[PATCH] USB: ehci: use packed, aligned(4) instead of removing the packed attribute

Nicolas Pitre nico at fluxnic.net
Tue Jun 28 14:51:17 EDT 2011

On Sat, 25 Jun 2011, Arnd Bergmann wrote:

> On Saturday 25 June 2011, Nicolas Pitre wrote:
> > > which means that the dma_buf variable is dereferenced before the
> > > volatile mmio_reg variable, which opens up a race: An interrupt may have
> > > signalled us that a DMA is in progress, so we read a MMIO register from
> > > the device (this is guaranteed to flush the DMA on PCI and similar buses).
> > > If we read the dma_buf before we read the mmio register, the data we get
> > > back may be stale.
> > > 
> > > Adding a barrier() between the two turns the assembly into the expected
> > > 
> > >         ldr     r3, [r1, #0]
> > >         ldr     r0, [r0, #0]
> > >         bx      lr
> > 
> > But isn't the usual dma_unmap_*() API call providing that barrier 
> > already?
> Yes, for the streaming mapping that would be sufficient, but not
> for a coherent mapping, which doesn't need dma_unmap_* or dma_sync*
> calls before accessing the buffer.

OK, so that leaves only that case.  Obviously that must not be all that 
critical in practice given the time it has been "broken" already.

So I'm wondering if this might be a better idea to augment the API with 
explicit barriers to cover the case above which is still a much less 
frequent pattern than successive readl()/writel()'s where the implicit 
memory barrier is really badly affecting the generated code.


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