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

[Alan Stern]

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