christoph,

On 19.08.2020 08:55, Christoph Hellwig wrote:
> this series replaced the DMA_ATTR_NON_CONSISTENT flag to dma_alloc_attrs
> with a separate new dma_alloc_pages API, which is available on all
> platforms.  In addition to cleaning up the convoluted code path, this
> ensures that other drivers that have asked for better support for
> non-coherent DMA to pages with incurring bounce buffering over can finally
> be properly supported.
>
> 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.  The other alternative would be to name the API
> dma_alloc_noncoherent, but the whole non-coherent naming seems to put
> people off.  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..)
>
> In addition to the conversions of the existing non-coherent DMA users
> the last three patches also convert the DMA coherent allocations in
> the NVMe driver to use this new framework through a dmapool addition.
> This was both to give me a good testing vehicle, but also because it
> should speed up the NVMe driver on platforms with non-coherent DMA
> nicely, without a downside on platforms with cache coherent DMA.

I really wonder what is the difference between this new API and 
alloc_pages(GFP_DMA, n). Is this API really needed? I thought that this 
is legacy thing to be removed one day...

Maybe it would make more sense to convert the few remaining drivers to 
regular dma_map_page()/dma_sync_*()/dma_unmap_page() or have I missed 
something?

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland