linux-kernel - Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

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Message-ID: <20170704151502.GH13824@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 4 Jul 2017 10:15:02 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        Mark Rutland <mark.rutland@....com>, Mason <slash.tmp@...e.fr>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

On Mon, Jul 03, 2017 at 07:11:28PM +0100, Russell King - ARM Linux wrote:
> On Mon, Jul 03, 2017 at 08:40:31AM -0500, Bjorn Helgaas wrote:
> > The problem is serializing vs. memory accesses, since they don't use
> > any wrappers.  However, they are ioremapped(), so it's at least
> > conceivable that another solution would be to use VM to trap those
> > accesses.  I'm not a VM person, so I don't know whether that's
> > feasible in Linux.
> 
> Bjorn,
> 
> You're forgetting that MMIO (iow, memory returned by ioremap()) must
> be accessed through the appropriate accessors, and must not be
> directly dereferenced in C.  (We do have buggy drivers that do that
> but they are buggy, and in many cases are getting attention to fix
> that.)

Oh, you're right, thank you!  I guess you're referring to readb()
and friends.  I haven't found an actual prohibition on directly
dereferencing addresses returned from ioremap(), but
Documentation/driver-api/device-io.rst is clear that they're
suitable for passing to readb(), etc.

I recently told someone else my mistaken idea that ioremap() must
return a valid virtual address.  I wish I remembered who it was, so I
could correct that.  Documentation/DMA-API-HOWTO.txt also suggests
that ioremap() returns a virtual address -- I think I wrote that, and
maybe that virtual address reference should be tweaked a bit.

Another wrinkle is that the pci_mmap_resource() interface is exposed
via sysfs and allows direct userspace mmap of PCI MMIO resources.  In
that case, there is no accessor available.  I wonder if we need some
way to disable this mmap when readb() is non-trivial.

Bjorn