From: Robin Murphy <robin.murphy@arm.com>
To: "Christian König" <christian.koenig@amd.com>,
"Christoph Hellwig" <hch@lst.de>
Cc: Sinan Kaya <okaya@codeaurora.org>,
amd-gfx@lists.freedesktop.org, timur@codeaurora.org,
sulrich@codeaurora.org, Tom St Denis <tom.stdenis@amd.com>,
"David (ChunMing) Zhou" <David1.Zhou@amd.com>,
Emily Deng <Emily.Deng@amd.com>, David Airlie <airlied@linux.ie>,
linux-arm-msm@vger.kernel.org,
Felix Kuehling <Felix.Kuehling@amd.com>,
open list <linux-kernel@vger.kernel.org>,
"open list:DRM DRIVERS" <dri-devel@lists.freedesktop.org>,
David Panariti <David.Panariti@amd.com>, Jim Qu <Jim.Qu@amd.com>,
Huang Rui <ray.huang@amd.com>, Roger He <Hongbo.He@amd.com>,
Monk Liu <Monk.Liu@amd.com>, Feifei Xu <Feifei.Xu@amd.com>,
Alex Deucher <alexander.deucher@amd.com>,
linux-arm-kernel@lists.infradead.org,
iommu@lists.linux-foundation.org
Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers
Date: Thu, 12 Apr 2018 14:51:00 +0100 [thread overview]
Message-ID: <7a3e7459-2965-54c8-ff0c-6edb3a6aecb5@arm.com> (raw)
In-Reply-To: <6802bd0c-7da6-659a-7e34-8f05d7c6422c@amd.com>
On 12/04/18 10:42, Christian König wrote:
> Am 12.04.2018 um 08:26 schrieb Christoph Hellwig:
>> On Wed, Apr 11, 2018 at 01:03:59PM +0100, Robin Murphy wrote:
>>> On 10/04/18 21:59, Sinan Kaya wrote:
>>>> Code is expecing to observe the same number of buffers returned from
>>>> dma_map_sg() function compared to sg_alloc_table_from_pages(). This
>>>> doesn't hold true universally especially for systems with IOMMU.
>>> So why not fix said code? It's clearly not a real hardware
>>> limitation, and
>>> the map_sg() APIs have potentially returned fewer than nents since
>>> forever,
>>> so there's really no excuse.
>> Yes, relying on dma_map_sg returning the same number of entries as passed
>> it is completely bogus.
>
> I agree that the common DRM functions should be able to deal with both,
> but we should let the driver side decide if it wants multiple addresses
> combined or not.
>
>>
>>>> IOMMU driver tries to combine buffers into a single DMA address as much
>>>> as it can. The right thing is to tell the DMA layer how much combining
>>>> IOMMU can do.
>>> Disagree; this is a dodgy hack, since you'll now end up passing
>>> scatterlists into dma_map_sg() which already violate max_seg_size to
>>> begin
>>> with, and I think a conscientious DMA API implementation would be at
>>> rights
>>> to fail the mapping for that reason (I know arm64 happens not to, but
>>> that
>>> was a deliberate design decision to make my life easier at the time).
>> Agreed.
>
> Sounds like my understanding of max_seg_size is incorrect, what exactly
> should that describe?
The segment size and boundary mask are there to represent a device's
hardware scatter-gather capabilities - a lot of things have limits on
the size and alignment of the data pointed to by a single descriptor
(e.g. an xHCI TRB, where the data buffer must not cross a 64KB boundary)
- although they can also be relevant to non-scatter-gather devices if
they just have limits on how big/aligned a single DMA transfer can be.
Robin.
prev parent reply other threads:[~2018-04-12 13:51 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-10 20:59 [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers Sinan Kaya
2018-04-11 6:26 ` Huang Rui
2018-04-11 9:17 ` Christian König
2018-04-11 12:03 ` Robin Murphy
2018-04-11 14:33 ` Sinan Kaya
2018-04-11 15:32 ` Robin Murphy
2018-04-12 6:26 ` Christoph Hellwig
2018-04-12 9:42 ` Christian König
2018-04-12 13:51 ` Robin Murphy [this message]
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