From: Robin Murphy <robin.murphy@arm.com>
To: Sinan Kaya <okaya@codeaurora.org>,
amd-gfx@lists.freedesktop.org, timur@codeaurora.org,
sulrich@codeaurora.org
Cc: "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>,
"Christian König" <christian.koenig@amd.com>,
linux-arm-kernel@lists.infradead.org,
iommu@lists.linux-foundation.org, hch@lst.de
Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers
Date: Wed, 11 Apr 2018 16:32:13 +0100 [thread overview]
Message-ID: <2d812da5-1c0d-0e53-83f2-ba1781a12fe9@arm.com> (raw)
In-Reply-To: <1a1d0069-c8d3-cc0d-cf1c-8e90e4a2b0d7@codeaurora.org>
On 11/04/18 15:33, Sinan Kaya wrote:
> On 4/11/2018 8:03 AM, 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.
>
> Sure, I'll take a better fix if there is one.
>
>>
>>> 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).
>>
>> As a short-term fix, at least do something like what i915 does and
>> constrain the table allocation to the desired segment size as well,
>> so things remain self-consistent. But still never claim that faking
>> a hardware constraint as a workaround for a driver shortcoming is
>> "the right thing to do" ;)
>
> You are asking for something like this from here, right?
>
> https://elixir.bootlin.com/linux/v4.16.1/source/drivers/gpu/drm/i915/i915_gem_dmabuf.c#L58
I just looked for callers of __sg_alloc_table_from_pages() with a limit
other than SCATTERLIST_MAX_SEGMENT, and found
__i915_gem_userptr_alloc_pages(), which at first glance appears to be
doing something vaguely similar to amdgpu_ttm_tt_pin_userptr().
Robin.
next prev parent reply other threads:[~2018-04-11 15:32 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 [this message]
2018-04-12 6:26 ` Christoph Hellwig
2018-04-12 9:42 ` Christian König
2018-04-12 13:51 ` Robin Murphy
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