linux-kernel - Re: [PATCH v2 2/9] mailbox: Add NVIDIA Tegra XUSB mailbox driver

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Date:	Tue, 26 Aug 2014 10:09:25 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	Andrew Bresticker <abrestic@...omium.org>,
	linux-tegra@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Jassi Brar <jassisinghbrar@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mathias Nyman <mathias.nyman@...el.com>,
	Grant Likely <grant.likely@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Kishon Vijay Abraham I <kishon@...com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-usb@...r.kernel.org,
	Jonas Bonn <jonas@...thpole.se>,
	David Howells <dhowells@...hat.com>,
	Koichi Yasutake <yasutake.koichi@...panasonic.com>
Subject: Re: [PATCH v2 2/9] mailbox: Add NVIDIA Tegra XUSB mailbox driver

On Tuesday 26 August 2014 09:50:25 Thierry Reding wrote:
> On Tue, Aug 26, 2014 at 09:43:50AM +0200, Arnd Bergmann wrote:
> > On Tuesday 26 August 2014 08:57:31 Thierry Reding wrote:
> > > On Mon, Aug 25, 2014 at 01:01:52PM -0600, Stephen Warren wrote:
> > > > On 08/18/2014 11:08 AM, Andrew Bresticker wrote:
> > > [...]
> > > > >+static int tegra_xusb_mbox_probe(struct platform_device *pdev)
> > > > 
> > > > >+    res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > > > >+    if (!res)
> > > > >+            return -ENODEV;
> > > > 
> > > > Should devm_request_mem_region() be called here to claim the region?
> > > > 
> > > > >+    mbox->regs = devm_ioremap_nocache(&pdev->dev, res->start,
> > > > >+                                      resource_size(res));
> > > > >+    if (!mbox->regs)
> > > > >+            return -ENOMEM;
> > > > 
> > > > Is _nocache required? I don't see other drivers using it. I assume there's
> > > > nothing special about the mbox registers.
> > > 
> > > Most drivers should be using devm_ioremap_resource() which will use the
> > > _nocache variant of devm_ioremap() when appropriate. Usually the region
> > > will not be marked cacheable (IORESOURCE_CACHEABLE) and therefore be
> > > remapped uncached.
> > > 
> > 
> > Note that ioremap() and ioremap_nocache() are the same. We really shouldn't
> > ever call ioremap_nocache().
> 
> Perhaps we should remove ioremap_nocache() in that case. Or ioremap(),
> really, and keep only those variants that do what they claim to do.

That would be good, but there are many instances of either one:

arnd@...rfel:/git/arm-soc$ git grep -w ioremap | wc
   2156   13402  183732
arnd@...rfel:/git/arm-soc$ git grep -w ioremap_nocache | wc
    485    2529   42955

FWIW, I just looked through all architectures and found three on
which ioremap and ioremap_nocache are not the same, and ioremap
defaults to cacheable:

- OpenRISC so far only supports running in a simulator, so this
  is likely to be a bug that will get hit on actual hardware with
  MMIO. Jonas should probably look into this.

- mn10300 has no MMU and doesn't really use ioremap, but it should
  still be fixed for PCI drivers using it on the one board that
  supports PCI.

- cris seems to have been broken forever.

> > devm_ioremap_resource() and pci_iomap() checking for IORESOURCE_CACHEABLE is
> > rather silly, since it doesn't call ioremap_cache() in that case.
> 
> Then that should be fixed.

Yes. I'd suggest we just ignore that flag and always call ioremap here.

When I checked this before, IORESOURCE_CACHEABLE only ever gets set for
PCI ROM BARs, which we don't map into the kernel.

	Arnd
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