Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers

From: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
To: "Christian König" <christian.koenig-5C7GfCeVMHo@public.gmane.org>,
	"Christoph Hellwig" <hch-jcswGhMUV9g@public.gmane.org>
Cc: Tom St Denis <tom.stdenis-5C7GfCeVMHo@public.gmane.org>,
	"David (ChunMing) Zhou"
	<David1.Zhou-5C7GfCeVMHo@public.gmane.org>,
	sulrich-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org,
	David Panariti <David.Panariti-5C7GfCeVMHo@public.gmane.org>,
	David Airlie <airlied-cv59FeDIM0c@public.gmane.org>,
	linux-arm-msm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	timur-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org,
	open list <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	amd-gfx-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org,
	Sinan Kaya <okaya-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>,
	Emily Deng <Emily.Deng-5C7GfCeVMHo@public.gmane.org>,
	iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org,
	Huang Rui <ray.huang-5C7GfCeVMHo@public.gmane.org>,
	Roger He <Hongbo.He-5C7GfCeVMHo@public.gmane.org>,
	"open list:DRM DRIVERS"
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	Feifei Xu <Feifei.Xu-5C7GfCeVMHo@public.gmane.org>,
	Alex Deucher <alexander.deucher-5C7GfCeVMHo@public.gmane.org>,
	Jim Qu <Jim.Qu-5C7GfCeVMHo@public.gmane.org>,
	Felix Kuehling <Felix.Kuehling-5C7GfCeVMHo@public.gmane.org>,
	Monk Liu <Monk.Liu-5C7GfCeVMHo@public.gmane.org>,
	linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.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-5C7GfCeVMHo@public.gmane.org>

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.
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