On Tue, Mar 8, 2016 at 8:59 AM, Timur Tabi <timur@xxxxxxxxxxxxxx> wrote:
>> + /* Try setting the coherent_dma_mask to 64 bits, then try 32 bits
>> */
>> + ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
>> + if (ret) {
>> + ret = dma_set_mask_and_coherent(&pdev->dev,
>> DMA_BIT_MASK(32));
>> + if (ret)
>> + return ret;
>> + }
>
>
> I still do not understand what situation would cause a 64-bit mask to fail,
> but a 32-bit mask to succeed. If the driver says that the device can
> support 64-bit DMA, why would that ever fail? That means that the device
> can DMA anywhere, so there are no restrictions.
Can anyone help me understand this? It's been a month since I posted
this question, and I haven't gotten an answer. I see the whole
try-64-and-then-32-bit-masks code in a number of places, but I've
never understood why it's necessary.
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