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

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

[Javier Martinez Canillas]

[ resend since dri-devel wasn't Cc'ed on all patches, sorry for the noise ]

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 reverted the mentioned
hack. This will also unblock a pending patch to not make num_registered_fb
visible to drivers anymore, since that's really 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).

I'm sending as an RFC since there are many changes to the locking scheme
and that is always tricky to get right. Please let me know what you think.

Best regards,
Javier


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

 drivers/firmware/sysfb.c          | 51 ++++++++++++++++++++++++++-----
 drivers/firmware/sysfb_simplefb.c | 24 +++++++++------
 drivers/video/fbdev/core/fbmem.c  | 38 ++++++++++++++++++-----
 drivers/video/fbdev/efifb.c       | 11 -------
 drivers/video/fbdev/simplefb.c    | 11 -------
 include/linux/fb.h                |  1 -
 include/linux/sysfb.h             | 29 +++++++++++++++---
 7 files changed, 112 insertions(+), 53 deletions(-)

-- 
2.35.1

[RESEND RFC PATCH 1/5] firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer

[Javier Martinez Canillas]

This function just returned 0 on success or an errno code on error, but it
could be useful to sysfb_init() to get a pointer to the device registered.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

 drivers/firmware/sysfb.c          |  4 ++--
 drivers/firmware/sysfb_simplefb.c | 24 +++++++++++++++---------
 include/linux/sysfb.h             | 10 +++++-----
 3 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
index 2bfbb05f7d89..b032f40a92de 100644
--- a/drivers/firmware/sysfb.c
+++ b/drivers/firmware/sysfb.c
@@ -46,8 +46,8 @@ static __init int sysfb_init(void)
 	/* try to create a simple-framebuffer device */
 	compatible = sysfb_parse_mode(si, &mode);
 	if (compatible) {
-		ret = sysfb_create_simplefb(si, &mode);
-		if (!ret)
+		pd = sysfb_create_simplefb(si, &mode);
+		if (!IS_ERR(pd))
 			return 0;
 	}
 
diff --git a/drivers/firmware/sysfb_simplefb.c b/drivers/firmware/sysfb_simplefb.c
index 76c4abc42a30..c42648ed3aad 100644
--- a/drivers/firmware/sysfb_simplefb.c
+++ b/drivers/firmware/sysfb_simplefb.c
@@ -57,8 +57,8 @@ __init bool sysfb_parse_mode(const struct screen_info *si,
 	return false;
 }
 
-__init int sysfb_create_simplefb(const struct screen_info *si,
-				 const struct simplefb_platform_data *mode)
+__init struct platform_device *sysfb_create_simplefb(const struct screen_info *si,
+						     const struct simplefb_platform_data *mode)
 {
 	struct platform_device *pd;
 	struct resource res;
@@ -76,7 +76,7 @@ __init int sysfb_create_simplefb(const struct screen_info *si,
 		base |= (u64)si->ext_lfb_base << 32;
 	if (!base || (u64)(resource_size_t)base != base) {
 		printk(KERN_DEBUG "sysfb: inaccessible VRAM base\n");
-		return -EINVAL;
+		return ERR_PTR(-EINVAL);
 	}
 
 	/*
@@ -93,7 +93,7 @@ __init int sysfb_create_simplefb(const struct screen_info *si,
 	length = mode->height * mode->stride;
 	if (length > size) {
 		printk(KERN_WARNING "sysfb: VRAM smaller than advertised\n");
-		return -EINVAL;
+		return ERR_PTR(-EINVAL);
 	}
 	length = PAGE_ALIGN(length);
 
@@ -104,25 +104,31 @@ __init int sysfb_create_simplefb(const struct screen_info *si,
 	res.start = base;
 	res.end = res.start + length - 1;
 	if (res.end <= res.start)
-		return -EINVAL;
+		return ERR_PTR(-EINVAL);
 
 	pd = platform_device_alloc("simple-framebuffer", 0);
 	if (!pd)
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 
 	sysfb_apply_efi_quirks(pd);
 
 	ret = platform_device_add_resources(pd, &res, 1);
 	if (ret) {
 		platform_device_put(pd);
-		return ret;
+		return ERR_PTR(ret);
 	}
 
 	ret = platform_device_add_data(pd, mode, sizeof(*mode));
 	if (ret) {
 		platform_device_put(pd);
-		return ret;
+		return ERR_PTR(ret);
 	}
 
-	return platform_device_add(pd);
+	ret = platform_device_add(pd);
+	if (ret) {
+		platform_device_put(pd);
+		return ERR_PTR(ret);
+	}
+
+	return pd;
 }
diff --git a/include/linux/sysfb.h b/include/linux/sysfb.h
index b0dcfa26d07b..708152e9037b 100644
--- a/include/linux/sysfb.h
+++ b/include/linux/sysfb.h
@@ -72,8 +72,8 @@ static inline void sysfb_apply_efi_quirks(struct platform_device *pd)
 
 bool sysfb_parse_mode(const struct screen_info *si,
 		      struct simplefb_platform_data *mode);
-int sysfb_create_simplefb(const struct screen_info *si,
-			  const struct simplefb_platform_data *mode);
+struct platform_device *sysfb_create_simplefb(const struct screen_info *si,
+					      const struct simplefb_platform_data *mode);
 
 #else /* CONFIG_SYSFB_SIMPLE */
 
@@ -83,10 +83,10 @@ static inline bool sysfb_parse_mode(const struct screen_info *si,
 	return false;
 }
 
-static inline int sysfb_create_simplefb(const struct screen_info *si,
-					 const struct simplefb_platform_data *mode)
+static inline struct platform_device *sysfb_create_simplefb(const struct screen_info *si,
+							    const struct simplefb_platform_data *mode)
 {
-	return -EINVAL;
+	return ERR_PTR(-EINVAL);
 }
 
 #endif /* CONFIG_SYSFB_SIMPLE */
-- 
2.35.1

Re: [RESEND RFC PATCH 1/5] firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer

[Daniel Vetter]

On Wed, Apr 06, 2022 at 11:39:15PM +0200, Javier Martinez Canillas wrote:
> This function just returned 0 on success or an errno code on error, but it
> could be useful to sysfb_init() to get a pointer to the device registered.
> 
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

You need to rebase this onto 202c08914ba5 ("firmware: sysfb: fix
platform-device leak in error path") which fixes the same error path leak
you are fixing in here too. Or we just have a neat conflict when merging
:-) But in that case please mention that you fix the error path leak too
so it's less confusing when Linus or someone needs to resolve the
conflict.

Anyway Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
> 
>  drivers/firmware/sysfb.c          |  4 ++--
>  drivers/firmware/sysfb_simplefb.c | 24 +++++++++++++++---------
>  include/linux/sysfb.h             | 10 +++++-----
>  3 files changed, 22 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
> index 2bfbb05f7d89..b032f40a92de 100644
> --- a/drivers/firmware/sysfb.c
> +++ b/drivers/firmware/sysfb.c
> @@ -46,8 +46,8 @@ static __init int sysfb_init(void)
>  	/* try to create a simple-framebuffer device */
>  	compatible = sysfb_parse_mode(si, &mode);
>  	if (compatible) {
> -		ret = sysfb_create_simplefb(si, &mode);
> -		if (!ret)
> +		pd = sysfb_create_simplefb(si, &mode);
> +		if (!IS_ERR(pd))
>  			return 0;
>  	}
>  
> diff --git a/drivers/firmware/sysfb_simplefb.c b/drivers/firmware/sysfb_simplefb.c
> index 76c4abc42a30..c42648ed3aad 100644
> --- a/drivers/firmware/sysfb_simplefb.c
> +++ b/drivers/firmware/sysfb_simplefb.c
> @@ -57,8 +57,8 @@ __init bool sysfb_parse_mode(const struct screen_info *si,
>  	return false;
>  }
>  
> -__init int sysfb_create_simplefb(const struct screen_info *si,
> -				 const struct simplefb_platform_data *mode)
> +__init struct platform_device *sysfb_create_simplefb(const struct screen_info *si,
> +						     const struct simplefb_platform_data *mode)
>  {
>  	struct platform_device *pd;
>  	struct resource res;
> @@ -76,7 +76,7 @@ __init int sysfb_create_simplefb(const struct screen_info *si,
>  		base |= (u64)si->ext_lfb_base << 32;
>  	if (!base || (u64)(resource_size_t)base != base) {
>  		printk(KERN_DEBUG "sysfb: inaccessible VRAM base\n");
> -		return -EINVAL;
> +		return ERR_PTR(-EINVAL);
>  	}
>  
>  	/*
> @@ -93,7 +93,7 @@ __init int sysfb_create_simplefb(const struct screen_info *si,
>  	length = mode->height * mode->stride;
>  	if (length > size) {
>  		printk(KERN_WARNING "sysfb: VRAM smaller than advertised\n");
> -		return -EINVAL;
> +		return ERR_PTR(-EINVAL);
>  	}
>  	length = PAGE_ALIGN(length);
>  
> @@ -104,25 +104,31 @@ __init int sysfb_create_simplefb(const struct screen_info *si,
>  	res.start = base;
>  	res.end = res.start + length - 1;
>  	if (res.end <= res.start)
> -		return -EINVAL;
> +		return ERR_PTR(-EINVAL);
>  
>  	pd = platform_device_alloc("simple-framebuffer", 0);
>  	if (!pd)
> -		return -ENOMEM;
> +		return ERR_PTR(-ENOMEM);
>  
>  	sysfb_apply_efi_quirks(pd);
>  
>  	ret = platform_device_add_resources(pd, &res, 1);
>  	if (ret) {
>  		platform_device_put(pd);
> -		return ret;
> +		return ERR_PTR(ret);
>  	}
>  
>  	ret = platform_device_add_data(pd, mode, sizeof(*mode));
>  	if (ret) {
>  		platform_device_put(pd);
> -		return ret;
> +		return ERR_PTR(ret);
>  	}
>  
> -	return platform_device_add(pd);
> +	ret = platform_device_add(pd);
> +	if (ret) {
> +		platform_device_put(pd);
> +		return ERR_PTR(ret);
> +	}
> +
> +	return pd;
>  }
> diff --git a/include/linux/sysfb.h b/include/linux/sysfb.h
> index b0dcfa26d07b..708152e9037b 100644
> --- a/include/linux/sysfb.h
> +++ b/include/linux/sysfb.h
> @@ -72,8 +72,8 @@ static inline void sysfb_apply_efi_quirks(struct platform_device *pd)
>  
>  bool sysfb_parse_mode(const struct screen_info *si,
>  		      struct simplefb_platform_data *mode);
> -int sysfb_create_simplefb(const struct screen_info *si,
> -			  const struct simplefb_platform_data *mode);
> +struct platform_device *sysfb_create_simplefb(const struct screen_info *si,
> +					      const struct simplefb_platform_data *mode);
>  
>  #else /* CONFIG_SYSFB_SIMPLE */
>  
> @@ -83,10 +83,10 @@ static inline bool sysfb_parse_mode(const struct screen_info *si,
>  	return false;
>  }
>  
> -static inline int sysfb_create_simplefb(const struct screen_info *si,
> -					 const struct simplefb_platform_data *mode)
> +static inline struct platform_device *sysfb_create_simplefb(const struct screen_info *si,
> +							    const struct simplefb_platform_data *mode)
>  {
> -	return -EINVAL;
> +	return ERR_PTR(-EINVAL);
>  }
>  
>  #endif /* CONFIG_SYSFB_SIMPLE */
> -- 
> 2.35.1
> 

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

Re: [RESEND RFC PATCH 1/5] firmware: sysfb: Make sysfb_create_simplefb() return a pdev pointer

[Javier Martinez Canillas]

Hello Daniel,

On 4/7/22 11:03, Daniel Vetter wrote:
> On Wed, Apr 06, 2022 at 11:39:15PM +0200, Javier Martinez Canillas wrote:
>> This function just returned 0 on success or an errno code on error, but it
>> could be useful to sysfb_init() to get a pointer to the device registered.
>>
>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> 
> You need to rebase this onto 202c08914ba5 ("firmware: sysfb: fix
> platform-device leak in error path") which fixes the same error path leak
> you are fixing in here too. Or we just have a neat conflict when merging
> :-) But in that case please mention that you fix the error path leak too
> so it's less confusing when Linus or someone needs to resolve the
> conflict.
>

Ups, I thought that had my local tree up-to-date but it seems that was a few
days old. I've updated my remote now and rebased, so will have this fixed in
the next revision of the series.

And this patch becomes smaller indeed :)
 
> Anyway Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 

Thanks!

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat

[RESEND RFC PATCH 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>
---

 drivers/firmware/sysfb.c | 47 +++++++++++++++++++++++++++++++++++-----
 include/linux/sysfb.h    | 19 ++++++++++++++++
 2 files changed, 60 insertions(+), 6 deletions(-)

diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
index b032f40a92de..08ae78c083f1 100644
--- a/drivers/firmware/sysfb.c
+++ b/drivers/firmware/sysfb.c
@@ -34,21 +34,52 @@
 #include <linux/screen_info.h>
 #include <linux/sysfb.h>
 
+static struct platform_device *pd;
+static DEFINE_MUTEX(load_lock);
+static bool disabled;
+
+void sysfb_disable(void)
+{
+	mutex_lock(&load_lock);
+	disabled = true;
+	mutex_unlock(&load_lock);
+}
+EXPORT_SYMBOL_GPL(sysfb_disable);
+
+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 +91,10 @@ 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 +106,11 @@ 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: [RESEND RFC PATCH 2/5] firmware: sysfb: Add helpers to unregister a pdev and disable registration

[Daniel Vetter]

On Wed, Apr 06, 2022 at 11:39:16PM +0200, Javier Martinez Canillas wrote:
> 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>
> ---
> 
>  drivers/firmware/sysfb.c | 47 +++++++++++++++++++++++++++++++++++-----
>  include/linux/sysfb.h    | 19 ++++++++++++++++
>  2 files changed, 60 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
> index b032f40a92de..08ae78c083f1 100644
> --- a/drivers/firmware/sysfb.c
> +++ b/drivers/firmware/sysfb.c
> @@ -34,21 +34,52 @@
>  #include <linux/screen_info.h>
>  #include <linux/sysfb.h>
>  
> +static struct platform_device *pd;
> +static DEFINE_MUTEX(load_lock);
> +static bool disabled;
> +
> +void sysfb_disable(void)
> +{
> +	mutex_lock(&load_lock);
> +	disabled = true;
> +	mutex_unlock(&load_lock);
> +}
> +EXPORT_SYMBOL_GPL(sysfb_disable);
> +
> +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);

Kerneldoc for these plus adding that to
Documentation/firmware/other_interfaces.rst would be really neat.

With that added Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>


> +
>  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 +91,10 @@ 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 +106,11 @@ 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
> 

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

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

[Javier Martinez Canillas]

On 4/7/22 11:06, Daniel Vetter wrote:
> On Wed, Apr 06, 2022 at 11:39:16PM +0200, Javier Martinez Canillas wrote:

[snip]

>> +}
>> +EXPORT_SYMBOL_GPL(sysfb_try_unregister);
> 
> Kerneldoc for these plus adding that to
> Documentation/firmware/other_interfaces.rst would be really neat.
> 
> With that added Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
>

I will, thanks for pointing this out.
 -- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat

[RESEND RFC PATCH 3/5] fbdev: Restart conflicting fb removal loop when unregistering devices

[Javier Martinez Canillas]

Drivers that want to remove registered conflicting framebuffers prior to
register their own framebuffer, calls remove_conflicting_framebuffers().

This function takes the registration_lock mutex, to prevent a races when
drivers register framebuffer devices. But if a conflicting framebuffer
device is found, the underlaying platform device is unregistered and this
will lead to the platform driver .remove callback to be called, which in
turn will call to the unregister_framebuffer() that takes the same lock.

To prevent this, a struct fb_info.forced_out field was used as indication
to unregister_framebuffer() whether the mutex has to be grabbed or not.

A cleaner solution is to drop the lock before platform_device_unregister()
so unregister_framebuffer() can take it when called from the fbdev driver,
and just grab the lock again after the device has been registered and do
a removal loop restart.

Since the framebuffer devices will already be removed, the loop would just
finish when no more conflicting framebuffers are found.

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

 drivers/video/fbdev/core/fbmem.c | 21 ++++++++++++++-------
 include/linux/fb.h               |  1 -
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index b585339509b0..c1bfb8df9cba 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1555,6 +1555,7 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
 {
 	int i;
 
+restart_removal:
 	/* check all firmware fbs and kick off if the base addr overlaps */
 	for_each_registered_fb(i) {
 		struct apertures_struct *gen_aper;
@@ -1582,8 +1583,18 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
 			 * fix would add code to remove the device from the system.
 			 */
 			if (dev_is_platform(device)) {
-				registered_fb[i]->forced_out = true;
+				/*
+				 * Drop the lock since the driver will call to the
+				 * unregister_framebuffer() function that takes it.
+				 */
+				mutex_unlock(&registration_lock);
 				platform_device_unregister(to_platform_device(device));
+				mutex_lock(&registration_lock);
+				/*
+				 * Restart the removal now that the platform device
+				 * has been unregistered and its associated fb gone.
+				 */
+				goto restart_removal;
 			} else {
 				pr_warn("fb%d: cannot remove device\n", i);
 				do_unregister_framebuffer(registered_fb[i]);
@@ -1917,13 +1928,9 @@ EXPORT_SYMBOL(register_framebuffer);
 void
 unregister_framebuffer(struct fb_info *fb_info)
 {
-	bool forced_out = fb_info->forced_out;
-
-	if (!forced_out)
-		mutex_lock(&registration_lock);
+	mutex_lock(&registration_lock);
 	do_unregister_framebuffer(fb_info);
-	if (!forced_out)
-		mutex_unlock(&registration_lock);
+	mutex_unlock(&registration_lock);
 }
 EXPORT_SYMBOL(unregister_framebuffer);
 
diff --git a/include/linux/fb.h b/include/linux/fb.h
index 39baa9a70779..f1e0cd751b06 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -503,7 +503,6 @@ struct fb_info {
 	} *apertures;
 
 	bool skip_vt_switch; /* no VT switch on suspend/resume required */
-	bool forced_out; /* set when being removed by another driver */
 };
 
 static inline struct apertures_struct *alloc_apertures(unsigned int max_num) {
-- 
2.35.1

Re: [RESEND RFC PATCH 3/5] fbdev: Restart conflicting fb removal loop when unregistering devices

[Daniel Vetter]

On Wed, Apr 06, 2022 at 11:39:17PM +0200, Javier Martinez Canillas wrote:
> Drivers that want to remove registered conflicting framebuffers prior to
> register their own framebuffer, calls remove_conflicting_framebuffers().
> 
> This function takes the registration_lock mutex, to prevent a races when
> drivers register framebuffer devices. But if a conflicting framebuffer
> device is found, the underlaying platform device is unregistered and this
> will lead to the platform driver .remove callback to be called, which in
> turn will call to the unregister_framebuffer() that takes the same lock.
> 
> To prevent this, a struct fb_info.forced_out field was used as indication
> to unregister_framebuffer() whether the mutex has to be grabbed or not.
> 
> A cleaner solution is to drop the lock before platform_device_unregister()
> so unregister_framebuffer() can take it when called from the fbdev driver,
> and just grab the lock again after the device has been registered and do
> a removal loop restart.
> 
> Since the framebuffer devices will already be removed, the loop would just
> finish when no more conflicting framebuffers are found.
> 
> Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

It's always entertaining with these things since they can go boom in funny
ways, but need to a least try :-) Recursive locks are just a bit too evil.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
> 
>  drivers/video/fbdev/core/fbmem.c | 21 ++++++++++++++-------
>  include/linux/fb.h               |  1 -
>  2 files changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index b585339509b0..c1bfb8df9cba 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1555,6 +1555,7 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
>  {
>  	int i;
>  
> +restart_removal:
>  	/* check all firmware fbs and kick off if the base addr overlaps */
>  	for_each_registered_fb(i) {
>  		struct apertures_struct *gen_aper;
> @@ -1582,8 +1583,18 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
>  			 * fix would add code to remove the device from the system.
>  			 */
>  			if (dev_is_platform(device)) {
> -				registered_fb[i]->forced_out = true;
> +				/*
> +				 * Drop the lock since the driver will call to the
> +				 * unregister_framebuffer() function that takes it.
> +				 */
> +				mutex_unlock(&registration_lock);
>  				platform_device_unregister(to_platform_device(device));
> +				mutex_lock(&registration_lock);
> +				/*
> +				 * Restart the removal now that the platform device
> +				 * has been unregistered and its associated fb gone.
> +				 */
> +				goto restart_removal;
>  			} else {
>  				pr_warn("fb%d: cannot remove device\n", i);
>  				do_unregister_framebuffer(registered_fb[i]);
> @@ -1917,13 +1928,9 @@ EXPORT_SYMBOL(register_framebuffer);
>  void
>  unregister_framebuffer(struct fb_info *fb_info)
>  {
> -	bool forced_out = fb_info->forced_out;
> -
> -	if (!forced_out)
> -		mutex_lock(&registration_lock);
> +	mutex_lock(&registration_lock);
>  	do_unregister_framebuffer(fb_info);
> -	if (!forced_out)
> -		mutex_unlock(&registration_lock);
> +	mutex_unlock(&registration_lock);
>  }
>  EXPORT_SYMBOL(unregister_framebuffer);
>  
> diff --git a/include/linux/fb.h b/include/linux/fb.h
> index 39baa9a70779..f1e0cd751b06 100644
> --- a/include/linux/fb.h
> +++ b/include/linux/fb.h
> @@ -503,7 +503,6 @@ struct fb_info {
>  	} *apertures;
>  
>  	bool skip_vt_switch; /* no VT switch on suspend/resume required */
> -	bool forced_out; /* set when being removed by another driver */
>  };
>  
>  static inline struct apertures_struct *alloc_apertures(unsigned int max_num) {
> -- 
> 2.35.1
> 

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

[RESEND RFC PATCH 4/5] fbdev: Fix some race conditions between fbmem and sysfb

[Javier Martinez Canillas]

The platform devices registered in sysfb match with a firmware-based fbdev
or DRM driver, that are used to have early graphics using framebuffers set
up by the system firmware.

Real DRM drivers later are probed and remove all conflicting framebuffers,
leading to these platform devices for generic drivers to be unregistered.

But the current solution has two issues that this patch fixes:

1) It is a layering violation for the fbdev core to unregister a device
   that was registered by sysfb.

   Instead, the sysfb_try_unregister() helper function can be called for
   sysfb to attempt unregistering the device if is the one registered.

2) The sysfb_init() function could be called after a DRM driver is probed
   and requested to unregister devices for drivers with a conflicting fb.

   To prevent this, disable any future sysfb platform device registration
   by calling sysfb_disable(), if a driver requested to remove conflicting
   framebuffers with remove_conflicting_framebuffers().

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

 drivers/video/fbdev/core/fbmem.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index c1bfb8df9cba..acf641b05d11 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -19,6 +19,7 @@
 #include <linux/kernel.h>
 #include <linux/major.h>
 #include <linux/slab.h>
+#include <linux/sysfb.h>
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/vt.h>
@@ -1588,7 +1589,10 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
 				 * unregister_framebuffer() function that takes it.
 				 */
 				mutex_unlock(&registration_lock);
-				platform_device_unregister(to_platform_device(device));
+				if (!sysfb_try_unregister(device)) {
+					/* sysfb didn't register this device, unregister it */
+					platform_device_unregister(to_platform_device(device));
+				}
 				mutex_lock(&registration_lock);
 				/*
 				 * Restart the removal now that the platform device
@@ -1781,6 +1785,17 @@ int remove_conflicting_framebuffers(struct apertures_struct *a,
 		do_free = true;
 	}
 
+	/*
+	 * If a driver asked to unregister a platform device registered by
+	 * sysfb, then can be assumed that this is a driver for a display
+	 * that is set up by the system firmware and has a generic driver.
+	 *
+	 * Drivers for devices that don't have a generic driver will never
+	 * ask for this, so let's assume that a real driver for the display
+	 * was already probed and prevent sysfb to register devices later.
+	 */
+	sysfb_disable();
+
 	mutex_lock(&registration_lock);
 	do_remove_conflicting_framebuffers(a, name, primary);
 	mutex_unlock(&registration_lock);
-- 
2.35.1

Re: [RESEND RFC PATCH 4/5] fbdev: Fix some race conditions between fbmem and sysfb

[Daniel Vetter]

On Wed, Apr 06, 2022 at 11:39:18PM +0200, Javier Martinez Canillas wrote:
> The platform devices registered in sysfb match with a firmware-based fbdev
> or DRM driver, that are used to have early graphics using framebuffers set
> up by the system firmware.
> 
> Real DRM drivers later are probed and remove all conflicting framebuffers,
> leading to these platform devices for generic drivers to be unregistered.
> 
> But the current solution has two issues that this patch fixes:
> 
> 1) It is a layering violation for the fbdev core to unregister a device
>    that was registered by sysfb.
> 
>    Instead, the sysfb_try_unregister() helper function can be called for
>    sysfb to attempt unregistering the device if is the one registered.
> 
> 2) The sysfb_init() function could be called after a DRM driver is probed
>    and requested to unregister devices for drivers with a conflicting fb.
> 
>    To prevent this, disable any future sysfb platform device registration
>    by calling sysfb_disable(), if a driver requested to remove conflicting
>    framebuffers with remove_conflicting_framebuffers().
> 
> Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> ---
> 
>  drivers/video/fbdev/core/fbmem.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index c1bfb8df9cba..acf641b05d11 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -19,6 +19,7 @@
>  #include <linux/kernel.h>
>  #include <linux/major.h>
>  #include <linux/slab.h>
> +#include <linux/sysfb.h>
>  #include <linux/mm.h>
>  #include <linux/mman.h>
>  #include <linux/vt.h>
> @@ -1588,7 +1589,10 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
>  				 * unregister_framebuffer() function that takes it.
>  				 */
>  				mutex_unlock(&registration_lock);
> -				platform_device_unregister(to_platform_device(device));
> +				if (!sysfb_try_unregister(device)) {
> +					/* sysfb didn't register this device, unregister it */

Maybe explain in the commit message that this is still needed for drivers
which set up their platform_dev themselves, like vga16fb.

Also I'm not sure we want to have an assumption encoded in fbmem.c here
that the sysfb device is always a platform device. I think it would be
better to call sysfb_try_unregister on any device, and then fall back to
the forced removal on our own if it's a platform device.

Also maybe change the comment to /* FIXME: Not all platform fb drivers use sysfb yet */

> +					platform_device_unregister(to_platform_device(device));
> +				}
>  				mutex_lock(&registration_lock);
>  				/*
>  				 * Restart the removal now that the platform device
> @@ -1781,6 +1785,17 @@ int remove_conflicting_framebuffers(struct apertures_struct *a,
>  		do_free = true;
>  	}
>  
> +	/*
> +	 * If a driver asked to unregister a platform device registered by
> +	 * sysfb, then can be assumed that this is a driver for a display
> +	 * that is set up by the system firmware and has a generic driver.
> +	 *
> +	 * Drivers for devices that don't have a generic driver will never
> +	 * ask for this, so let's assume that a real driver for the display
> +	 * was already probed and prevent sysfb to register devices later.
> +	 */

Yeah it's disappointing, but no worse than the piles of hacks we have now.

With the bikesheds addressed above:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> +	sysfb_disable();
> +
>  	mutex_lock(&registration_lock);
>  	do_remove_conflicting_framebuffers(a, name, primary);
>  	mutex_unlock(&registration_lock);
> -- 
> 2.35.1
> 

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

Re: [RESEND RFC PATCH 4/5] fbdev: Fix some race conditions between fbmem and sysfb

[Javier Martinez Canillas]

On 4/7/22 11:11, Daniel Vetter wrote:
> On Wed, Apr 06, 2022 at 11:39:18PM +0200, Javier Martinez Canillas wrote:

[snip]

> 
> Yeah it's disappointing, but no worse than the piles of hacks we have now.
> 
> With the bikesheds addressed above:
>

Agree with all your comments and will address in the next version.
 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>

Thanks for reviewing these patches so quickly!

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat

[RESEND RFC PATCH 5/5] Revert "fbdev: Prevent probing generic drivers if a FB is already registered"

[Javier Martinez Canillas]

From: Daniel Vetter <daniel.vetter@ffwll.ch>

This reverts commit fb561bf9abde49f7e00fdbf9ed2ccf2d86cac8ee.

With

commit 27599aacbaefcbf2af7b06b0029459bbf682000d
Author: Thomas Zimmermann <tzimmermann@suse.de>
Date:   Tue Jan 25 10:12:18 2022 +0100

    fbdev: Hot-unplug firmware fb devices on forced removal

this should be fixed properly and we can remove this somewhat hackish
check here (e.g. this won't catch drm drivers if fbdev emulation isn't
enabled).

Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Zack Rusin <zackr@vmware.com>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Zack Rusin <zackr@vmware.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Ilya Trukhanov <lahvuun@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Peter Jones <pjones@redhat.com>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

 drivers/video/fbdev/efifb.c    | 11 -----------
 drivers/video/fbdev/simplefb.c | 11 -----------
 2 files changed, 22 deletions(-)

diff --git a/drivers/video/fbdev/efifb.c b/drivers/video/fbdev/efifb.c
index ea42ba6445b2..edca3703b964 100644
--- a/drivers/video/fbdev/efifb.c
+++ b/drivers/video/fbdev/efifb.c
@@ -351,17 +351,6 @@ static int efifb_probe(struct platform_device *dev)
 	char *option = NULL;
 	efi_memory_desc_t md;
 
-	/*
-	 * Generic drivers must not be registered if a framebuffer exists.
-	 * If a native driver was probed, the display hardware was already
-	 * taken and attempting to use the system framebuffer is dangerous.
-	 */
-	if (num_registered_fb > 0) {
-		dev_err(&dev->dev,
-			"efifb: a framebuffer is already registered\n");
-		return -EINVAL;
-	}
-
 	if (screen_info.orig_video_isVGA != VIDEO_TYPE_EFI || pci_dev_disabled)
 		return -ENODEV;
 
diff --git a/drivers/video/fbdev/simplefb.c b/drivers/video/fbdev/simplefb.c
index 94fc9c6d0411..0ef41173325a 100644
--- a/drivers/video/fbdev/simplefb.c
+++ b/drivers/video/fbdev/simplefb.c
@@ -413,17 +413,6 @@ static int simplefb_probe(struct platform_device *pdev)
 	struct simplefb_par *par;
 	struct resource *res, *mem;
 
-	/*
-	 * Generic drivers must not be registered if a framebuffer exists.
-	 * If a native driver was probed, the display hardware was already
-	 * taken and attempting to use the system framebuffer is dangerous.
-	 */
-	if (num_registered_fb > 0) {
-		dev_err(&pdev->dev,
-			"simplefb: a framebuffer is already registered\n");
-		return -EINVAL;
-	}
-
 	if (fb_get_options("simplefb", NULL))
 		return -ENODEV;
 
-- 
2.35.1