Re: [Aarch64-laptops] [PATCH 1/2] iommu: add support for drivers that manage iommu explicitly

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