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

Joerg Roedel
2019-07-04T08:20:14Z

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.

> + /*
> + * 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.

> - 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?

Joerg