linux-kernel - Re: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

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Message-ID: <CAAFQd5CsX-YJdwQUS+eEK6kj1xU94AiGHY0QX=QGnf67JcKyaQ@mail.gmail.com>
Date:   Thu, 20 Dec 2018 12:23:46 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        "Matwey V. Kornilov" <matwey@....msu.ru>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Matwey V. Kornilov" <matwey.kornilov@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Ezequiel Garcia <ezequiel@...labora.com>, hdegoede@...hat.com,
        Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        rostedt@...dmis.org, mingo@...hat.com,
        Mike Isely <isely@...ox.com>,
        Bhumika Goyal <bhumirks@...il.com>,
        Colin King <colin.king@...onical.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        keiichiw@...omium.org
Subject: Re: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers
 for ISO transfer

On Wed, Dec 19, 2018 at 11:51 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Wed, Dec 19, 2018 at 05:18:35PM +0900, Tomasz Figa wrote:
> > The existing code that deals with dma_alloc_attrs() without
> > DMA_ATTR_NON_CONSISTENT would just call dma_get_sgtable_attrs() like
> > here:
>
> I know.  And dma_get_sgtable_attrs is fundamentally flawed and we
> need to kill this interface as it just can't worked with virtually
> tagged cases.  It is a prime example for an interface that looks
> nice and simple but is plain wrong.

Got it, thanks.

I haven't been following the problems with virtually tagged cases,
would you mind sharing some background, so that we can consider it
when adding non-consistent allocations to VB2?

Best regards,
Tomasz