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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Fri, 31 Aug 2018 15:51:54 +0900
Message-ID: <CAAFQd5DhFr8ywjc41oK9q+zZXH9zsOKh_7DxWmjzcE0+5Q3hGA@mail.gmail.com>
Subject: Re: [RFC 2/3] USB: core: Add non-coherent buffer allocation helpers
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Ezequiel Garcia <ezequiel@collabora.com>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        linux-usb@vger.kernel.org,
        "list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>, Joerg
        Roedel <joro@8bytes.org>," <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        "Matwey V. Kornilov" <matwey@sai.msu.ru>,
        Alan Stern <stern@rowland.harvard.edu>, kernel@collabora.com,
        keiichiw@chromium.org
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On Fri, Aug 31, 2018 at 2:50 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Thu, Aug 30, 2018 at 07:11:35PM -0300, Ezequiel Garcia wrote:
> > On Thu, 2018-08-30 at 10:58 -0700, Christoph Hellwig wrote:
> > > Please don't introduce new DMA_ATTR_NON_CONSISTENT users, it is
> > > a rather horrible interface, and I plan to kill it off rather sooner
> > > than later.  I plan to post some patches for a better interface
> > > that can reuse the normal dma_sync_single_* interfaces for ownership
> > > transfers.  I can happily include usb in that initial patch set based
> > > on your work here if that helps.
> >
> > Please do. Until we have proper allocators that go thru the DMA API,
> > drivers will have to kmalloc the USB transfer buffers, and have
> > streaming mappings. Which in turns mean not using IOMMU or CMA.
>
> dma_map_page will of course use the iommu.

Sure, dma_map*() will, but using kmalloc() defeats (half of) the
purpose of it, since contiguous memory would be allocated
unnecessarily, risking failures due to fragmentation.

Best regards,
Tomasz