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Hi, sent here by the IOMMU upstream maintainer:

This is likely a bug in the DRM code for your GPU. The recent changes in the AMD IOMMU driver might cause sg-list entries to be merged by the DMA-API. Some drivers probably don't handle this correctly. I've seen a similar problem with RDMA devices, caused by the same change.

(RDMA thread mentioned above: https://www.spinics.net/lists/linux-nfs/msg76396.html)

Bug report:

Windows of OpenGL applications ran with DRI_PRIME=1 environment variable (hybrid graphics) are filled with white backgrounds when scaled instead of being rendered properly.

How reproducible:

Always

Steps to Reproduce:

1. Boot with kernel version 5.5rc1 or higher and login to your xorg session (I use xmonad without a compositor)
2. Run any OpenGL application with DRI_PRIME=1 set. I used glxgears for this test (tested mpv with --vo=opengl, radium are known to be affected as well) For example:

DRI_PRIME=1 glxgears

Actual results:

Half of the window is filled with white, white pixels do not go away as you resize the window.

Expected results:

Application is rendered correctly.

Bisect results:

commit be62dbf554c5b50718a54a359372c148cd9975c7
Author: Tom Murphy <murphyt7@tcd.ie>
Date:   Sun Sep 8 09:56:41 2019 -0700

    iommu/amd: Convert AMD iommu driver to the dma-iommu api

    Convert the AMD iommu driver to the dma-iommu api. Remove the iova
    handling and reserve region code from the AMD iommu driver.

    Signed-off-by: Tom Murphy <murphyt7@tcd.ie>
    Signed-off-by: Joerg Roedel <jroedel@suse.de>

 drivers/iommu/Kconfig     |   1 +
 drivers/iommu/amd_iommu.c | 692 +++++-----------------------------------------
 2 files changed, 68 insertions(+), 625 deletions(-)

Extra info:

dmesg prints this from radeon/amd-vi once when glxgears is started:

AMD-Vi: Event logged [IO_PAGE_FAULT device=08:00.0 domain=0x0000 address=0xfffffff2c0 flags=0x0010]

awk -f scripts/ver_linux

GNU C                   9.2.0
GNU Make                4.3
Binutils                2.33.1
Util-linux              2.35.1
Mount                   2.35.1
Module-init-tools       26
E2fsprogs               1.45.5
Jfsutils                1.1.15
Reiserfsprogs           3.6.27
Xfsprogs                5.4.0
Bison                   3.5.1
Flex                    2.6.4
Linux C Library         2.30
Dynamic linker (ldd)    2.30
Linux C++ Library       6.0.27
Procps                  3.3.15
Net-tools               2.10
Kbd                     2.2.0
Console-tools           2.2.0
Sh-utils                8.31
Udev                    244
Modules Loaded          acpi_cpufreq aesni_intel agpgart ahci amdgpu asus_wmi async_memcpy async_pq async_raid6_recov async_tx async_xor battery bcache blake2b_generic btrfs ccp crc32c_ge
neric crc32c_intel crc32_pclmul crc64 crct10dif_pclmul cryptd crypto_simd dca dm_mod dm_raid drm drm_kms_helper eeepc_wmi evdev fat fb_sys_fops ghash_clmulni_intel glue_helper gpio_amdpt
gpu_sched hid hid_generic i2c_algo_bit i2c_piix4 igb input_leds ip_tables irqbypass jc42 joydev k10temp kvm kvm_amd libahci libata libcrc32c mac_hid macvlan macvtap mc md_mod mousedev mxm
_wmi nls_cp437 nls_iso8859_1 pcspkr pinctrl_amd radeon raid1 raid456 raid6_pq rfkill rng_core scsi_mod sd_mod snd snd_aloop snd_hda_codec snd_hda_codec_hdmi snd_hda_core snd_hda_intel snd
_hwdep snd_intel_dspcfg snd_pcm snd_rawmidi snd_timer snd_usb_audio snd_usbmidi_lib soundcore sparse_keymap syscopyarea sysfillrect sysimgblt tap ttm tun usbhid vfat vfio vfio_iommu_type1
 vfio_pci vfio_virqfd vhost vhost_net wmi wmi_bmof xfs xhci_hcd xhci_pci xor x_tables

Known fixes:

Reverting commit be62dbf554c5b50718a54a359372c148cd9975c7 on 5.5 tag or disabling IOMMU in bios resolves the issue and glxgears is rendered correctly.

git-bisect.log lspci_-vv.txt dmesg-output.txt