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Date:   Tue, 25 Aug 2020 13:30:41 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Christoph Hellwig <hch@....de>,
        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>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Pawel Osciak <pawel@...iak.com>,
        Matt Porter <mporter@...nel.crashing.org>,
        iommu@...ts.linux-foundation.org
Cc:     Tom Lendacky <thomas.lendacky@....com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.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-nvme@...ts.infradead.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

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

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