Hi Russell,
From: Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>
Subject: Re: [PATCH v3 0/2] ARM: IOMMU: tegra: Add iommu_ops for GART/SMMU driver
Date: Thu, 5 Jan 2012 13:53:26 +0100
Message-ID: <20120105125326.GT11810@xxxxxxxxxxxxxxxxxxxxxx>
> On Thu, Jan 05, 2012 at 09:17:18AM +0200, Hiroshi Doyu wrote:
> > Just for DMA mapping test from MPU side, the following one is used.
>
> This patch is buggy.
>
> > +static void dmaapi_test_map_page(struct device *dev)
> > +{
> > + struct page *page;
> > + dma_addr_t dma_addr;
> > + void *cpu_addr;
> > +
> > + page = alloc_page(GFP_KERNEL);
> > + BUG_ON(!page);
> > +
> > + dma_addr = dma_map_page(dev, page, 0, PAGE_SIZE, DMA_TO_DEVICE);
> > + BUG_ON(!dma_addr);
> > +
> > + cpu_addr = kmap(page);
> > + BUG_ON(!cpu_addr);
> > + memset(cpu_addr, 0xa5, PAGE_SIZE);
> > + kunmap(cpu_addr);
>
> The DMA API works like this:
>
> - The CPU owns the page or buffer and can access it.
> - You map the page or buffer.
> - The device owns the page or buffer; the CPU must explicitly access it.
> - You unmap the page or buffer.
> - The CPU again owns the page/buffer and can access it.
>
> Please respect the DMA API rules.
Right, I do.
> So. Once dma_map_page() has returned, you must not kmap() or otherwise
> access the data contained in that page until after you have unmapped it.
Ok, the above function order should be as below?
page = alloc_page(GFP_KERNEL);
cpu_addr = kmap(page);
memset(cpu_addr, 0xa5, PAGE_SIZE);
dma_addr = dma_map_page(dev, page, 0, PAGE_SIZE, DMA_TO_DEVICE);
< expect GPU does something >
dma_unmap_page(dev, dma_addr, PAGE_SIZE, DMA_TO_DEVICE);
< CPU can access data here again>
kunmap(cpu_addr);
__free_page(page);
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