On Thu, Jan 5, 2012 at 9:53 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> 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.
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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