Hi,
On Thu, Jul 5, 2018 at 7:31 AM, Antti Seppälä <a.sepp...@gmail.com> wrote:
> 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")
>
> Signed-off-by: Antti Seppälä <a.sepp...@gmail.com>
> ---
> drivers/usb/dwc2/hcd.c | 44 +++++++++++++++++++++++---------------------
> 1 file changed, 23 insertions(+), 21 deletions(-)
Thanks for tracking this down and sorry for the original regression.
Seems like a good fix. With this fix, I'd be curious of your
observations on how dwc2 performs (both performance and compatibility
under stress) with the newest driver compared to whatever you were
using before.
Also: you're using the dwc2_set_ltq_params() parameters? Have you
checked if removing the "max_transfer_size" limit boosts your
performance?
Cc: sta...@vger.kernel.org
Reviewed-by: Douglas Anderson <diand...@chromium.org>
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