linux-kernel - Re: [PATCH] dma-direct: Set SG_DMA

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Message-ID: <1be83f24-15bd-43a4-b310-f62c720cf064@arm.com>
Date: Thu, 9 May 2024 13:54:56 +0100
From: Robin Murphy <robin.murphy@....com>
To: Christoph Hellwig <hch@....de>, "T.J. Mercier" <tjmercier@...gle.com>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>, isaacmanjarres@...gle.com,
 Catalin Marinas <catalin.marinas@....com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct

On 08/05/2024 12:33 pm, Christoph Hellwig wrote:
> On Tue, May 07, 2024 at 01:07:25PM -0700, T.J. Mercier wrote:
>> On Mon, May 6, 2024 at 10:43???PM Christoph Hellwig <hch@....de> wrote:
>>>
>>> On Mon, May 06, 2024 at 09:39:53AM -0700, T.J. Mercier wrote:
>>>>> You should not check, you simply must handle it by doing the proper
>>>>> DMA API based ownership management.
>>>>
>>>> That doesn't really work for uncached buffers.
>>>
>>> What uncached buffers?
>>
>> For example these ones:
>> https://android.googlesource.com/kernel/common/+/refs/heads/android-mainline/drivers/dma-buf/heaps/system_heap.c#141
> 
> Whatever that code is doing is probably not upstream because it's too
> broken to live.

Indeed, at a glance it appears to be trying to reinvent 
dma_alloc_noncontiguous(). What's not immediately obvious is whether 
it's particular about allocations being DMA-contiguous; if not then I 
think it comes down to the same thing as vb2-dma-sg and the ideas we 
were tossing around for that[1].

Thanks,
Robin.

[1] 
https://lore.kernel.org/linux-media/20231228074645.765yytb2a7hvz7ti@chromium.org/