Re: [RFC 10/10] mmc: sdhci-tegra: Add IOMMU support

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Fri, Jun 27, 2014 at 12:46:02PM +0300, Hiroshi DOyu wrote:
> 
> Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
> 
> > From: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >
> > Attach to the device's master interface of the IOMMU at .probe() time.
> > IOMMU support becomes available via the DMA mapping API interoperation
> > code, but this explicit attachment is necessary to ensure proper probe
> > order.
> >
> > Signed-off-by: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > ---
> >  drivers/mmc/host/sdhci-tegra.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/mmc/host/sdhci-tegra.c b/drivers/mmc/host/sdhci-tegra.c
> > index 33100d10d176..b884614fa4e6 100644
> > --- a/drivers/mmc/host/sdhci-tegra.c
> > +++ b/drivers/mmc/host/sdhci-tegra.c
> > @@ -15,6 +15,7 @@
> >  #include <linux/err.h>
> >  #include <linux/module.h>
> >  #include <linux/init.h>
> > +#include <linux/iommu.h>
> >  #include <linux/platform_device.h>
> >  #include <linux/clk.h>
> >  #include <linux/io.h>
> > @@ -237,6 +238,11 @@ static int sdhci_tegra_probe(struct platform_device *pdev)
> >  	match = of_match_device(sdhci_tegra_dt_match, &pdev->dev);
> >  	if (!match)
> >  		return -EINVAL;
> > +
> > +	rc = iommu_attach(&pdev->dev);
> > +	if (rc < 0)
> > +		return rc;
> > +
> 
> I thought that, if we consider that ->probe() should include minimal H/W
> probing so that DMA API call in ->probe() could be deferred after
> ->probe() and till it's in use actually, like opening a device node. For
> me this decision(minimal h/w probe) seemed logical but it would add a
> new restriction. One advantage is that we could still keep all drivers
> wihtout any IOMMU code if it doesn't call DMA API in ->probe().

This isn't immediately apparent in this case, but I think that in the
future we may need to have this kind of explicit attachment to an IOMMU
for example once devices start to appear that have multiple master
interfaces (possibly on different IOMMUs). For easy cases like this
SDMMC driver we may be able to get away more easily by hooking this up
within the driver core for example. I'd have to look into how exactly
this would work, though.

Thierry

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