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Date: Fri, 19 Jun 2015 10:50:54 +0800 From: Mark yao <mark.yao@...k-chips.com> To: Russell King - ARM Linux <linux@....linux.org.uk> CC: dri-devel@...ts.freedesktop.org, zwl@...k-chips.com, Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>, linux-kernel@...r.kernel.org, Daniel Kurtz <djkurtz@...omium.org>, tfiga@...omium.org, linux-rockchip@...ts.infradead.org, Rob Clark <robdclark@...il.com>, Philipp Zabel <p.zabel@...gutronix.de>, xw@...k-chips.com, dkm@...k-chips.com, sandy.huang@...k-chips.com, linux-arm-kernel@...ts.infradead.org, Heiko Stuebner <heiko@...ech.de> Subject: Re: [PATCH 1/6] drm/rockchip: import dma_buf to gem Thanks Russell On 2015年06月18日 18:57, Russell King - ARM Linux wrote: > This is wrong. > > First, if you can only cope with a single scatterlist entry, you need to > enforce that. You can do that in your gem_prime_import_sg_table() method > by checking sgt->nents. I'm confuse that how to get coherent iova address from nocoherent scatterlist with iommu. I saw the arm_iommu_map_sg, it says that: The scatter gather list elements are merged together (if possible) and tagged with the appropriate dma address and length. I guess the map_sg maybe merge scatterlist into coherent iova address, but I don't know how to check the "if possible", and the sgt->nents not be 1 when already map to coherent iova address. checking sgt-nents = 1 can sure that iova is coherent, but when coherent iova address from nocoherent scatterlist, I think sgt->nents maybe greater than 1, is there a method can deal it? All above is my guess, maybe wrong, I only tested it with CMA buffer import. > Secondly, you're mapping an already mapped scatterlist - scatterlists > are mapped by the exporter inside dma_buf_map_attachment() for your > device. Right, found the dma_map_sg on dma_buf_map_attachment(). > > Thirdly, I hate drm_gem_prime_import() being used on ARM... it forces > drivers to do something very buggy: the DMA buffer is mapped for DMA > when the buffer is imported. If the buffer is a write-combine or cached > buffer, writes to the buffer after the import will not become visible to > the display hardware until sometime later (when they're evicted from the > caches and/or pushed out of the bus structure.) The DMA mapping should > be performed as close to the start of DMA as possible. However, this is > a long-standing issue I have with dma_buf itself and is not something you > should be too concerned with in your patch. Just bear it in mind if you > start to see corruption of imported buffers - the answer is not more > dma_map_sg() calls, but to get dma_buf fixed. Yeah, Got it. -- Mark Yao -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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