linux-kernel - Re: [PATCH 1/7] iommu/omap: Do bus_set

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Date:	Mon, 23 Dec 2013 22:07:04 +0100
From:	Florian Vaussard <florian.vaussard@...l.ch>
To:	"Anna, Suman" <s-anna@...com>, Joerg Roedel <joro@...tes.org>,
	Tony Lindgren <tony@...mide.com>,
	Benoît Cousson <bcousson@...libre.com>
CC:	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Rob Landley <rob@...dley.net>,
	Grant Likely <grant.likely@...aro.org>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/7] iommu/omap: Do bus_set_iommu() only if probe() succeeds

Hi Suman,

On 12/23/2013 08:02 PM, Anna, Suman wrote:
> Hi Florian,
> 
> On 12/17/2013 06:53 AM, Florian Vaussard wrote:
>> Currently, bus_set_iommu() is done in omap_iommu_init(). However,
>> omap_iommu_probe() can fail in a number of ways, leaving the platform
>> bus with a dangling reference to a non-initialized iommu. Perform
>> bus_set_iommu() only if omap_iommu_probe() succeed.
> 
> Can you clarify a bit more on what kind of issues you were seeing
> specifically? In general, there can be multiple instances of the iommu,
> so setting it in probe may not be fixing whatever issue you were seeing.
> The current OMAP3 code has only the ISP MMU enabled, but there is also
> another one for the IVA MMU (currently not configured by default).
> Moving the bus_set_iommu to probe makes sense if only one iommu is
> present, so this patch may not be needed at all.
> 

If omap_iommu_probe() fails, the init will have called bus_set_iommu()
anyways. Thus, when a driver request the iommu by calling
iommu_domain_alloc(), it will succeed (but iommu_attach_device() will
fail if I remember). Leaving a driver with a dangling reference to
a phantom iommu is not good IMHO. It will lead to strange behaviours
one day or another.

As for the multiple iommu case, as I do not think it is currently
possible, as bus_type (in this case &platform_bus_type) has only
one struct iommu_ops. bus_set_iommu() will return EBUSY on the
second call. Am I missing something?

> Also, the main change in this patch is moving the bus_set_iommu from
> omap_iommu_init to omap_iommu_probe, so you should probably leave out
> moving the function. The omap_iommu_probe function would anyway need
> conversion to using devm_ functions.
> 

This was my first try also, but as bus_set_iommu() needs omap_iommu_ops
(itself depending on several functions), its call must come after the
declaration of omap_iommu_ops. Thus I moved omap_iommu_probe() after
the declaration of omap_iommu_ops. But I can probably use a forward
declaration for omap_iommu_ops, this would be better.

Indeed, we can also convert to devm_.

Regards,

Florian
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