On Thu, Jan 5, 2012 at 9:53 PM, Russell King - ARM Linux
[off-list ref] wrote:

On Thu, Jan 05, 2012 at 09:17:18AM +0200, Hiroshi Doyu wrote:

quoted

Just for DMA mapping test from MPU side, the following one is used.

This patch is buggy.

quoted

+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.

I don't understand what  "CPU must __explicitly__ access it" means.

Do you mean that CPU must access the mapped buffer when
it __explicitly__ knows that the access does not cause any side effect?

- You unmap the page or buffer.
- The CPU again owns the page/buffer and can access it.

Please respect the DMA API rules.

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.
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