Re: [PATCH 0/3] RFC: addition to DMA API

Alan Stern
2011-09-01T16:48:21Z

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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