From mboxrd@z Thu Jan  1 00:00:00 1970
From: christian.koenig@amd.com (=?UTF-8?Q?Christian_K=c3=b6nig?=)
Date: Thu, 12 Apr 2018 11:42:35 +0200
Subject: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for
 scatter-gather buffers
In-Reply-To: <20180412062600.GB30499@lst.de>
References: <1523394001-4615-1-git-send-email-okaya@codeaurora.org>
 <32b82296-bba5-b5f1-266b-45c1ed66da94@arm.com>
 <20180412062600.GB30499@lst.de>
Message-ID: <6802bd0c-7da6-659a-7e34-8f05d7c6422c@amd.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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?

Thanks,
Christian.