Re: [PATCH v2] arm64: pci: add support for pci_mmap_page_range

From: David Woodhouse <dwmw2@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Jerin Jacob <jerin.jacob@caviumnetworks.com>,
	lorenzo.pieralisi@arm.com, Arnd Bergmann <arnd@arndb.de>,
	david.daney@cavium.com, catalin.marinas@arm.com,
	Liviu.Dudau@arm.com, rrichter@cavium.com, hanjun.guo@linaro.org,
	linux-pci <linux-pci@vger.kernel.org>,
	bhelgaas@google.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2] arm64: pci: add support for pci_mmap_page_range
Date: Mon, 20 Mar 2017 14:07:38 +0000	[thread overview]
Message-ID: <1490018858.5036.19.camel@infradead.org> (raw)
In-Reply-To: <20170320131836.GM17263@arm.com>

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On Mon, 2017-03-20 at 13:18 +0000, Will Deacon wrote:
> > The thing is, this *isn't* an architecture-specific interface where you
> > get a clean slate. It's a cross-platform interface. Legacy and horrid,
> > sure. But it does you no harm.

> I don't agree with that: it provides (privileged) userspace with a way to
> map non-prefetchable BARs using write-combining memory attributes, which
> could lead to mismatched memory attributes against a kernel mapping of the
> same BAR and is something that you can't achieve using the sysfs API.

I think that's just a bug. I'll add it to my list. We shouldn't be
allowing a WC mapping on a non-prefetchable resource, should we?

For that matter, I think it allows you mmap a range of MMIO addresses
that correspond to an I/O BAR, and on platforms which allow
pci_mmap_io, the converse. That seems... suboptimal.

> > What *else* don't you like having in /proc? Shall we have a clean slate
> > and eliminate *everything* other than actual processes from /proc for
> > the next architecture we add to the tree? If not, why not?
> It's not about what we like and don't like in /proc, it's about not
> promoting legacy that we don't need. If somebody actually needs the /proc
> interface, fine, we can support it. But all the people asking for this have
> been concerned solely about the sysfs interface, so I'd just like the two
> divorced from each other so that we can provide what people are asking for
> without pulling in a deprecated interface at the same time.
> 
> This should be straightforward.

Sure, but fairly much orthogonal. I'll roll it in. It's fairly much in
the noise now I'm this far down the rabbithole...

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