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Subject: Re: [PATCH] arm64: Enable PCI write-combine resources under sysfs
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     Jason Gunthorpe <jgg@nvidia.com>, Clint Sbisa <csbisa@amazon.com>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        linux-pci@vger.kernel.org, Bjorn Helgaas <helgaas@kernel.org>,
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        catalin.marinas@arm.com, Leon Romanovsky <leon@kernel.org>
Date:   Tue, 15 Sep 2020 07:41:48 +1000
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On Mon, 2020-09-14 at 11:17 -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?
> 
> > 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 :(

Well.. to be frank it does work "well enough" for simple cases like
frame buffers :-)

Cheers
Ben.