Re: [PATCH 1/2] iommu: add support for drivers that manage iommu explicitly

Rob Clark
2019-07-04T13:51:43Z

On Thu, Jul 4, 2019 at 1:20 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> Hi Rob,
>
> On Tue, Jul 02, 2019 at 01:26:18PM -0700, Rob Clark wrote:
> > 1) In some cases the bootloader takes the iommu out of bypass and
> > enables the display. This is in particular a problem on the aarch64
> > laptops that exist these days, and modern snapdragon android devices.
> > (Older devices also enabled the display in bootloader but did not
> > take the iommu out of bypass.) Attaching a DMA or IDENTITY domain
> > while scanout is active, before the driver has a chance to intervene,
> > makes things go *boom*
>
> Just to make sure I get this right: The bootloader inializes the SMMU
> and creates non-identity mappings for the GPU? And when the SMMU driver
> in Linux takes over this breaks display output.

correct

> > + /*
> > + * If driver is going to manage iommu directly, then avoid
> > + * attaching any non driver managed domain. There could
> > + * be already active dma underway (ie. scanout in case of
> > + * bootloader enabled display), and interfering with that
> > + * will make things go *boom*
> > + */
> > + if ((domain->type != IOMMU_DOMAIN_UNMANAGED) &&
> > + dev->driver && dev->driver->driver_manages_iommu)
> > + return 0;
> > +
>
> When the default domain is attached, there is usually no driver attached
> yet. I think this needs to be communicated by the firmware to Linux and
> the code should check against that.

At least for the OF case, it happens in the of_dma_configure() which
happens from really_probe(), so there is normally a driver. There are
a few exceptional cases, where drivers call of_dma_configure() on
their own sub-device without a driver attached (hence the need to
check if dev->driver is NULL).

I'm also interested in the ACPI case eventually... the aarch64
"windows" laptops do have ACPI. But for now we are booting with DT
since there is quite a lot of work before we get to point of using
ACPI. (In particular, under windows, device power management is done
thru a Platform Extension Plugin (PEP), but so far linux has no such
mechanism.)

We really don't have control of the firmware. But when arm-smmu is
probed it can read back the hw state and figure out what is going on
(with an RFC series[1] from Bjorn which was posted earlier), so we
don't really need to depend on the firmware.

> > - bool suppress_bind_attrs; /* disables bind/unbind via sysfs */
> > + bool suppress_bind_attrs:1; /* disables bind/unbind via sysfs */
> > + bool driver_manages_iommu:1; /* driver manages IOMMU explicitly */
>
> How does this field get set?

It is set in the driver in the second patch[2] in this series.

BR,
-R

[1] https://www.spinics.net/lists/arm-kernel/msg732246.html
[2] https://patchwork.freedesktop.org/patch/315291/