linux-kernel - Re: [PATCH 10/29] drivers: add DRIVER_HAS_OWN_IOMMU

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Date:	Mon, 01 Sep 2014 11:38:31 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	Thierry Reding <treding@...dia.com>,
	Rob Herring <robh@...nel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Shaik Ameer Basha <shaik.ameer@...sung.com>,
	Inki Dae <inki.dae@...sung.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joerg Roedel <joro@...tes.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tomasz Figa <tomasz.figa@...il.com>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Olof Johansson <olof@...om.net>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Cho KyongHo <pullip.cho@...sung.com>
Subject: Re: [PATCH 10/29] drivers: add DRIVER_HAS_OWN_IOMMU_MANAGER flag

On Monday 01 September 2014 09:53:29 Marek Szyprowski wrote:
> On 2014-09-01 09:07, Thierry Reding wrote:
> > On Mon, Sep 01, 2014 at 07:22:32AM +0200, Marek Szyprowski wrote:
> >> Hi Greg,
> >>
> >> On 2014-08-05 12:47, Marek Szyprowski wrote:
> >>> This patch adds a new flags for device drivers. This flag instructs
> >>> kernel that the device driver does it own management of IOMMU assisted
> >>> IO address space translations, so no default dma-mapping structures
> >>> should be initialized.
> >>>
> >>> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> >>> ---
> >>>    include/linux/device.h | 2 ++
> >>>    1 file changed, 2 insertions(+)
> >>>
> >>> diff --git a/include/linux/device.h b/include/linux/device.h
> >>> index 5f4ff02..2e62371 100644
> >>> --- a/include/linux/device.h
> >>> +++ b/include/linux/device.h
> >>> @@ -253,6 +253,8 @@ struct device_driver {
> >>>    
> >>>    /* disables bind/unbind via sysfs */
> >>>    #define DRIVER_SUPPRESS_BIND_ATTRS       (1 << 0)
> >>> +/* driver uses own methods to manage IO address space */
> >>> +#define DRIVER_HAS_OWN_IOMMU_MANAGER       (1 << 1)
> >>>    
> >>>    extern int __must_check driver_register(struct device_driver *drv);
> >>>    extern void driver_unregister(struct device_driver *drv);
> >> Could you comment if the approach of using flags in the struct driver
> >> could be accepted? I've converted suppress_bind_attrs entry to flags to
> >> avoid extending the structure, please see patch "[PATCH 05/29] drivers:
> >> convert suppress_bind_attrs parameter into flags".
> > Is this really necessary? What I did as part of an RFC series for Tegra
> > IOMMU support is keep this knowledge within the IOMMU driver rather than
> > export it to the driver core.i
> 
> The problem with embedding the list of drivers that you would need to update
> it everytime when you modify or extend iommu support in the other drivers.
> I've tried also other approach, like adding respective notifiers to 
> individual
> drivers to initialize custom iommu support (or disable default iommu 
> mapping)
> before iommu driver gets initialized, but such solution is in my opinion too
> complex and hard to understand if one is not familiar will all this stuff.
> 
> All in all it turned out that the simplest and most generic way is to simply
> add the flag to the driver core. Flags might be also used in the future
> to model other kinds of dependencies between device drivers and/or driver
> core.

I don't get it yet. I would expect that a driver doing its own management
of the iommu gets to use the linux/iommu.h interfaces, while a driver
using the default iommu setup uses linux/dma-mapping.h. Who do you think
needs to set this flag, and who needs to read it?

	Arnd
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