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

Clint Sbisa
2020-09-14T14:24:47Z

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.

>
> > As Jason noted in the other reply to this email, the Linux ENA driver makes use
> > of WC by using devm_ioremap_wc().
>
> As Ben noted we don't have kernel accessors to make this portable or
> safe :(

And therein lies our dilemma...

Clint