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

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Date:   Wed, 12 Dec 2018 18:34:25 +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 12, 2018 at 6:09 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Wed, Dec 12, 2018 at 05:57:02PM +0900, Tomasz Figa wrote:
> > How about dma_sync_sg_*()? I'd expect some drivers to export/import
> > such memory via sg, since that's the typical way of describing memory
> > in DMA-buf.
>
> The way it is implemented dma_sync_sg_* would just work, however there
> really should be no need to have sglists for buffers created by this
> API.

The typical DMA-buf import/export flow is as follows:
1) Driver X allocates buffer A using this API for device x and gets a
DMA address inside x's DMA (or IOVA) address space.
2) Driver X creates a dma_buf D(A), backed by buffer A and gives the
user space process a file descriptor FD(A) referring to it.
3) Driver Y gets FD(A) from the user space and needs to map it into
the DMA/IOVA address space of device y. It doe it by calling
dma_buf_map_attachment() which returns an sg_table describing the
mapping.

And then I realized that actually there is no need for the importer
(driver Y) to call dma_sync_*() on its own, since the exporter (driver
X) is expected to map and sync in its .map_dma_buf() dma_buf_ops
callback. But there is still a need to have an sg_table created for
the buffer, because it's what dma_buf_map_attachment() returns.

>
> > Sounds good to me. Thanks for working on this. I'd be happy to be on
> > CC and help with review when you post the patches later.
>
> The patches were already posted here:
>
> https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031982.html

Okay, thanks. I can see it in my inbox.

Best regards,
Tomasz