linux-kernel - Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE

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Date:   Wed, 11 Apr 2018 10:33:14 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Robin Murphy <robin.murphy@....com>, amd-gfx@...ts.freedesktop.org,
        timur@...eaurora.org, sulrich@...eaurora.org
Cc:     Tom St Denis <tom.stdenis@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        Emily Deng <Emily.Deng@....com>,
        David Airlie <airlied@...ux.ie>, linux-arm-msm@...r.kernel.org,
        Felix Kuehling <Felix.Kuehling@....com>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        David Panariti <David.Panariti@....com>,
        Jim Qu <Jim.Qu@....com>, Huang Rui <ray.huang@....com>,
        Roger He <Hongbo.He@....com>, Monk Liu <Monk.Liu@....com>,
        Feifei Xu <Feifei.Xu@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, hch@....de
Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for
 scatter-gather buffers

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


	ret = sg_alloc_table(st, obj->mm.pages->nents, GFP_KERNEL);
	if (ret)
		goto err_free;

	src = obj->mm.pages->sgl;
	dst = st->sgl;
	for (i = 0; i < obj->mm.pages->nents; i++) {
		sg_set_page(dst, sg_page(src), src->length, 0);
		dst = sg_next(dst);
		src = sg_next(src);
	}

This seems to allocate the scatter gather list and fill it in manually before passing it
to dma_map_sg(). I'll give it a try. 

Just double checking.

> 
> Robin.
> 
>> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.