Re: [PATCH v3 8/8] drm/vkms: convert to use faux_device

From: Louis Chauvet
Date: Fri Feb 07 2025 - 12:01:32 EST

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In reply to: Greg Kroah-Hartman: "Re: [PATCH v3 8/8] drm/vkms: convert to use faux_device"
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On 06/02/25 - 18:38, 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
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
> v3: new patch in the series. For an example of the api working, does
> not have to be merged at this time, but I can take it if the
> maintainers give an ack.

Hi,

This patch cannot be merged into drm-misc-next because we modified the
vkms_device structure in commit 49a167c393b0 ("drm/vkms: Switch to dynamic
allocation for CRTC"), which is present in linux-next.

Once this conflict is resolved, I agree with changing from platform_device
to faux_device.

Apart from this minor conflict, I believe your patch has revealed an issue
in VKMS:

Without your patch:

bash-5.2# modprobe vkms
[drm] Initialized vkms 1.0.0 for vkms on minor 0
bash-5.2#

With your patch:

bash-5.2# modprobe vkms
faux vkms: Resources present before probing
[drm] Initialized vkms 1.0.0 for vkms on minor 0
bash-5.2#

After some investigation, I found that the issue is not caused by your
patch but by VKMS itself:

During faux_device_create, the device core postpones the device probe to
run it later [0]. This probe checks if the devres list is empty [1] and
fails if it is not.

[0]:https://elixir.bootlin.com/linux/v6.13.1/source/drivers/base/bus.c#L534
[1]:https://elixir.bootlin.com/linux/v6.13.1/source/drivers/base/dd.c#L626

With a platform driver, the order of execution was:

platform_device_register_simple();
device_add();
*async* device_probe(); /* no issue, the devres is untouched */
devres_open_group();

But with faux-device, the order is:

faux_device_create();
device_add();
devres_open_group();
*async* device_probe(); /* issue here, because of the previous
devres_open_group */

How do you think this should be solved? I would like to keep a simple
solution, given that:
- we want to have multiple vkms devices (configfs [2])
- we need to ensure that device_probe is called before devres_open_group
and devm_drm_dev_alloc to avoid this error

[2]:https://lore.kernel.org/all/20250121-google-config-fs-v3-0-8154a6945142@xxxxxxxxxxx/

I found two other drm driver that may be broken in the same way (very
similar code pattern):
https://elixir.bootlin.com/linux/v6.13.1/source/drivers/gpu/drm/amd/amdxcp/amdgpu_xcp_drv.c#L64
https://elixir.bootlin.com/linux/v6.13.1/source/drivers/gpu/drm/vgem/vgem_drv.c#L138

Thanks a lot,
Louis Chauvet

Change to hide the issue:

diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
index 89ccf0d6419a..84777d6ba889 100644
--- a/drivers/gpu/drm/vkms/vkms_drv.c
+++ b/drivers/gpu/drm/vkms/vkms_drv.c
@@ -174,7 +174,7 @@ static int vkms_create(struct vkms_config *config)
fdev = faux_device_create(DRIVER_NAME, NULL);
if (!fdev)
return -ENODEV;
-
+ pr_err("%s:%d\n", __FILE__, __LINE__);
if (!devres_open_group(&fdev->dev, NULL, GFP_KERNEL)) {
ret = -ENOMEM;
goto out_unregister;

> drivers/gpu/drm/vkms/vkms_drv.c | 28 ++++++++++++++--------------
> drivers/gpu/drm/vkms/vkms_drv.h | 4 ++--
> 2 files changed, 16 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> index e0409aba9349..b1269f984886 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.c
> +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> @@ -10,7 +10,7 @@
> */
>
> #include <linux/module.h>
> -#include <linux/platform_device.h>
> +#include <linux/device/faux.h>
> #include <linux/dma-mapping.h>
>
> #include <drm/clients/drm_client_setup.h>
> @@ -177,25 +177,25 @@ static int vkms_modeset_init(struct vkms_device *vkmsdev)
> static int vkms_create(struct vkms_config *config)
> {
> int ret;
> - struct platform_device *pdev;
> + struct faux_device *fdev;
> struct vkms_device *vkms_device;
>
> - pdev = platform_device_register_simple(DRIVER_NAME, -1, NULL, 0);
> - if (IS_ERR(pdev))
> - return PTR_ERR(pdev);
> + fdev = faux_device_create(DRIVER_NAME, NULL);
> + if (!fdev)
> + return -ENODEV;
>
> - if (!devres_open_group(&pdev->dev, NULL, GFP_KERNEL)) {
> + if (!devres_open_group(&fdev->dev, NULL, GFP_KERNEL)) {
> ret = -ENOMEM;
> goto out_unregister;
> }
>
> - vkms_device = devm_drm_dev_alloc(&pdev->dev, &vkms_driver,
> + vkms_device = devm_drm_dev_alloc(&fdev->dev, &vkms_driver,
> struct vkms_device, drm);
> if (IS_ERR(vkms_device)) {
> ret = PTR_ERR(vkms_device);
> goto out_devres;
> }
> - vkms_device->platform = pdev;
> + vkms_device->faux_dev = fdev;
> vkms_device->config = config;
> config->dev = vkms_device;
>
> @@ -229,9 +229,9 @@ static int vkms_create(struct vkms_config *config)
> return 0;
>
> out_devres:
> - devres_release_group(&pdev->dev, NULL);
> + devres_release_group(&fdev->dev, NULL);
> out_unregister:
> - platform_device_unregister(pdev);
> + faux_device_destroy(fdev);
> return ret;
> }
>
> @@ -259,19 +259,19 @@ static int __init vkms_init(void)
>
> static void vkms_destroy(struct vkms_config *config)
> {
> - struct platform_device *pdev;
> + struct faux_device *fdev;
>
> if (!config->dev) {
> DRM_INFO("vkms_device is NULL.\n");
> return;
> }
>
> - pdev = config->dev->platform;
> + fdev = config->dev->faux_dev;
>
> drm_dev_unregister(&config->dev->drm);
> drm_atomic_helper_shutdown(&config->dev->drm);
> - devres_release_group(&pdev->dev, NULL);
> - platform_device_unregister(pdev);
> + devres_release_group(&fdev->dev, NULL);
> + faux_device_destroy(fdev);
>
> config->dev = NULL;
> }
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index 00541eff3d1b..4668b0e29a84 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -209,13 +209,13 @@ struct vkms_config {
> * struct vkms_device - Description of a VKMS device
> *
> * @drm - Base device in DRM
> - * @platform - Associated platform device
> + * @faux_dev- Associated faux device
> * @output - Configuration and sub-components of the VKMS device
> * @config: Configuration used in this VKMS device
> */
> struct vkms_device {
> struct drm_device drm;
> - struct platform_device *platform;
> + struct faux_device *faux_dev;
> struct vkms_output output;
> const struct vkms_config *config;
> };
> --
> 2.48.1
>

Next message: Dmitry Baryshkov: "Re: [PATCH v2 1/2] dt-bindings: display: qcom,sm8550-mdss: only document the mdp0-mem interconnect path"
Previous message: Faizal Rahim: "[PATCH iwl-next v3 9/9] igc: Add support to get frame preemption statistics via ethtool"
In reply to: Greg Kroah-Hartman: "Re: [PATCH v3 8/8] drm/vkms: convert to use faux_device"
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