linux-kernel - Re: [PATCH 2/2] iommu: dma: Use of_iommu_get_resv

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Date:   Mon, 2 Sep 2019 16:52:45 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Joerg Roedel <joro@...tes.org>, Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Will Deacon <will@...nel.org>,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iommu: dma: Use of_iommu_get_resv_regions()

On Mon, Sep 02, 2019 at 03:22:35PM +0100, Robin Murphy wrote:
> On 29/08/2019 12:14, Thierry Reding wrote:
> > From: Thierry Reding <treding@...dia.com>
> > 
> > For device tree nodes, use the standard of_iommu_get_resv_regions()
> > implementation to obtain the reserved memory regions associated with a
> > device.
> 
> This covers the window between iommu_probe_device() setting up a default
> domain and the device's driver finally probing and taking control, but
> iommu_probe_device() represents the point that the IOMMU driver first knows
> about this device - there's still a window from whenever the IOMMU driver
> itself probed up to here where the "unidentified" traffic may have already
> been disrupted. Some IOMMU drivers have no option but to make the necessary
> configuration during their own probe routine, at which point a struct device
> for the display/etc. endpoint may not even exist yet.

Yeah, I think I'm actually running into this issue with the ARM SMMU
driver. The above works fine with the Tegra SMMU driver, though, because
it doesn't touch the SMMU configuration until a device is attached to a
domain.

For anything earlier than iommu_probe_device(), I don't see a way of
doing this generically. I've been working on a prototype to make these
reserved memory regions early on for ARM SMMU but I've been failing so
far. I think it would possibly work if we just switched the default for
stream IDs to be "bypass" if they have any devices that have reserved
memory regions, but again, this isn't quite working (yet).

Thierry

> > Cc: Rob Herring <robh+dt@...nel.org>
> > Cc: Frank Rowand <frowand.list@...il.com>
> > Cc: devicetree@...r.kernel.org
> > Signed-off-by: Thierry Reding <treding@...dia.com>
> > ---
> >   drivers/iommu/dma-iommu.c | 3 +++
> >   1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index de68b4a02aea..31d48e55ab55 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -19,6 +19,7 @@
> >   #include <linux/iova.h>
> >   #include <linux/irq.h>
> >   #include <linux/mm.h>
> > +#include <linux/of_iommu.h>
> >   #include <linux/pci.h>
> >   #include <linux/scatterlist.h>
> >   #include <linux/vmalloc.h>
> > @@ -164,6 +165,8 @@ void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list)
> >   	if (!is_of_node(dev_iommu_fwspec_get(dev)->iommu_fwnode))
> >   		iort_iommu_msi_get_resv_regions(dev, list);
> > +	if (dev->of_node)
> > +		of_iommu_get_resv_regions(dev, list);
> >   }
> >   EXPORT_SYMBOL(iommu_dma_get_resv_regions);
> > 

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