[PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[Javier Martinez Canillas]

Hello,

The patches in this series are mostly changes suggested by Daniel Vetter
to fix some race conditions that exists between the fbdev core (fbmem)
and sysfb with regard to device registration and removal.

For example, it is currently possible for sysfb to register a platform
device after a real DRM driver was registered and requested to remove the
conflicting framebuffers.

A symptom of this issue, was worked around with by commit fb561bf9abde
("fbdev: Prevent probing generic drivers if a FB is already registered")
but that's really a hack and should be reverted.

This series attempt to fix it more properly and revert the mentioned hack.
That will also unblock a pending patch to not make the num_registered_fb
variable visible to drivers anymore, since that's internal to fbdev core.

Patch #1 is just a trivial preparatory change.

Patch #2 add sysfb_disable() and sysfb_try_unregister() helpers for fbmem
to use them.

Patch #3 changes how is dealt with conflicting framebuffers unregistering,
rather than having a variable to determine if a lock should be take, it
just drops the lock before unregistering the platform device.

Patch #4 fixes the mentioned race conditions and finally patch #5 is the
revert patch that was posted by Daniel before but he dropped from his set.

The patches were tested on a rpi4 using different video configurations:
(simpledrm -> vc4 both builtin, only vc4 builtin, only simpledrm builtin
and simpledrm builtin with vc4 built as a module).

Best regards,
Javier

Changes in v3:
- Rebase on top of latest drm-misc-next branch.

Changes in v2:
- Rebase on top of latest drm-misc-next and fix conflicts (Daniel Vetter).
- Add kernel-doc comments and include in other_interfaces.rst (Daniel Vetter).
- Explain in the commit message that fbmem has to unregister the device
  as fallback if a driver registered the device itself (Daniel Vetter).
- Also explain that fallback in a comment in the code (Daniel Vetter).
- Don't encode in fbmem the assumption that sysfb will always register
  platform devices (Daniel Vetter).
- Add a FIXME comment about drivers registering devices (Daniel Vetter).
- Drop RFC prefix since patches were already reviewed by Daniel Vetter.
- Add Daniel Reviewed-by tags to the patches.

Daniel Vetter (1):
  Revert "fbdev: Prevent probing generic drivers if a FB is already
    registered"

Javier Martinez Canillas (4):
  firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer
  firmware: sysfb: Add helpers to unregister a pdev and disable
    registration
  fbdev: Restart conflicting fb removal loop when unregistering devices
  fbdev: Fix some race conditions between fbmem and sysfb

 .../driver-api/firmware/other_interfaces.rst  |  6 ++
 drivers/firmware/sysfb.c                      | 77 +++++++++++++++++--
 drivers/firmware/sysfb_simplefb.c             | 16 ++--
 drivers/video/fbdev/core/fbmem.c              | 62 ++++++++++++---
 drivers/video/fbdev/efifb.c                   | 11 ---
 drivers/video/fbdev/simplefb.c                | 11 ---
 include/linux/fb.h                            |  1 -
 include/linux/sysfb.h                         | 29 +++++--
 8 files changed, 158 insertions(+), 55 deletions(-)

-- 
2.35.1

[PATCH v3 2/5] firmware: sysfb: Add helpers to unregister a pdev and disable registration

[Javier Martinez Canillas]

These can be used by subsystems to unregister a platform device registered
by sysfb and also to disable future platform device registration in sysfb.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---

(no changes since v2)

Changes in v2:
- Add kernel-doc comments and include in other_interfaces.rst (Daniel Vetter).

 .../driver-api/firmware/other_interfaces.rst  |  6 ++
 drivers/firmware/sysfb.c                      | 73 +++++++++++++++++--
 include/linux/sysfb.h                         | 19 +++++
 3 files changed, 92 insertions(+), 6 deletions(-)

diff --git a/Documentation/driver-api/firmware/other_interfaces.rst b/Documentation/driver-api/firmware/other_interfaces.rst
index b81794e0cfbb..06ac89adaafb 100644
--- a/Documentation/driver-api/firmware/other_interfaces.rst
+++ b/Documentation/driver-api/firmware/other_interfaces.rst
@@ -13,6 +13,12 @@ EDD Interfaces
 .. kernel-doc:: drivers/firmware/edd.c
    :internal:
 
+Generic System Framebuffers Interface
+-------------------------------------
+
+.. kernel-doc:: drivers/firmware/sysfb.c
+   :export:
+
 Intel Stratix10 SoC Service Layer
 ---------------------------------
 Some features of the Intel Stratix10 SoC require a level of privilege
diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
index b032f40a92de..a50d2858ce4d 100644
--- a/drivers/firmware/sysfb.c
+++ b/drivers/firmware/sysfb.c
@@ -34,21 +34,78 @@
 #include <linux/screen_info.h>
 #include <linux/sysfb.h>
 
+static struct platform_device *pd;
+static DEFINE_MUTEX(load_lock);
+static bool disabled;
+
+/**
+ * sysfb_disable() - disable the Generic System Framebuffers support
+ *
+ * This disables the registration of system framebuffer devices that match the
+ * generic drivers that make use of the system framebuffer set up by firmware.
+ *
+ * Context: The function can sleep. A @load_lock mutex is acquired to serialize
+ *          against sysfb_init(), that registers a system framebuffer device and
+ *          sysfb_try_unregister(), that tries to unregister framebuffer devices.
+ */
+void sysfb_disable(void)
+{
+	mutex_lock(&load_lock);
+	disabled = true;
+	mutex_unlock(&load_lock);
+}
+EXPORT_SYMBOL_GPL(sysfb_disable);
+
+/**
+ * sysfb_try_unregister() - attempt to unregister a system framebuffer device
+ * @dev: device to unregister
+ *
+ * This tries to unregister a system framebuffer device if this was registered
+ * by the Generic System Framebuffers. The device will only be unregistered if
+ * it was registered by sysfb_init(), otherwise it will not be unregistered.
+ *
+ * Context: The function can sleep. a @load_lock mutex is acquired to serialize
+ *          against sysfb_init(), that registers a simple framebuffer device and
+ *          sysfb_disable(), that disables the Generic System Framebuffers support.
+ *
+ * Return:
+ * * true          - the device was unregistered successfully
+ * * false         - the device was not unregistered
+ */
+bool sysfb_try_unregister(struct device *dev)
+{
+	bool ret = true;
+
+	mutex_lock(&load_lock);
+	if (!pd || pd != to_platform_device(dev))
+		return false;
+
+	platform_device_unregister(to_platform_device(dev));
+	pd = NULL;
+	mutex_unlock(&load_lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(sysfb_try_unregister);
+
 static __init int sysfb_init(void)
 {
 	struct screen_info *si = &screen_info;
 	struct simplefb_platform_data mode;
-	struct platform_device *pd;
 	const char *name;
 	bool compatible;
-	int ret;
+	int ret = 0;
+
+	mutex_lock(&load_lock);
+	if (disabled)
+		goto unlock_mutex;
 
 	/* try to create a simple-framebuffer device */
 	compatible = sysfb_parse_mode(si, &mode);
 	if (compatible) {
 		pd = sysfb_create_simplefb(si, &mode);
 		if (!IS_ERR(pd))
-			return 0;
+			goto unlock_mutex;
 	}
 
 	/* if the FB is incompatible, create a legacy framebuffer device */
@@ -60,8 +117,10 @@ static __init int sysfb_init(void)
 		name = "platform-framebuffer";
 
 	pd = platform_device_alloc(name, 0);
-	if (!pd)
-		return -ENOMEM;
+	if (!pd) {
+		ret = -ENOMEM;
+		goto unlock_mutex;
+	}
 
 	sysfb_apply_efi_quirks(pd);
 
@@ -73,9 +132,11 @@ static __init int sysfb_init(void)
 	if (ret)
 		goto err;
 
-	return 0;
+	goto unlock_mutex;
 err:
 	platform_device_put(pd);
+unlock_mutex:
+	mutex_unlock(&load_lock);
 	return ret;
 }
 
diff --git a/include/linux/sysfb.h b/include/linux/sysfb.h
index 708152e9037b..e8c0313fac8f 100644
--- a/include/linux/sysfb.h
+++ b/include/linux/sysfb.h
@@ -55,6 +55,25 @@ struct efifb_dmi_info {
 	int flags;
 };
 
+#ifdef CONFIG_SYSFB
+
+void sysfb_disable(void);
+bool sysfb_try_unregister(struct device *dev);
+
+#else /* CONFIG_SYSFB */
+
+static inline void sysfb_disable(void)
+{
+
+}
+
+static inline bool sysfb_try_unregister(struct device *dev)
+{
+	return false;
+}
+
+#endif /* CONFIG_SYSFB */
+
 #ifdef CONFIG_EFI
 
 extern struct efifb_dmi_info efifb_dmi_list[];
-- 
2.35.1

Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[Greg Kroah-Hartman]

On Wed, Apr 20, 2022 at 10:52:58AM +0200, Javier Martinez Canillas wrote:
> Hello,
> 
> The patches in this series are mostly changes suggested by Daniel Vetter
> to fix some race conditions that exists between the fbdev core (fbmem)
> and sysfb with regard to device registration and removal.
> 
> For example, it is currently possible for sysfb to register a platform
> device after a real DRM driver was registered and requested to remove the
> conflicting framebuffers.
> 
> A symptom of this issue, was worked around with by commit fb561bf9abde
> ("fbdev: Prevent probing generic drivers if a FB is already registered")
> but that's really a hack and should be reverted.
> 
> This series attempt to fix it more properly and revert the mentioned hack.
> That will also unblock a pending patch to not make the num_registered_fb
> variable visible to drivers anymore, since that's internal to fbdev core.
> 
> Patch #1 is just a trivial preparatory change.
> 
> Patch #2 add sysfb_disable() and sysfb_try_unregister() helpers for fbmem
> to use them.
> 
> Patch #3 changes how is dealt with conflicting framebuffers unregistering,
> rather than having a variable to determine if a lock should be take, it
> just drops the lock before unregistering the platform device.
> 
> Patch #4 fixes the mentioned race conditions and finally patch #5 is the
> revert patch that was posted by Daniel before but he dropped from his set.
> 
> The patches were tested on a rpi4 using different video configurations:
> (simpledrm -> vc4 both builtin, only vc4 builtin, only simpledrm builtin
> and simpledrm builtin with vc4 built as a module).
> 
> Best regards,
> Javier

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Re: [PATCH v3 2/5] firmware: sysfb: Add helpers to unregister a pdev and disable registration

[Thomas Zimmermann]

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Hi

Am 20.04.22 um 10:53 schrieb Javier Martinez Canillas:
> These can be used by subsystems to unregister a platform device registered
> by sysfb and also to disable future platform device registration in sysfb.
> 
> Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
> 
> (no changes since v2)
> 
> Changes in v2:
> - Add kernel-doc comments and include in other_interfaces.rst (Daniel Vetter).
> 
>   .../driver-api/firmware/other_interfaces.rst  |  6 ++
>   drivers/firmware/sysfb.c                      | 73 +++++++++++++++++--
>   include/linux/sysfb.h                         | 19 +++++
>   3 files changed, 92 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/driver-api/firmware/other_interfaces.rst b/Documentation/driver-api/firmware/other_interfaces.rst
> index b81794e0cfbb..06ac89adaafb 100644
> --- a/Documentation/driver-api/firmware/other_interfaces.rst
> +++ b/Documentation/driver-api/firmware/other_interfaces.rst
> @@ -13,6 +13,12 @@ EDD Interfaces
>   .. kernel-doc:: drivers/firmware/edd.c
>      :internal:
>   
> +Generic System Framebuffers Interface
> +-------------------------------------
> +
> +.. kernel-doc:: drivers/firmware/sysfb.c
> +   :export:
> +
>   Intel Stratix10 SoC Service Layer
>   ---------------------------------
>   Some features of the Intel Stratix10 SoC require a level of privilege
> diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
> index b032f40a92de..a50d2858ce4d 100644
> --- a/drivers/firmware/sysfb.c
> +++ b/drivers/firmware/sysfb.c
> @@ -34,21 +34,78 @@
>   #include <linux/screen_info.h>
>   #include <linux/sysfb.h>
>   
> +static struct platform_device *pd;

You could have multiple instances of simple-framebuffer because you 
don't know what the firmware set up. It originated from DT and EFI/VESA 
is really just an afterthought. If there is more than 1 instance, your 
entire unregistering breaks down.

(I've been told that such multi-output cases exist in the context of OF 
framebuffers. Something similar could exist for simple-frambuffer as well.)

Best regards
Thomas

> +static DEFINE_MUTEX(load_lock);
> +static bool disabled;
> +
> +/**
> + * sysfb_disable() - disable the Generic System Framebuffers support
> + *
> + * This disables the registration of system framebuffer devices that match the
> + * generic drivers that make use of the system framebuffer set up by firmware.
> + *
> + * Context: The function can sleep. A @load_lock mutex is acquired to serialize
> + *          against sysfb_init(), that registers a system framebuffer device and
> + *          sysfb_try_unregister(), that tries to unregister framebuffer devices.
> + */
> +void sysfb_disable(void)
> +{
> +	mutex_lock(&load_lock);
> +	disabled = true;
> +	mutex_unlock(&load_lock);
> +}
> +EXPORT_SYMBOL_GPL(sysfb_disable);
> +
> +/**
> + * sysfb_try_unregister() - attempt to unregister a system framebuffer device
> + * @dev: device to unregister
> + *
> + * This tries to unregister a system framebuffer device if this was registered
> + * by the Generic System Framebuffers. The device will only be unregistered if
> + * it was registered by sysfb_init(), otherwise it will not be unregistered.
> + *
> + * Context: The function can sleep. a @load_lock mutex is acquired to serialize
> + *          against sysfb_init(), that registers a simple framebuffer device and
> + *          sysfb_disable(), that disables the Generic System Framebuffers support.
> + *
> + * Return:
> + * * true          - the device was unregistered successfully
> + * * false         - the device was not unregistered
> + */
> +bool sysfb_try_unregister(struct device *dev)
> +{
> +	bool ret = true;
> +
> +	mutex_lock(&load_lock);
> +	if (!pd || pd != to_platform_device(dev))
> +		return false;
> +
> +	platform_device_unregister(to_platform_device(dev));
> +	pd = NULL;
> +	mutex_unlock(&load_lock);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(sysfb_try_unregister);
> +
>   static __init int sysfb_init(void)
>   {
>   	struct screen_info *si = &screen_info;
>   	struct simplefb_platform_data mode;
> -	struct platform_device *pd;
>   	const char *name;
>   	bool compatible;
> -	int ret;
> +	int ret = 0;
> +
> +	mutex_lock(&load_lock);
> +	if (disabled)
> +		goto unlock_mutex;
>   
>   	/* try to create a simple-framebuffer device */
>   	compatible = sysfb_parse_mode(si, &mode);
>   	if (compatible) {
>   		pd = sysfb_create_simplefb(si, &mode);
>   		if (!IS_ERR(pd))
> -			return 0;
> +			goto unlock_mutex;
>   	}
>   
>   	/* if the FB is incompatible, create a legacy framebuffer device */
> @@ -60,8 +117,10 @@ static __init int sysfb_init(void)
>   		name = "platform-framebuffer";
>   
>   	pd = platform_device_alloc(name, 0);
> -	if (!pd)
> -		return -ENOMEM;
> +	if (!pd) {
> +		ret = -ENOMEM;
> +		goto unlock_mutex;
> +	}
>   
>   	sysfb_apply_efi_quirks(pd);
>   
> @@ -73,9 +132,11 @@ static __init int sysfb_init(void)
>   	if (ret)
>   		goto err;
>   
> -	return 0;
> +	goto unlock_mutex;
>   err:
>   	platform_device_put(pd);
> +unlock_mutex:
> +	mutex_unlock(&load_lock);
>   	return ret;
>   }
>   
> diff --git a/include/linux/sysfb.h b/include/linux/sysfb.h
> index 708152e9037b..e8c0313fac8f 100644
> --- a/include/linux/sysfb.h
> +++ b/include/linux/sysfb.h
> @@ -55,6 +55,25 @@ struct efifb_dmi_info {
>   	int flags;
>   };
>   
> +#ifdef CONFIG_SYSFB
> +
> +void sysfb_disable(void);
> +bool sysfb_try_unregister(struct device *dev);
> +
> +#else /* CONFIG_SYSFB */
> +
> +static inline void sysfb_disable(void)
> +{
> +
> +}
> +
> +static inline bool sysfb_try_unregister(struct device *dev)
> +{
> +	return false;
> +}
> +
> +#endif /* CONFIG_SYSFB */
> +
>   #ifdef CONFIG_EFI
>   
>   extern struct efifb_dmi_info efifb_dmi_list[];

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[Thomas Zimmermann]

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Hi

Am 20.04.22 um 10:52 schrieb Javier Martinez Canillas:
> Hello,
> 
> The patches in this series are mostly changes suggested by Daniel Vetter
> to fix some race conditions that exists between the fbdev core (fbmem)
> and sysfb with regard to device registration and removal.
> 
> For example, it is currently possible for sysfb to register a platform
> device after a real DRM driver was registered and requested to remove the
> conflicting framebuffers.
> 
> A symptom of this issue, was worked around with by commit fb561bf9abde
> ("fbdev: Prevent probing generic drivers if a FB is already registered")
> but that's really a hack and should be reverted.

As I mentioned on IRC, I think this series should be merged for the 
reasons I give in the other comments.

> 
> This series attempt to fix it more properly and revert the mentioned hack.
> That will also unblock a pending patch to not make the num_registered_fb
> variable visible to drivers anymore, since that's internal to fbdev core.

Here's as far as I understand the problem:

  1) build DRM/fbdev and sysfb code into the kernel
  2) during boot, load the DRM/fbdev modules and have them acquire I/O 
ranges
  3) afterwards load sysfb and have it register platform devices for the 
generic framebuffers
  4) these devices now conflict with the already-registered DRM/fbdev 
devices

If that is the problem here, let's simply set a sysfb_disable flag in 
sysfb code when the first DRM/fbdev driver first loads. With the flag 
set, sysfb won't create any platform devices. We assume that there are 
now DRM/fbdev drivers for the framebuffers and sysfb won't be needed.

We can set the flag internally from drm_aperture_detach_drivers() [1] 
and do_remove_conflicting_framebuffers() [2].

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/v5.17.4/source/drivers/gpu/drm/drm_aperture.c#L253
[2] 
https://elixir.bootlin.com/linux/v5.17.4/source/drivers/video/fbdev/core/fbmem.c#L1559

> 
> Patch #1 is just a trivial preparatory change.
> 
> Patch #2 add sysfb_disable() and sysfb_try_unregister() helpers for fbmem
> to use them.
> 
> Patch #3 changes how is dealt with conflicting framebuffers unregistering,
> rather than having a variable to determine if a lock should be take, it
> just drops the lock before unregistering the platform device.
> 
> Patch #4 fixes the mentioned race conditions and finally patch #5 is the
> revert patch that was posted by Daniel before but he dropped from his set.
> 
> The patches were tested on a rpi4 using different video configurations:
> (simpledrm -> vc4 both builtin, only vc4 builtin, only simpledrm builtin
> and simpledrm builtin with vc4 built as a module).
> 
> Best regards,
> Javier
> 
> Changes in v3:
> - Rebase on top of latest drm-misc-next branch.
> 
> Changes in v2:
> - Rebase on top of latest drm-misc-next and fix conflicts (Daniel Vetter).
> - Add kernel-doc comments and include in other_interfaces.rst (Daniel Vetter).
> - Explain in the commit message that fbmem has to unregister the device
>    as fallback if a driver registered the device itself (Daniel Vetter).
> - Also explain that fallback in a comment in the code (Daniel Vetter).
> - Don't encode in fbmem the assumption that sysfb will always register
>    platform devices (Daniel Vetter).
> - Add a FIXME comment about drivers registering devices (Daniel Vetter).
> - Drop RFC prefix since patches were already reviewed by Daniel Vetter.
> - Add Daniel Reviewed-by tags to the patches.
> 
> Daniel Vetter (1):
>    Revert "fbdev: Prevent probing generic drivers if a FB is already
>      registered"
> 
> Javier Martinez Canillas (4):
>    firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer
>    firmware: sysfb: Add helpers to unregister a pdev and disable
>      registration
>    fbdev: Restart conflicting fb removal loop when unregistering devices
>    fbdev: Fix some race conditions between fbmem and sysfb
> 
>   .../driver-api/firmware/other_interfaces.rst  |  6 ++
>   drivers/firmware/sysfb.c                      | 77 +++++++++++++++++--
>   drivers/firmware/sysfb_simplefb.c             | 16 ++--
>   drivers/video/fbdev/core/fbmem.c              | 62 ++++++++++++---
>   drivers/video/fbdev/efifb.c                   | 11 ---
>   drivers/video/fbdev/simplefb.c                | 11 ---
>   include/linux/fb.h                            |  1 -
>   include/linux/sysfb.h                         | 29 +++++--
>   8 files changed, 158 insertions(+), 55 deletions(-)
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[Thomas Zimmermann]

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Hi

Am 25.04.22 um 10:54 schrieb Thomas Zimmermann:
> Hi
> 
> Am 20.04.22 um 10:52 schrieb Javier Martinez Canillas:
>> Hello,
>>
>> The patches in this series are mostly changes suggested by Daniel Vetter
>> to fix some race conditions that exists between the fbdev core (fbmem)
>> and sysfb with regard to device registration and removal.
>>
>> For example, it is currently possible for sysfb to register a platform
>> device after a real DRM driver was registered and requested to remove the
>> conflicting framebuffers.
>>
>> A symptom of this issue, was worked around with by commit fb561bf9abde
>> ("fbdev: Prevent probing generic drivers if a FB is already registered")
>> but that's really a hack and should be reverted.
> 
> As I mentioned on IRC, I think this series should be merged for the 
> reasons I give in the other comments.
> 
>>
>> This series attempt to fix it more properly and revert the mentioned 
>> hack.
>> That will also unblock a pending patch to not make the num_registered_fb
>> variable visible to drivers anymore, since that's internal to fbdev core.
> 
> Here's as far as I understand the problem:
> 
>   1) build DRM/fbdev and sysfb code into the kernel
>   2) during boot, load the DRM/fbdev modules and have them acquire I/O 
> ranges
>   3) afterwards load sysfb and have it register platform devices for the 
> generic framebuffers
>   4) these devices now conflict with the already-registered DRM/fbdev 
> devices
> 
> If that is the problem here, let's simply set a sysfb_disable flag in 
> sysfb code when the first DRM/fbdev driver first loads. With the flag 
> set, sysfb won't create any platform devices. We assume that there are 
> now DRM/fbdev drivers for the framebuffers and sysfb won't be needed.
> 
> We can set the flag internally from drm_aperture_detach_drivers() [1] 
> and do_remove_conflicting_framebuffers() [2].

And further thinking about it, it would be better to set such a flag 
after successfully registering a DRM/fbdev device.  So we know that 
there's at least one working display in the system. We don't have to 
rely on generic framebuffers after that.

Best regards
Thomas

> 
> Best regards
> Thomas
> 
> [1] 
> https://elixir.bootlin.com/linux/v5.17.4/source/drivers/gpu/drm/drm_aperture.c#L253 
> 
> [2] 
> https://elixir.bootlin.com/linux/v5.17.4/source/drivers/video/fbdev/core/fbmem.c#L1559 
> 
> 
>>
>> Patch #1 is just a trivial preparatory change.
>>
>> Patch #2 add sysfb_disable() and sysfb_try_unregister() helpers for fbmem
>> to use them.
>>
>> Patch #3 changes how is dealt with conflicting framebuffers 
>> unregistering,
>> rather than having a variable to determine if a lock should be take, it
>> just drops the lock before unregistering the platform device.
>>
>> Patch #4 fixes the mentioned race conditions and finally patch #5 is the
>> revert patch that was posted by Daniel before but he dropped from his 
>> set.
>>
>> The patches were tested on a rpi4 using different video configurations:
>> (simpledrm -> vc4 both builtin, only vc4 builtin, only simpledrm builtin
>> and simpledrm builtin with vc4 built as a module).
>>
>> Best regards,
>> Javier
>>
>> Changes in v3:
>> - Rebase on top of latest drm-misc-next branch.
>>
>> Changes in v2:
>> - Rebase on top of latest drm-misc-next and fix conflicts (Daniel 
>> Vetter).
>> - Add kernel-doc comments and include in other_interfaces.rst (Daniel 
>> Vetter).
>> - Explain in the commit message that fbmem has to unregister the device
>>    as fallback if a driver registered the device itself (Daniel Vetter).
>> - Also explain that fallback in a comment in the code (Daniel Vetter).
>> - Don't encode in fbmem the assumption that sysfb will always register
>>    platform devices (Daniel Vetter).
>> - Add a FIXME comment about drivers registering devices (Daniel Vetter).
>> - Drop RFC prefix since patches were already reviewed by Daniel Vetter.
>> - Add Daniel Reviewed-by tags to the patches.
>>
>> Daniel Vetter (1):
>>    Revert "fbdev: Prevent probing generic drivers if a FB is already
>>      registered"
>>
>> Javier Martinez Canillas (4):
>>    firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer
>>    firmware: sysfb: Add helpers to unregister a pdev and disable
>>      registration
>>    fbdev: Restart conflicting fb removal loop when unregistering devices
>>    fbdev: Fix some race conditions between fbmem and sysfb
>>
>>   .../driver-api/firmware/other_interfaces.rst  |  6 ++
>>   drivers/firmware/sysfb.c                      | 77 +++++++++++++++++--
>>   drivers/firmware/sysfb_simplefb.c             | 16 ++--
>>   drivers/video/fbdev/core/fbmem.c              | 62 ++++++++++++---
>>   drivers/video/fbdev/efifb.c                   | 11 ---
>>   drivers/video/fbdev/simplefb.c                | 11 ---
>>   include/linux/fb.h                            |  1 -
>>   include/linux/sysfb.h                         | 29 +++++--
>>   8 files changed, 158 insertions(+), 55 deletions(-)
>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[Javier Martinez Canillas]

Hello Thomas,

Thanks for the feedback. It was very useful.

On 4/25/22 11:15, Thomas Zimmermann wrote:
> Hi
> 
> Am 25.04.22 um 10:54 schrieb Thomas Zimmermann:
>> Hi
>>
>> Am 20.04.22 um 10:52 schrieb Javier Martinez Canillas:
>>> Hello,
>>>
>>> The patches in this series are mostly changes suggested by Daniel Vetter
>>> to fix some race conditions that exists between the fbdev core (fbmem)
>>> and sysfb with regard to device registration and removal.
>>>
>>> For example, it is currently possible for sysfb to register a platform
>>> device after a real DRM driver was registered and requested to remove the
>>> conflicting framebuffers.
>>>
>>> A symptom of this issue, was worked around with by commit fb561bf9abde
>>> ("fbdev: Prevent probing generic drivers if a FB is already registered")
>>> but that's really a hack and should be reverted.
>>
>> As I mentioned on IRC, I think this series should be merged for the 
>> reasons I give in the other comments.
>>

You meant that should *not* get merged, as we discussed over IRC.

>>>
>>> This series attempt to fix it more properly and revert the mentioned 
>>> hack.
>>> That will also unblock a pending patch to not make the num_registered_fb
>>> variable visible to drivers anymore, since that's internal to fbdev core.
>>
>> Here's as far as I understand the problem:
>>
>>   1) build DRM/fbdev and sysfb code into the kernel
>>   2) during boot, load the DRM/fbdev modules and have them acquire I/O 
>> ranges
>>   3) afterwards load sysfb and have it register platform devices for the 
>> generic framebuffers
>>   4) these devices now conflict with the already-registered DRM/fbdev 
>> devices
>>

That's correct, yes.

>> If that is the problem here, let's simply set a sysfb_disable flag in 
>> sysfb code when the first DRM/fbdev driver first loads. With the flag 
>> set, sysfb won't create any platform devices. We assume that there are 
>> now DRM/fbdev drivers for the framebuffers and sysfb won't be needed.
>>
>> We can set the flag internally from drm_aperture_detach_drivers() [1] 
>> and do_remove_conflicting_framebuffers() [2].
> 
> And further thinking about it, it would be better to set such a flag 
> after successfully registering a DRM/fbdev device.  So we know that 
> there's at least one working display in the system. We don't have to 
> rely on generic framebuffers after that.
>

Exactly, should be done when the device is registered rather than when
the driver is registered or a call is made to remove the conflicting FB.

I'll rework this series with only the bits for sysfb_disable() and drop
the rest. We can go back to the discussion of the remaining parts later
if that makes sense (I still think that patch 3/5 is a better approach,
but let's defer that for a different series).

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat

Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[Daniel Vetter]

On Mon, Apr 25, 2022 at 11:49:13AM +0200, Javier Martinez Canillas wrote:
> Hello Thomas,
> 
> Thanks for the feedback. It was very useful.
> 
> On 4/25/22 11:15, Thomas Zimmermann wrote:
> > Hi
> > 
> > Am 25.04.22 um 10:54 schrieb Thomas Zimmermann:
> >> Hi
> >>
> >> Am 20.04.22 um 10:52 schrieb Javier Martinez Canillas:
> >>> Hello,
> >>>
> >>> The patches in this series are mostly changes suggested by Daniel Vetter
> >>> to fix some race conditions that exists between the fbdev core (fbmem)
> >>> and sysfb with regard to device registration and removal.
> >>>
> >>> For example, it is currently possible for sysfb to register a platform
> >>> device after a real DRM driver was registered and requested to remove the
> >>> conflicting framebuffers.
> >>>
> >>> A symptom of this issue, was worked around with by commit fb561bf9abde
> >>> ("fbdev: Prevent probing generic drivers if a FB is already registered")
> >>> but that's really a hack and should be reverted.
> >>
> >> As I mentioned on IRC, I think this series should be merged for the 
> >> reasons I give in the other comments.
> >>
> 
> You meant that should *not* get merged, as we discussed over IRC.
> 
> >>>
> >>> This series attempt to fix it more properly and revert the mentioned 
> >>> hack.
> >>> That will also unblock a pending patch to not make the num_registered_fb
> >>> variable visible to drivers anymore, since that's internal to fbdev core.
> >>
> >> Here's as far as I understand the problem:
> >>
> >>   1) build DRM/fbdev and sysfb code into the kernel
> >>   2) during boot, load the DRM/fbdev modules and have them acquire I/O 
> >> ranges
> >>   3) afterwards load sysfb and have it register platform devices for the 
> >> generic framebuffers
> >>   4) these devices now conflict with the already-registered DRM/fbdev 
> >> devices
> >>
> 
> That's correct, yes.
> 
> >> If that is the problem here, let's simply set a sysfb_disable flag in 
> >> sysfb code when the first DRM/fbdev driver first loads. With the flag 
> >> set, sysfb won't create any platform devices. We assume that there are 
> >> now DRM/fbdev drivers for the framebuffers and sysfb won't be needed.
> >>
> >> We can set the flag internally from drm_aperture_detach_drivers() [1] 
> >> and do_remove_conflicting_framebuffers() [2].
> > 
> > And further thinking about it, it would be better to set such a flag 
> > after successfully registering a DRM/fbdev device.  So we know that 
> > there's at least one working display in the system. We don't have to 
> > rely on generic framebuffers after that.
> >
> 
> Exactly, should be done when the device is registered rather than when
> the driver is registered or a call is made to remove the conflicting FB.
> 
> I'll rework this series with only the bits for sysfb_disable() and drop
> the rest. We can go back to the discussion of the remaining parts later
> if that makes sense (I still think that patch 3/5 is a better approach,
> but let's defer that for a different series).

We need to kill sysfb _before_ the driver loads, otherwise you can have
two drivers fighting over each another. And yes that means you might end
up with black screen if the driver load goes wrong, but the two drivers
fighting over each another can also result in black screens. And the
latter isn't fixable any other way (in general at least) than by making
sure the fw stuff is gone before driver load starts in earnest.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH v3 0/5] Fix some race conditions that exists between fbmem and sysfb

[Javier Martinez Canillas]

Hello Daniel,

On 4/29/22 09:47, Daniel Vetter wrote:

[snip]

>>
>> Exactly, should be done when the device is registered rather than when
>> the driver is registered or a call is made to remove the conflicting FB.
>>
>> I'll rework this series with only the bits for sysfb_disable() and drop
>> the rest. We can go back to the discussion of the remaining parts later
>> if that makes sense (I still think that patch 3/5 is a better approach,
>> but let's defer that for a different series).
> 
> We need to kill sysfb _before_ the driver loads, otherwise you can have
> two drivers fighting over each another. And yes that means you might end
> up with black screen if the driver load goes wrong, but the two drivers
> fighting over each another can also result in black screens. And the
> latter isn't fixable any other way (in general at least) than by making
> sure the fw stuff is gone before driver load starts in earnest.

Yes, you are correct. I didn't realize all the possible cases when agreed
with Thomas about doing this but tried and found that it's not enough.

I've a full patch-set now and will post as a RFC so we can discuss more.

> -Daniel

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat