From: Jason Gunthorpe <jgg@ziepe.ca>
To: Jiri Pirko <jiri@resnulli.us>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org,
iommu@lists.linux.dev, linux-media@vger.kernel.org,
sumit.semwal@linaro.org, benjamin.gaignard@collabora.com,
Brian.Starkey@arm.com, jstultz@google.com, tjmercier@google.com,
christian.koenig@amd.com, m.szyprowski@samsung.com,
robin.murphy@arm.com, leon@kernel.org, sean.anderson@linux.dev,
ptesarik@suse.com, catalin.marinas@arm.com,
suzuki.poulose@arm.com, steven.price@arm.com,
thomas.lendacky@amd.com, john.allen@amd.com,
ashish.kalra@amd.com, suravee.suthikulpanit@amd.com,
linux-coco@lists.linux.dev
Subject: Re: [PATCH v5 1/2] dma-mapping: introduce DMA_ATTR_CC_SHARED for shared memory
Date: Tue, 21 Apr 2026 09:10:04 -0300 [thread overview]
Message-ID: <20260421121004.GA3611611@ziepe.ca> (raw)
In-Reply-To: <tteiecxfqy4k24wnzvp6ocxnuopyhmqtne2xwh5htwldlbzjnp@o6cbzdlurxld>
On Tue, Apr 21, 2026 at 01:53:31PM +0200, Jiri Pirko wrote:
> >> You reach there when is_swiotlb_force_bounce(dev) is true and
> >> DMA_ATTR_CC_SHARED is set. What am I missing?
> >>
> >
> >So a swiotlb_force_bounce will not use swiotlb bouncing if
> >DMA_ATTR_CC_SHARED is set ?
>
> Correct. Bouncing does not make sense in this case, as shared memory is
> already being mapped.
It is a little bit mangled, there are many reasons force_swiotlb can
be set, but we loose them as it flows through - swiotlb_init()
just has a simple SWIOTLB_FORCE
Ideally DMA_ATTR_CC_SHARED would skip swiotlb only if it is being
selected for CC reasons. For instance if you have the swiotlb force
command line parameter I would still expect it bounce shared memory.
Arguably I think this arch flow is misdesigned, the
is_swiotlb_force_bounce() should not be used for CC. dma_capable() is
the correct API to check if the device can DMA to the presented
address, and it will trigger swiotlb_map() just the same without
creating this gap.
Jason
next prev parent reply other threads:[~2026-04-21 12:10 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CGME20260325192400eucas1p2ae38ff4c2b3ab35a7047cfd680d9fda3@eucas1p2.samsung.com>
2026-03-25 19:23 ` [PATCH v5 0/2] dma-buf: heaps: system: add an option to allocate explicitly shared/decrypted memory Jiri Pirko
2026-03-25 19:23 ` [PATCH v5 1/2] dma-mapping: introduce DMA_ATTR_CC_SHARED for shared memory Jiri Pirko
2026-03-31 15:08 ` Jason Gunthorpe
2026-04-20 6:34 ` Aneesh Kumar K.V
2026-04-20 9:29 ` Jiri Pirko
2026-04-21 9:42 ` Aneesh Kumar K.V
2026-04-21 11:53 ` Jiri Pirko
2026-04-21 12:10 ` Jason Gunthorpe [this message]
2026-03-25 19:23 ` [PATCH v5 2/2] dma-buf: heaps: system: add system_cc_shared heap for explicitly " Jiri Pirko
2026-03-27 19:43 ` T.J. Mercier
2026-03-31 15:08 ` Jason Gunthorpe
2026-04-02 12:23 ` Maxime Ripard
2026-04-02 12:56 ` Jiri Pirko
2026-03-27 9:38 ` [PATCH v5 0/2] dma-buf: heaps: system: add an option to allocate explicitly shared/decrypted memory Marek Szyprowski
2026-03-27 12:10 ` Jason Gunthorpe
2026-03-27 19:43 ` T.J. Mercier
2026-04-02 4:41 ` Sumit Semwal
2026-04-02 5:35 ` Marek Szyprowski
2026-04-02 9:52 ` Brian Starkey
2026-04-02 12:02 ` Jason Gunthorpe
2026-04-02 12:58 ` Jiri Pirko
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