[PATCH 0/3] RFC: addition to DMA API
Alan Stern
stern at rowland.harvard.edu
Thu Sep 1 12:48:19 EDT 2011
On Thu, 1 Sep 2011, Ming Lei wrote:
> > Suppose A and B are _both_ part of the dma descriptor. The device
> > might see A==1 and B==0, if the memory accesses occur like this:
> >
> > CPU device
> > --- ------
> > A = 1;
> > wmb();
> > read B
> > read A
> > B = 2;
> >
> > When this happens, the device will observe a non-atomic update of the
> > descriptor. There's no way to prevent this.
>
> If device doesn't find that B is 2, it will not fetch descriptor of A,
> and will observe
> a atomic update, which is just EHCI does for many cases(such as 4.10.2).
You didn't read what I wrote above. Suppose A and B are _both_ part of
the same descriptor, like hw_token and hw_qtd_next.
> > The memory barrier in your qh_link_async() example can make sure that
> > the device always sees consistent data. It doesn't guarantee that the
> > write to head->hw->hw_next will be flushed to memory in a reasonably
> > short time, which is the problem you are trying to solve.
>
> Yes, up to now, it is the only case in which the flush can address to,
> and in which kind of cases device will poll DMA coherent memory contiguously,
> I am not sure if there are other devices except for EHCI(maybe have uhci/ohci).
Yes: UHCI, OHCI, EHCI, and XHCI all poll memory constantly.
> If there are many such kind of devices, the flush operation introduced will
> make sense.
>
> As far as I know, for most of devices, dma operation of bus master is triggered
> by writing into mmio register instead of writing into coherent memory, and the
> flush is not required in this case surely.
That's right. It is needed only when the device polls automatically.
Alan Stern
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