Re: [PATCH v4 9/9] drm/vkms: convert to use faux_device

From: Thomas Zimmermann
Date: Fri Jun 13 2025 - 08:33:54 EST

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Hi

Am 13.06.25 um 13:55 schrieb José Expósito:

Hi Thomas,

Thanks for the heads up, this issue fall through the cracks.

On Fri, Jun 13, 2025 at 10:15:05AM +0200, Thomas Zimmermann wrote:

Hi

Am 13.03.25 um 18:20 schrieb José Expósito:

On Thu, Mar 13, 2025 at 03:22:21PM +0100, Simona Vetter wrote:

On Wed, Mar 12, 2025 at 07:22:07AM +0100, Greg KH wrote:

On Tue, Mar 11, 2025 at 06:20:53PM +0100, José Expósito wrote:

Hi everyone,

On Tue, Feb 25, 2025 at 02:51:40PM +0100, Louis Chauvet wrote:

Le 25/02/2025 à 12:41, Thomas Zimmermann a écrit :

Hi

Am 10.02.25 um 15:37 schrieb Louis Chauvet:

On 10/02/25 - 13:30, Greg Kroah-Hartman wrote:

The vkms driver does not need to create a platform device, as there is
no real platform resources associated it, it only did so because it was
simple to do that in order to get a device to use for resource
management of drm resources. Change the driver to use the faux device
instead as this is NOT a real platform device.

Cc: Louis Chauvet <louis.chauvet@xxxxxxxxxxx>
Cc: Haneen Mohammed <hamohammed.sa@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: Melissa Wen <melissa.srw@xxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

Tested-by: Louis Chauvet <louis.chauvet@xxxxxxxxxxx>
Reviewed-by: Louis Chauvet <louis.chauvet@xxxxxxxxxxx>

Thanks for the modification, it seems to work.

Should this patch be merged through DRM trees? drm-misc-next is at
v6.14-rc4 and has struct faux_device.

Hi,

I was not aware the faux-device was merged, as it is a new feature, I
expected it to reach drm-misc-next on 6.15-rc1.

I added it to Linus's tree just so that DRM could get these changes into
their tree now :)

I plan to merge [1] today/tomorrow (well tested with platform_device), and
then I will submit an updated version of this patch (only trivial conflicts,
but never tested with multiple VKMS devices).

[1]:https://lore.kernel.org/all/20250218101214.5790-1-jose.exposito89@xxxxxxxxx/

Great, thanks!

greg k-h

Testing this patch again as part of some IGT tests I'm working on,
I noticed that, applying this patch on top of the latest drm-misc-next
triggers a warning at drivers/gpu/drm/drm_gem.c:571, in
drm_gem_get_pages():

if (WARN_ON(!obj->filp))
return ERR_PTR(-EINVAL);

I don't see how the faux bus change would have anything to do with a
filp as that's not related as far as I can tell. But I don't know the
drm layer at all, where does that filp come from?

Yeah that filp is the shmem file that backs gem bo. That's very far away
from anything device/driver related datastrctures. If this is a new
failure due to the aux bux conversion then it would be really surprising.

Agreed, I find it surprising, but reverting the patch removes the warning.

It's most likely an issue on my side, but I decided to double check just
in case someone else is also seeing this warning.

Any news on this issue?

I tested again with drm-misc-next. At the moment of writing this, the last
commit is 6bd90e700b42 ("drm/xe: Make dma-fences compliant with the safe
access rules") and I still see a similar warning. The stack trace changed,
but the warning is still present.

I'm going to detail the exact steps I followed. Let's see if someone else is
able to reproduce the issue:

I started by applying the patches from this series that are not already merged:

- [PATCH v4 4/9] x86/microcode: move away from using a fake platform
- [PATCH v4 5/9] wifi: cfg80211: move away from using a fake
- [PATCH v4 8/9] drm/vgem/vgem_drv convert to use faux_device
- [PATCH v4 9/9] drm/vkms: convert to use faux_device

The last patch has small conflict in vkms_drv.h that I solved like this:

struct vkms_device {
struct drm_device drm;
struct faux_device *faux_dev;
const struct vkms_config *config;
};

And in vkms_drv.c:

static int vkms_create(struct vkms_config *config)
{
int ret;
struct faux_device *fdev;
struct vkms_device *vkms_device;
const char *dev_name;

dev_name = vkms_config_get_device_name(config);
fdev = faux_device_create(dev_name, NULL, NULL);
if (!fdev)
return -ENODEV;

Next, I installed the new kernel in a QEMU virtual machine running Fedora 41.
There is nothing special about my Fedora, it is the regular desktop version.

After a reboot, "sudo modprobe vkms" shows a similar warning in dmesg.
For reference, the warning is at the end of my email.

Am I the only one sawing this warning?

Jose

---

[ 69.417850] [drm] Initialized vkms 1.0.0 for vkms on minor 1
[ 69.419446] faux_driver vkms: [drm] fb1: vkmsdrmfb frame buffer device
[ 69.520944] ------------[ cut here ]------------
[ 69.520954] WARNING: CPU: 2 PID: 1015 at drivers/dma-buf/dma-buf.c:1518 dma_buf_vmap+0x212/0x540
[ 69.520992] Modules linked in: vkms snd_seq_dummy snd_hrtimer snd_seq snd_seq_device snd_timer snd soundcore nf_conntrack_netbios_ns nf_conntrack_broadcast nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nf_tables rfkill qrtr sunrpc binfmt_misc ppdev pktcdvd parport_pc parport pcspkr i2c_piix4 e1000 i2c_smbus joydev loop nfnetlink vsock_loopback zram vmw_vsock_virtio_transport_common vmw_vsock_vmci_transport vmw_vmci vsock bochs serio_raw ata_generic pata_acpi fuse qemu_fw_cfg
[ 69.521082] CPU: 2 UID: 42 PID: 1015 Comm: KMS thread Not tainted 6.16.0-rc1+ #3 PREEMPT(voluntary)
[ 69.521092] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-4.fc42 04/01/2014
[ 69.521095] RIP: 0010:dma_buf_vmap+0x212/0x540

That's dmabuf. could be related to the fact that there's no real DMA possible with the faux_device. We now have support for dmabuf-without-DMA, sort of. Could you please replace DRM_GEM_SHMEM_DRIVER_OPS with the new DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT at [1]. That would avoid any actual hardware-DMA functionality.

[1] https://elixir.bootlin.com/linux/v6.15.1/source/drivers/gpu/drm/vkms/vkms_drv.c#L104

Best regards
Thomas

[ 69.521105] Code: 7c 41 ff 03 0f 85 0a 02 00 00 c9 e9 c8 47 0c 01 80 3c 06 00 0f 85 c4 01 00 00 48 c7 01 00 00 00 00 48 85 d2 0f 85 bd fe ff ff <0f> 0b b8 ea ff ff ff eb af 48 85 f6 0f 85 cf 01 00 00 48 89 4c 24
[ 69.521112] RSP: 0018:ffffc90006a5f690 EFLAGS: 00010246
[ 69.521125] RAX: dffffc0000000000 RBX: 1ffff92000d4beea RCX: ffff88811467dcc8
[ 69.521128] RDX: 0000000000000000 RSI: 1ffff110228cfb99 RDI: ffff88811467dcd0
[ 69.521131] RBP: ffffc90006a5f728 R08: 1ffff92000d4bed9 R09: fffff52000d4bef1
[ 69.521162] R10: fffff52000d4bef2 R11: ffff8881017f4e28 R12: ffff8881149594f0
[ 69.521165] R13: ffff888114959400 R14: 1ffff11023146b29 R15: ffff88811467dcc8
[ 69.521168] FS: 00007fbbdd1ff6c0(0000) GS:ffff888417580000(0000) knlGS:0000000000000000
[ 69.521172] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 69.521174] CR2: 00007fbbcc0345c8 CR3: 000000011ec5a000 CR4: 00000000000006f0
[ 69.521179] Call Trace:
[ 69.521182] <TASK>
[ 69.521185] ? __pfx_dma_buf_vmap+0x10/0x10
[ 69.521193] ? dma_resv_get_singleton+0x9a/0x2a0
[ 69.521197] drm_gem_shmem_vmap_locked+0xc2/0x5f0
[ 69.521208] ? __pfx_drm_gem_shmem_vmap_locked+0x10/0x10
[ 69.521212] ? __pfx_ww_mutex_lock+0x10/0x10
[ 69.521225] ? sched_clock_noinstr+0xd/0x20
[ 69.521230] ? local_clock_noinstr+0x13/0xf0
[ 69.521233] drm_gem_shmem_object_vmap+0xd/0x20
[ 69.521237] drm_gem_vmap_locked+0x70/0xf0
[ 69.521247] drm_gem_vmap+0x4c/0xa0
[ 69.521250] drm_gem_fb_vmap+0xb2/0x3b0
[ 69.521255] vkms_prepare_fb+0x6f/0x90 [vkms]
[ 69.521264] ? drm_atomic_helper_setup_commit+0xb7b/0x1320
[ 69.521268] drm_atomic_helper_prepare_planes+0x19f/0xb90
[ 69.521272] ? __pfx_drm_atomic_helper_commit+0x10/0x10
[ 69.521276] drm_atomic_helper_commit+0x126/0x2d0
[ 69.521279] ? __pfx_drm_atomic_helper_commit+0x10/0x10
[ 69.521282] drm_atomic_commit+0x205/0x2d0
[ 69.521290] ? _raw_spin_lock_irqsave+0x97/0xf0
[ 69.521295] ? __pfx_drm_atomic_commit+0x10/0x10
[ 69.521299] ? __pfx___drm_printfn_info+0x10/0x10
[ 69.521313] ? drm_event_reserve_init+0x1cd/0x260
[ 69.521318] drm_mode_atomic_ioctl+0x1c79/0x2d30
[ 69.521323] ? __pfx_drm_mode_atomic_ioctl+0x10/0x10
[ 69.521326] ? __kasan_check_write+0x18/0x20
[ 69.521339] drm_ioctl_kernel+0x17b/0x2f0
[ 69.521343] ? __pfx_drm_mode_atomic_ioctl+0x10/0x10
[ 69.521349] ? __pfx_drm_ioctl_kernel+0x10/0x10
[ 69.521353] ? __pfx_do_vfs_ioctl+0x10/0x10
[ 69.521361] ? __kasan_check_write+0x18/0x20
[ 69.521365] drm_ioctl+0x51b/0xbd0
[ 69.521369] ? __pfx_drm_mode_atomic_ioctl+0x10/0x10
[ 69.521373] ? __pfx_drm_ioctl+0x10/0x10
[ 69.521378] ? selinux_file_ioctl+0xfc/0x260
[ 69.521390] __x64_sys_ioctl+0x143/0x1d0
[ 69.521394] x64_sys_call+0xf4b/0x1d70
[ 69.521404] do_syscall_64+0x82/0x2a0
[ 69.521408] ? __kasan_check_write+0x18/0x20
[ 69.521411] ? do_user_addr_fault+0x491/0xa60
[ 69.521420] ? irqentry_exit+0x3f/0x50
[ 69.521423] ? exc_page_fault+0x8b/0xe0
[ 69.521426] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 69.521430] RIP: 0033:0x7fbc078fd8ed
[ 69.521441] Code: 04 25 28 00 00 00 48 89 45 c8 31 c0 48 8d 45 10 c7 45 b0 10 00 00 00 48 89 45 b8 48 8d 45 d0 48 89 45 c0 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1a 48 8b 45 c8 64 48 2b 04 25 28 00 00 00
[ 69.521444] RSP: 002b:00007fbbdd1fd9b0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[ 69.521449] RAX: ffffffffffffffda RBX: 00007fbbcc02af60 RCX: 00007fbc078fd8ed
[ 69.521452] RDX: 00007fbbdd1fda50 RSI: 00000000c03864bc RDI: 0000000000000035
[ 69.521455] RBP: 00007fbbdd1fda00 R08: 00000000000000e0 R09: 0000000000000001
[ 69.521457] R10: 0000000000000003 R11: 0000000000000246 R12: 00007fbbdd1fda50
[ 69.521459] R13: 00000000c03864bc R14: 0000000000000035 R15: 00007fbbcc02acf0
[ 69.521464] </TASK>
[ 69.521466] ---[ end trace 0000000000000000 ]---

Best regards
Thomas

Jose

-Sima

--
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Next message: Miao, Jun: "RE: [PATCH V2] net: usb: Convert tasklet API to new bottom half workqueue mechanism"
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In reply to: José Expósito: "Re: [PATCH v4 9/9] drm/vkms: convert to use faux_device"
Next in thread: José Expósito: "Re: [PATCH v4 9/9] drm/vkms: convert to use faux_device"
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