linux-kernel - Re: [RFC 2/3] USB: core: Add non-coherent buffer allocation helpers

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Date:   Fri, 31 Aug 2018 15:51:54 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Ezequiel Garcia <ezequiel@...labora.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        linux-usb@...r.kernel.org,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        "Matwey V. Kornilov" <matwey@....msu.ru>,
        Alan Stern <stern@...land.harvard.edu>, kernel@...labora.com,
        keiichiw@...omium.org
Subject: Re: [RFC 2/3] USB: core: Add non-coherent buffer allocation helpers

On Fri, Aug 31, 2018 at 2:50 PM Christoph Hellwig <hch@...radead.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