[alsa-devel] [PATCH] ASoC: omap-pcm: Lower the dma coherent mask to 32bits

[Russell King - ARM Linux]

On Thu, Dec 05, 2013 at 05:33:46PM +0200, Peter Ujfalusi wrote:
> I'm not that familiar with this part of the code (mm, dma-mapping) but so far
> this is what I found:
> 
> ret = dma_coerce_mask_and_coherent(card->dev, DMA_BIT_MASK(64));
> is successful, no failure to set the mask to 64.
> 
> later on when requesting the dma channel however:
> arm_dma_alloc() -> get_coherent_dma_mask()  fails.
> 
> As far I can see we have different checks in case of
> dma_coerce_mask_and_coherent() and arm_dma_alloc():
> 
> [1] dma_coerce_mask_and_coherent() -> dma_supported()
> 
>  	if (sizeof(mask) != sizeof(dma_addr_t) &&
>  	    mask > (dma_addr_t)~0 &&
>  	    dma_to_pfn(dev, ~0) > arm_dma_pfn_limit)  /* !!! */
>  		return 0;
> 
> 
> [2] arm_dma_alloc() -> get_coherent_dma_mask()
> 
>  	if (sizeof(mask) != sizeof(dma_addr_t) &&
>  	    mask > (dma_addr_t)~0 &&
>  	    dma_to_pfn(dev, ~0) > min(max_pfn, arm_dma_pfn_limit)) /* !!! */
>  		return 0;
> 
> On omap4:
> max_pfn: 0xc0000, arm_dma_pfn_limit: 0xfffff

Obviously, we should not be saying that the mask is okay, and then failing
with that same mask.

I've thinking about what the right fix here is - it's been quite a while
since I devised those tests and the knowledge I had back then has been
swapped out...

The point of these tests are to deal with masks which are larger than
dma_addr_t allows, and to only deny if we have sufficient system memory
that we may overflow dma_addr_t.

So...

	sizeof(u64) != sizeof(dma_addr_t)

detects when we have non-64 bit dma_addr_t.

	mask > (dma_addr_t)~0

detects if the mask is larger than 4GiB.

	dma_to_pfn(dev, ~0)

gives us the very last PFN which the bus associated with dev is able to
address.  Hmm - so I got the test wrong on both twice - they should both
be:

 	if (sizeof(mask) != sizeof(dma_addr_t) &&
 	    mask > (dma_addr_t)~0 &&
 	    dma_to_pfn(dev, ~0) < min(max_pfn, arm_dma_pfn_limit))
		return 0;

since we want to fail if we have _more_ memory than the bus will allow.
Can you try the above in both locations please?