[U-Boot] [PATCH v8] usb: align buffers at cacheline

Marek Vasut
2012-03-07T22:06:32Z

Dear puneets,

> Hi Mike,
>
> On Tuesday 06 March 2012 08:37 AM, Mike Frysinger wrote:
> > * PGP Signed by an unknown key
> >
> > On Monday 05 March 2012 09:46:21 Puneet Saxena wrote:
> >> As DMA expects the buffers to be equal and larger then
> >> cache lines, This aligns buffers at cacheline.
> >
> > i don't think this statement is true. DMA doesn't care about alignment
> > (well, some do, but it's not related to cache lines but rather some
> > other restriction in the peripheral DMA itself). what does matter is
> > that cache operations operate on cache lines and not individual bytes.
> > hence the core arm code was updated to warn when someone told it to
> > invalidate X bytes but the hardware literally could not, so it had to
> > invalidate X + Y bytes.
>
> Agreed, Will update the commit message in next patchset.
>
> >> --- a/drivers/usb/host/ehci-hcd.c
> >> +++ b/drivers/usb/host/ehci-hcd.c
> >>
> >> static void flush_invalidate(u32 addr, int size, int flush)
> >> {
> >>
> >> + /*
> >> + * Size is the bytes actually moved during transaction,
> >> + * which may not equal to the cache line. This results
> >> + * stop address passed for invalidating cache may not be aligned.
> >> + * Therfore making size as multiple of cache line size.
> >> + */
> >> + size = ALIGN(size, ARCH_DMA_MINALIGN);
> >> +
> >>
> >> if (flush)
> >>
> >> flush_dcache_range(addr, addr + size);
> >>
> >> else
> >
> > i think this is wrong and merely hides the errors from higher up instead
> > of fixing them. the point of the warning was to tell you that the code
> > was invalidating *too many* bytes. this code still invalidates too many
> > bytes without any justification as for why it's OK to do here. further,
> > this code path only matters to the invalidation logic, not the flush
> > logic. -mike
>
> The sole purpose of this patch to remove the warnings as start/stop
> address sent for invalidating
> is unaligned. Without this patch code works fine but with lots of
> spew...Which we don't want and discussed
> in earlier thread which Simon posted. Please have a look on following link.
>
> As I understood, you agree that we need to align start/stop buffer
> address and also agree that
> to align stop address we need to align size as start address is already
> aligned.
> Now, "why its OK to do here"?
> We could have aligned the size in two places, cache_qtd() and cache_qh()
> but then we need to place alignment check
> at all the places where size is passed. So I thought better Aligning at
> flush_invalidate() and "ALIGN" macro does not
> increase the size if size is already aligned.

Actually I have to agree with Mike here. Can you please remove that ALIGN() (and
all others you might have added)? If it does spew, that's ok and it tells us
something is wrong in the USB core subsystem. Such stuff can be fixed in
subsequent patch.

Best regards,
Marek Vasut