Re: [PATCH] arm64: Enable PCI write-combine resources under sysfs

Benjamin Herrenschmidt
2020-09-14T21:43:14Z

On Mon, 2020-09-14 at 11:38 -0300, Jason Gunthorpe wrote:
> On Mon, Sep 14, 2020 at 02:24:06PM +0000, Clint Sbisa wrote:
> > On Mon, Sep 14, 2020 at 11:17:26AM -0300, Jason Gunthorpe wrote:
> > > On Fri, Sep 11, 2020 at 09:42:25PM +0000, Clint Sbisa wrote:
> > >
> > > > There's no DMA involved with this BAR-- the driver writes a
> > > > portion of the
> > > > packet contents in addition to the descriptors, which generally
> > > > increases the
> > > > number of TLPs if write-combine isn't used. Furthermore, this
> > > > BAR is only used
> > > > for writes and never for reads.
> > >
> > > You use DPDK without DMA? How does receive work?
> >
> > That was not worded well. DMA is used, but the first X bytes of the
> > packet are
> > written directly to this BAR instead of being DMA'd-- the rest of
> > the data is
> > DMA'd.
>
> which is back to my original question, how do you do DMA using
> /sys/xx/resources? Why not use VFIO like everything else?

Note: All this doesnt' change the fact that sys/xx/resources_wc exists
for other archs and I see no reasons so far not to have it on ARM...

Cheers,
Ben.