Re: [PATCH] usb: dwc2: Fix DMA alignment to start at allocated boundary

[Greg KH <greg@kroah.com>]

On Fri, Jul 27, 2018 at 09:52:17PM +0300, Antti Sepp�l� wrote:
> commit 56406e017a883b54b339207b230f85599f4d70ae upstream.
> 
> The commit 3bc04e28a030 ("usb: dwc2: host: Get aligned DMA in a more
> supported way") introduced a common way to align DMA allocations.
> The code in the commit aligns the struct dma_aligned_buffer but the
> actual DMA address pointed by data[0] gets aligned to an offset from
> the allocated boundary by the kmalloc_ptr and the old_xfer_buffer
> pointers.
> 
> This is against the recommendation in Documentation/DMA-API.txt which
> states:
> 
>   Therefore, it is recommended that driver writers who don't take
>   special care to determine the cache line size at run time only map
>   virtual regions that begin and end on page boundaries (which are
>   guaranteed also to be cache line boundaries).
> 
> The effect of this is that architectures with non-coherent DMA caches
> may run into memory corruption or kernel crashes with Unhandled
> kernel unaligned accesses exceptions.
> 
> Fix the alignment by positioning the DMA area in front of the allocation
> and use memory at the end of the area for storing the orginal
> transfer_buffer pointer. This may have the added benefit of increased
> performance as the DMA area is now fully aligned on all architectures.
> 
> Tested with Lantiq xRX200 (MIPS) and RPi Model B Rev 2 (ARM).
> 
> Fixes: 3bc04e28a030 ("usb: dwc2: host: Get aligned DMA in a more supported way")
> Cc: <stable@vger.kernel.org>
> Reviewed-by: Douglas Anderson <dianders@chromium.org>
> [ Antti: backported to 4.9: edited difference in whitespace ]
> Signed-off-by: Antti Sepp�l� <a.seppala@gmail.com>
> Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
> ---
> 
> Notes:
>     This is the same patch already applied upstream and queued for stable kernels
>     4.14 and 4.17 but with a minor whitespace edit to make it apply also on 4.9.

Now applied, thanks.

greg k-h