linux-kernel - Re: [PATCHv3 2/4] ARM: tegra: Add SMMU enabler in AHB

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Date:	Thu, 26 Apr 2012 23:26:14 +0300
From:	Felipe Balbi <balbi@...com>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Hiroshi DOYU <hdoyu@...dia.com>, swarren@...dia.com,
	linux-tegra@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Felipe Balbi <balbi@...com>, Colin Cross <ccross@...roid.com>,
	Olof Johansson <olof@...om.net>,
	Russell King <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	H Hartley Sweeten <hsweeten@...ionengravers.com>,
	Jamie Iles <jamie@...ieiles.com>,
	Rob Herring <rob.herring@...xeda.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 2/4] ARM: tegra: Add SMMU enabler in AHB

Hi,

On Thu, Apr 26, 2012 at 01:55:13PM -0600, Stephen Warren wrote:
> On 04/25/2012 05:07 AM, Hiroshi DOYU wrote:
> > Add extern func, "tegra_ahb_enable_smmu()" to inform AHB that SMMU is
> > ready.
> 
> > +#ifdef CONFIG_ARCH_TEGRA_3x_SOC
> > +static int __tegra_ahb_enable_smmu(struct device *dev, void *data)
> ...
> > +int tegra_ahb_enable_smmu(void)
> > +{
> > +	return driver_for_each_device(&tegra_ahb_driver.driver, NULL, NULL,
> > +				      __tegra_ahb_enable_smmu);
> > +}
> > +EXPORT_SYMBOL(tegra_ahb_enable_smmu);
> > +#endif
> 
> That looks like a neat solution to avoid having a global device object.

except that it won't work always. If you happen to have two AHB bridges,
each using a separate smmu but only one smmu is ready, this will set
SMMU_INIT_DONE on both bridges.

> However, if that driver_for_each_device finds no devices, the function
> still succeeds. That doesn't seem right, and doesn't allow e.g. the SMMU
> to defer its probe until the AHB driver has completed.
> 
> Perhaps add a local int variable to tegra_ahb_enable_smmu(), pass the
> address to __tegra_ahb_enable_smmu, and have it increment the int. Then,
> after calling driver_for_each_device,:
> 
> if (!ahb_device_count)
>     return -EPROBE_DEFER
> if (WARN_ON(ahb_device_count != 1))
>     return -EINVAL;
> return 0;

that would look, well, weird. Why don't you just different initcall
leves for this ? Maybe smmu goes into postcore_initcall() and tegra_ahb
goes into postcore_initcall_sync() ?? then you know that SMMU will be
ready by the time you call tegra_ahb probe. Well, unless smmu's probe
fail, but then again, IIUC it won't work anyway...

-- 
balbi

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