From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932617AbdIRJpH (ORCPT ); Mon, 18 Sep 2017 05:45:07 -0400 Received: from foss.arm.com ([217.140.101.70]:36740 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754179AbdIRJpC (ORCPT ); Mon, 18 Sep 2017 05:45:02 -0400 Subject: Re: [V5, 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN in non-coherent DMA mode To: Huacai Chen Cc: Andrew Morton , Fuxin Zhang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <1505708548-4750-1-git-send-email-chenhc@lemote.com> From: Robin Murphy Message-ID: <601437ae-2860-c48a-aa7c-4da37aeb6256@arm.com> Date: Mon, 18 Sep 2017 10:44:54 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: <1505708548-4750-1-git-send-email-chenhc@lemote.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/09/17 05:22, Huacai Chen wrote: > In non-coherent DMA mode, kernel uses cache flushing operations to > maintain I/O coherency, so the dmapool objects should be aligned to > ARCH_DMA_MINALIGN. Otherwise, it will cause data corruption, at least > on MIPS: > > Step 1, dma_map_single > Step 2, cache_invalidate (no writeback) > Step 3, dma_from_device > Step 4, dma_unmap_single This is a massive red warning flag for the whole series, because DMA pools don't work like that. At best, this will do nothing, and at worst it is papering over egregious bugs elsewhere. Streaming mappings of coherent allocations means completely broken code. > If a DMA buffer and a kernel structure share a same cache line, and if > the kernel structure has dirty data, cache_invalidate (no writeback) > will cause data lost. DMA pools are backed by coherent allocations, and those should already be at *page* granularity, so this doubly cannot happen for correct code. More generally, the whole point of having the DMA APIs is that drivers and subsystems should not have to be aware of details like hardware coherency. Besides, cache line sharing that could pose a correctness issue for non-hardware-coherent systems could still be a performance issue in the presence of hardware coherency (due to unnecessary line migration), so there's still an argument for not treating them differently. Robin. > Cc: stable@vger.kernel.org > Signed-off-by: Huacai Chen > --- > mm/dmapool.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/dmapool.c b/mm/dmapool.c > index 4d90a64..6263905 100644 > --- a/mm/dmapool.c > +++ b/mm/dmapool.c > @@ -140,6 +140,9 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev, > else if (align & (align - 1)) > return NULL; > > + if (!device_is_coherent(dev)) > + align = max_t(size_t, align, dma_get_cache_alignment()); > + > if (size == 0) > return NULL; > else if (size < 4) >