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From: Claire Chang <tientzu@chromium.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Nicolas Boichat <drinkcat@chromium.org>,
	devicetree@vger.kernel.org, heikki.krogerus@linux.intel.com,
	Saravana Kannan <saravanak@google.com>,
	suzuki.poulose@arm.com, Robin Murphy <robin.murphy@arm.com>,
	lkml <linux-kernel@vger.kernel.org>,
	bgolaszewski@baylibre.com, iommu@lists.linux-foundation.org,
	Rob Herring <robh+dt@kernel.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	dan.j.williams@intel.com, treding@nvidia.com,
	frowand.list@gmail.com
Subject: Re: [PATCH 1/4] dma-mapping: Add bounced DMA ops
Date: Wed, 15 Jul 2020 17:04:36 +0800
Message-ID: <CALiNf28LVobHKLuXAf7P7Avi6n1oU+tbbFJ55ZPanJuq8Q1Ysg@mail.gmail.com> (raw)
In-Reply-To: <CALiNf2-9b5LMjv+KCqFJ9oz2FocT6oQ1zVY_MBaFgNG1DQxZ=Q@mail.gmail.com>

On Wed, Jul 15, 2020 at 11:46 AM Claire Chang <tientzu@chromium.org> wrote:
>
> On Tue, Jul 14, 2020 at 7:01 PM Christoph Hellwig <hch@lst.de> wrote:
> >
> > On Mon, Jul 13, 2020 at 12:55:43PM +0100, Robin Murphy wrote:
> > > On 2020-07-13 10:12, Claire Chang wrote:
> > >> The bounced DMA ops provide an implementation of DMA ops that bounce
> > >> streaming DMA in and out of a specially allocated region. Only the
> > >> operations relevant to streaming DMA are supported.
> > >
> > > I think there are too many implicit assumptions here - apparently that
> > > coherent allocations will always be intercepted by
> > > dma_*_from_dev_coherent(), and that calling into dma-direct won't actually
> > > bounce things a second time beyond where you thought they were going,
> > > manage coherency for a different address, and make it all go subtly wrong.
> > > Consider "swiotlb=force", for instance...
If I understand it correctly, reusing SWIOTLB won't prevent the
coherent allocations
from always being intercepted by dma_*_from_dev_coherent(), right?
Since we can't bounce the coherent memory, we still need to rely on
dma_*_from_dev_coherent() and a reserved-memory region for coherent DMA to
restrict the device DMA access.

As for calling into dma-direct, in this version, I use set_dma_ops to set the
dma_bounced_ops, so I just bypass dma-direct and SWIOTLB. "swiotlb=force"
won't bounce things a second time and the data will still be bounced
to the region
set in dts.
Besides, I did a quick search and found that only two *-iommu.c directly use
dma_direct_map_page.
https://elixir.bootlin.com/linux/latest/C/ident/dma_direct_map_page
Since bounced DMA is to mitigate the lack of DMA access control on systems
without an IOMMU (see patch#4, only call of_dma_set_bounce_buffer for the
devices not behind an IOMMU), can we assume no one will use dma-direct?
(I understand that if we build bounced DMA on top of SWIOTLB, we don't need
to worry about this.)

> > >
> > > Again, plumbing this straight into dma-direct so that SWIOTLB can simply
> > > target a different buffer and always bounce regardless of masks would seem
> > > a far better option.
> >
> > I haven't really had time to read through the details, but I agree that
> > any bouncing scheme should reuse the swiotlb code and not invent a
> > parallel infrastructure.
> Thanks for the feedback. I'll try to reuse SWIOTLB.
My current plan is to first change the buffers management logic in SWIOTLB to
use gen_pool like this patch (i.e., gen_pool_dma_alloc, gen_pool_free, ect), and
then make SWIOTLB use the device's private pool for regular DMA to/from system
memory if possible.
Does this sound right?

Thanks!
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  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-13  9:12 [PATCH 0/4] Bounced DMA support Claire Chang
2020-07-13  9:12 ` [PATCH 1/4] dma-mapping: Add bounced DMA ops Claire Chang
2020-07-13 11:55   ` Robin Murphy
2020-07-14 11:01     ` Christoph Hellwig
2020-07-15  3:46       ` Claire Chang
2020-07-15  9:04         ` Claire Chang [this message]
2020-07-28  5:05           ` Claire Chang
2020-07-13  9:12 ` [PATCH 2/4] dma-mapping: Add bounced DMA pool Claire Chang
2020-07-13  9:12 ` [PATCH 3/4] dt-bindings: of: Add plumbing for " Claire Chang
2020-07-13  9:12 ` [PATCH 4/4] " Claire Chang
2020-07-13 11:39 ` [PATCH 0/4] Bounced DMA support Robin Murphy
2020-07-15  3:43   ` Claire Chang
2020-07-15 17:53     ` Robin Murphy

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