Re: [V5, 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode
Oh, I know, I've make a mistake, dmapool doesn't need to change. Huacai -- Original -- From: "Christoph Hellwig"<h...@infradead.org>; Date: Mon, Sep 18, 2017 11:51 PM To: "Robin Murphy"<robin.mur...@arm.com>; Cc: "Huacai Chen"<che...@lemote.com>; "Andrew Morton"<a...@linux-foundation.org>; "Fuxin Zhang"<zhan...@lemote.com>; "linux-mm"<linux...@kvack.org>; "linux-kernel"<linux-kernel@vger.kernel.org>; "stable"<sta...@vger.kernel.org>; Subject: Re: [V5, 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode On Mon, Sep 18, 2017 at 10:44:54AM +0100, Robin Murphy wrote: > 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. Oh, I hadn't even seen that part. Yes, dma coherent (and pool) allocations must never be used for streaming mappings. I wish we'd have some debug infrastructure to warn on such uses.
Re: [V5, 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode
Oh, I know, I've make a mistake, dmapool doesn't need to change. Huacai -- Original -- From: "Christoph Hellwig"; Date: Mon, Sep 18, 2017 11:51 PM To: "Robin Murphy"; Cc: "Huacai Chen"; "Andrew Morton"; "Fuxin Zhang"; "linux-mm"; "linux-kernel"; "stable"; Subject: Re: [V5, 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode On Mon, Sep 18, 2017 at 10:44:54AM +0100, Robin Murphy wrote: > 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. Oh, I hadn't even seen that part. Yes, dma coherent (and pool) allocations must never be used for streaming mappings. I wish we'd have some debug infrastructure to warn on such uses.
Re: [PATCH V5 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode
Hi, Christoph, Maybe you missed something. 1, pool_alloc_page() use dma_alloc_coherent() to allocate pool pages, and of course these pages are aligned to ARCH_DMA_MINALIGN. 2, dma_pool_alloc() is the element allocator, but it doesn't use dma_alloc_coherent(). Elements only align to pool->size, but pool->size is usually less than ARCH_DMA_MINALIGN. 3, ARCH_DMA_MINALIGN is now only used in serveral drivers, no dma_ops use it. Huacai -- Original -- From: "Christoph Hellwig"<h...@infradead.org>; Date: Mon, Sep 18, 2017 01:22 PM To: "Huacai Chen"<che...@lemote.com>; Cc: "Andrew Morton"<a...@linux-foundation.org>; "Fuxin Zhang"<zhan...@lemote.com>; "linux-mm"<linux...@kvack.org>; "linux-kernel"<linux-kernel@vger.kernel.org>; "stable"<sta...@vger.kernel.org>; Subject: Re: [PATCH V5 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode The dmapool code uses dma_alloc_coherent to allocate each element, and dma_alloc_coherent must align to ARCH_DMA_MINALIGN already. If you implementation doesn't do that it needs to be fixed.
Re: [PATCH V5 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode
Hi, Christoph, Maybe you missed something. 1, pool_alloc_page() use dma_alloc_coherent() to allocate pool pages, and of course these pages are aligned to ARCH_DMA_MINALIGN. 2, dma_pool_alloc() is the element allocator, but it doesn't use dma_alloc_coherent(). Elements only align to pool->size, but pool->size is usually less than ARCH_DMA_MINALIGN. 3, ARCH_DMA_MINALIGN is now only used in serveral drivers, no dma_ops use it. Huacai -- Original -- From: "Christoph Hellwig"; Date: Mon, Sep 18, 2017 01:22 PM To: "Huacai Chen"; Cc: "Andrew Morton"; "Fuxin Zhang"; "linux-mm"; "linux-kernel"; "stable"; Subject: Re: [PATCH V5 2/3] mm: dmapool: Align to ARCH_DMA_MINALIGN innon-coherent DMA mode The dmapool code uses dma_alloc_coherent to allocate each element, and dma_alloc_coherent must align to ARCH_DMA_MINALIGN already. If you implementation doesn't do that it needs to be fixed.
