[PATCH] usb: ehci: make HC see up-to-date qh/qtd descriptor ASAP

Alan Stern stern at rowland.harvard.edu
Tue Aug 30 13:15:42 EDT 2011


On Tue, 30 Aug 2011, Mark Salter wrote:

> On Wed, 2011-08-31 at 00:03 +0800, ming.lei at canonical.com wrote:
> > +/*
> > + * Writing to dma coherent memory on ARM may be delayed via L2
> > + * writing buffer, so introduce the helper which can flush L2 writing
> > + * buffer into memory immediately, especially used to flush ehci
> > + * descriptor to memory.
> > + * */
> > +#ifdef CONFIG_ARM_DMA_MEM_BUFFERABLE
> > +static inline void ehci_sync_mem()
> > +{
> > +       mb();
> > +}
> > +#else
> > +static inline void ehci_sync_mem()
> > +{
> > +}
> > +#endif
> > +
> 
> I'm wondering if this doesn't really belong in the DMA API for any
> future architectures that can't avoid prolonged write buffering to DMA
> coherent memory. IIUC, ARM mitigates this for most drivers by including
> an implicit write buffer flush in the mmio write routines. This takes
> care of the drivers which write to a mmio device register after writing
> something to shared DMA memory. IIUC, this doesn't help ehci because the
> host controller is polling to see what the cpu writes to the shared
> memory. Other hardware which polls shared memory like that will likely
> have the same problem and could use buffer drain helpers as well.

This would be a good thing to define centrally.  Would you like to 
post an RFC on LKML?

Do you know of any other examples of hardware that polls shared DMA 
memory?

Alan Stern




More information about the linux-arm-kernel mailing list