On Sat, Jun 15, 2013 at 6:19 PM, Jussi Kivilinna [off-list ref] wrote:

On 15.06.2013 10:41, Ming Lei wrote:

quoted

Cc: netdev

On Fri, Jun 14, 2013 at 9:38 PM, Jussi Kivilinna [off-list ref] wrote:

quoted

Appearently some out-of-tree USB host drivers do not handle DMA alignment for

Looks these host drivers have to face the fact that the transfer buffer is often
DMA non-aligned from network device drivers(in fact, the buffer is from
network protocol stack), if you run usbnet, then you will get the added warning
immediately.

Yes, getting warning immediately, but once, and blaming host driver seems ok.

We do know the fact of non-aligned transfer buffer from network, which has been
for long time, so does it make sense to print warning and annoy people?

quoted

quoted

URB buffers and let core/hcd.c to do the mapping on architectures that have
minimum DMA alignment requirements. This leads to random memory corruptions
and crashes when using USB device drivers that use unaligned URB buffers.

Maybe you should check the dma mapping/unmapping implementation of
the arch, non-aligned buffer should have be covered by the API easily.

Also USB Host controller should have supported non-aligned DMA buffer.

From what I found, there was some discussion about these issues around 2010:
 http://lists.infradead.org/pipermail/linux-arm-kernel/2010-August/022983.html

From the discussion,  people think that HCD should handle the unaligned buffer,

right?

To me, it seems that non-aligned buffers cannot be easily handled by all archs
at dma mapping/unmapping phase and that HCD driver should do the alignment on

If the memory which shares cache line with transfer buffer can't be
accessed during
DMA transfer(between URB submit and complete), dma mapping/unmapping
should have handled it.

About the network transfer buffer case, I think it should be true,
otherwise there
should have lots of memory corruption reports about usb network drivers.
Fortunately, there are seldom such reports.

archs that set ARCH_DMA_MINALIGN. For example, ehci_tegra does copy unaligned
transfer buffers to temporary aligned buffers before letting them to USB core.

Yes, if host controller can't handle this, the HCD has to work around
the problem. Anyway, most of host controllers can deal with the it,
can't they?

quoted

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Instead of fixing host drivers, users end up posting bug reports against
those USB device drivers that use unaligned buffers for URB; such as with
rtl8192cu (http://thread.gmane.org/gmane.linux.kernel.wireless.general/105631).

Not only rtl8192cu driver, all USB network device drivers have the problem.

quoted

Patch makes this issue more visible at core level, and hopefully gives hint
for future hcd driver implementors about this problem.

So please find the root cause first, and don't add the noise now.

I think the root cause is that host driver is letting pass non-aligned buffers
to core on archs that have ARCH_DMA_MINALIGN set.

No, I don't think so, about the problem, the dma alignment requirement should
be from your host controller.

As I said above, dma mapping/unmapping should be capable of dealing with
the unaligned buffer if no one touches memory which shares cacheline with
URB->transfer_buffer during URB transfer.

Looks you need to know why the memory corruption happens. Is it caused
by non-aligned arch mapping/unmapping? or by host controller hardware when
dealing with non-aligned transfer buffer?

The warning given just before such unaligned buffer is passed to dma_map_single,
which requires ARCH_DMA_MINALIGN alignment. This seems reasonable to me.

ARCH_DMA_MINALIGN means that kmalloc() should return aligned dma buffer.

Again, you have to accept the fact in which transfer buffer from
network stack is
non-aligned.


Thanks,
-- 
Ming Lei
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