vga_switcheroo: Add support for switching only the DDC

Originally by Seth Forshee <seth.forshee@canonical.com>, 2012-10-04:
    During graphics driver initialization it's useful to be able to mux
    only the DDC to the inactive client in order to read the EDID. Add
    a switch_ddc callback to allow capable handlers to provide this
    functionality, and add vga_switcheroo_switch_ddc() to allow DRM
    to mux only the DDC.

Modified by Dave Airlie <airlied@gmail.com>, 2012-12-22:
    I can't figure out why I didn't like this, but I rewrote this [...]
    to lock/unlock the ddc lines [...]. I think I'd prefer something
    like that otherwise the interface got really ugly.

Modified by Lukas Wunner <lukas@wunner.de>, 2015-04 - 2015-10:
    Change semantics of ->switch_ddc handler callback to return previous
    DDC owner. Original version tried to determine previous DDC owner
    with find_active_client() but this fails if the inactive client
    registers before the active client.

    Don't lock vgasr_mutex in _lock_ddc() / _unlock_ddc(), it can cause
    deadlocks because (a) during switch (with vgasr_mutex already held),
    GPU is woken and probes its outputs, tries to re-acquire vgasr_mutex
    to lock DDC lines; (b) Likewise during switch, GPU is suspended and
    calls cancel_delayed_work_sync() to stop output polling, if poll
    task is running at this moment we may wait forever for it to finish.

    Instead, lock mux_hw_lock when unregistering the handler because
    the only reason why we'd want to lock vgasr_mutex in _lock_ddc() /
    _unlock_ddc() is to block the handler from disappearing while DDC
    lines are switched.

    Also acquire mux_hw_lock in stage2 to avoid race condition where
    reading the EDID and switching happens simultaneously. Likewise on
    MIGD / MDIS commands and on runtime suspend.

    v2.1: Overhaul locking, squash commits (Daniel Vetter)

    v2.2: Readability improvements (Thierry Reding)

    v2.3: Overhaul locking once more

    v2.4: Retain semantics of ->switchto handler callback to switch all
          pins, including DDC (Daniel Vetter)

    v5:   Rename ddc_lock to mux_hw_lock: Since we acquire this both
          when calling ->switch_ddc and ->switchto, it protects not just
          access to the DDC lines but to the mux in general. This is in
          line with the DRM convention to use low-level locks to avoid
          concurrent hw access (e.g. i2c, dp_aux) which are often called
          hw_lock (Daniel Vetter)

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115
Tested-by: Lukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/e81ae9722b84c5ed591805fee3ea6dbf5dc6c4b3.1452525860.git.lukas@wunner.de

drivers/gpu/vga/vga_switcheroo.c

@@ -74,9 +74,17 @@
  * there can thus be up to three clients: Two vga clients (GPUs) and one audio
  * client (on the discrete GPU). The code is mostly prepared to support
  * machines with more than two GPUs should they become available.
+ *
  * The GPU to which the outputs are currently switched is called the
  * active client in vga_switcheroo parlance. The GPU not in use is the
- * inactive client.
+ * inactive client. When the inactive client's DRM driver is loaded,
+ * it will be unable to probe the panel's EDID and hence depends on
+ * VBIOS to provide its display modes. If the VBIOS modes are bogus or
+ * if there is no VBIOS at all (which is common on the MacBook Pro),
+ * a client may alternatively request that the DDC lines are temporarily
+ * switched to it, provided that the handler supports this. Switching
+ * only the DDC lines and not the entire output avoids unnecessary
+ * flickering.
  */
 
 /**
@@ -127,6 +135,9 @@ static DEFINE_MUTEX(vgasr_mutex);
  * @clients: list of registered clients
  * @handler: registered handler
  * @handler_flags: flags of registered handler
+ * @mux_hw_lock: protects mux state
+ *	(in particular while DDC lines are temporarily switched)
+ * @old_ddc_owner: client to which DDC lines will be switched back on unlock
  *
  * vga_switcheroo private data. Currently only one vga_switcheroo instance
  * per system is supported.
@@ -144,6 +155,8 @@ struct vgasr_priv {
 
 	const struct vga_switcheroo_handler *handler;
 	enum vga_switcheroo_handler_flags_t handler_flags;
+	struct mutex mux_hw_lock;
+	int old_ddc_owner;
 };
 
 #define ID_BIT_AUDIO		0x100
@@ -158,6 +171,7 @@ static void vga_switcheroo_debugfs_fini(struct vgasr_priv *priv);
 /* only one switcheroo per system */
 static struct vgasr_priv vgasr_priv = {
 	.clients = LIST_HEAD_INIT(vgasr_priv.clients),
+	.mux_hw_lock = __MUTEX_INITIALIZER(vgasr_priv.mux_hw_lock),
 };
 
 static bool vga_switcheroo_ready(void)
@@ -227,6 +241,7 @@ EXPORT_SYMBOL(vga_switcheroo_register_handler);
 void vga_switcheroo_unregister_handler(void)
 {
 	mutex_lock(&vgasr_mutex);
+	mutex_lock(&vgasr_priv.mux_hw_lock);
 	vgasr_priv.handler_flags = 0;
 	vgasr_priv.handler = NULL;
 	if (vgasr_priv.active) {
@@ -234,6 +249,7 @@ void vga_switcheroo_unregister_handler(void)
 		vga_switcheroo_debugfs_fini(&vgasr_priv);
 		vgasr_priv.active = false;
 	}
+	mutex_unlock(&vgasr_priv.mux_hw_lock);
 	mutex_unlock(&vgasr_mutex);
 }
 EXPORT_SYMBOL(vga_switcheroo_unregister_handler);
@@ -432,6 +448,76 @@ void vga_switcheroo_client_fb_set(struct pci_dev *pdev,
 }
 EXPORT_SYMBOL(vga_switcheroo_client_fb_set);
 
+/**
+ * vga_switcheroo_lock_ddc() - temporarily switch DDC lines to a given client
+ * @pdev: client pci device
+ *
+ * Temporarily switch DDC lines to the client identified by @pdev
+ * (but leave the outputs otherwise switched to where they are).
+ * This allows the inactive client to probe EDID. The DDC lines must
+ * afterwards be switched back by calling vga_switcheroo_unlock_ddc(),
+ * even if this function returns an error.
+ *
+ * Return: Previous DDC owner on success or a negative int on error.
+ * Specifically, %-ENODEV if no handler has registered or if the handler
+ * does not support switching the DDC lines. Also, a negative value
+ * returned by the handler is propagated back to the caller.
+ * The return value has merely an informational purpose for any caller
+ * which might be interested in it. It is acceptable to ignore the return
+ * value and simply rely on the result of the subsequent EDID probe,
+ * which will be %NULL if DDC switching failed.
+ */
+int vga_switcheroo_lock_ddc(struct pci_dev *pdev)
+{
+	enum vga_switcheroo_client_id id;
+
+	mutex_lock(&vgasr_priv.mux_hw_lock);
+	if (!vgasr_priv.handler || !vgasr_priv.handler->switch_ddc) {
+		vgasr_priv.old_ddc_owner = -ENODEV;
+		return -ENODEV;
+	}
+
+	id = vgasr_priv.handler->get_client_id(pdev);
+	vgasr_priv.old_ddc_owner = vgasr_priv.handler->switch_ddc(id);
+	return vgasr_priv.old_ddc_owner;
+}
+EXPORT_SYMBOL(vga_switcheroo_lock_ddc);
+
+/**
+ * vga_switcheroo_unlock_ddc() - switch DDC lines back to previous owner
+ * @pdev: client pci device
+ *
+ * Switch DDC lines back to the previous owner after calling
+ * vga_switcheroo_lock_ddc(). This must be called even if
+ * vga_switcheroo_lock_ddc() returned an error.
+ *
+ * Return: Previous DDC owner on success (i.e. the client identifier of @pdev)
+ * or a negative int on error.
+ * Specifically, %-ENODEV if no handler has registered or if the handler
+ * does not support switching the DDC lines. Also, a negative value
+ * returned by the handler is propagated back to the caller.
+ * Finally, invoking this function without calling vga_switcheroo_lock_ddc()
+ * first is not allowed and will result in %-EINVAL.
+ */
+int vga_switcheroo_unlock_ddc(struct pci_dev *pdev)
+{
+	enum vga_switcheroo_client_id id;
+	int ret = vgasr_priv.old_ddc_owner;
+
+	if (WARN_ON_ONCE(!mutex_is_locked(&vgasr_priv.mux_hw_lock)))
+		return -EINVAL;
+
+	if (vgasr_priv.old_ddc_owner >= 0) {
+		id = vgasr_priv.handler->get_client_id(pdev);
+		if (vgasr_priv.old_ddc_owner != id)
+			ret = vgasr_priv.handler->switch_ddc(
+						     vgasr_priv.old_ddc_owner);
+	}
+	mutex_unlock(&vgasr_priv.mux_hw_lock);
+	return ret;
+}
+EXPORT_SYMBOL(vga_switcheroo_unlock_ddc);
+
 /**
  * DOC: Manual switching and manual power control
  *
@@ -569,7 +655,9 @@ static int vga_switchto_stage2(struct vga_switcheroo_client *new_client)
 		console_unlock();
 	}
 
+	mutex_lock(&vgasr_priv.mux_hw_lock);
 	ret = vgasr_priv.handler->switchto(new_client->id);
+	mutex_unlock(&vgasr_priv.mux_hw_lock);
 	if (ret)
 		return ret;
 
@@ -684,7 +772,9 @@ vga_switcheroo_debugfs_write(struct file *filp, const char __user *ubuf,
 	vgasr_priv.delayed_switch_active = false;
 
 	if (just_mux) {
+		mutex_lock(&vgasr_priv.mux_hw_lock);
 		ret = vgasr_priv.handler->switchto(client_id);
+		mutex_unlock(&vgasr_priv.mux_hw_lock);
 		goto out;
 	}
 
@@ -896,8 +986,11 @@ static int vga_switcheroo_runtime_suspend(struct device *dev)
 	if (ret)
 		return ret;
 	mutex_lock(&vgasr_mutex);
-	if (vgasr_priv.handler->switchto)
+	if (vgasr_priv.handler->switchto) {
+		mutex_lock(&vgasr_priv.mux_hw_lock);
 		vgasr_priv.handler->switchto(VGA_SWITCHEROO_IGD);
+		mutex_unlock(&vgasr_priv.mux_hw_lock);
+	}
 	vga_switcheroo_power_switch(pdev, VGA_SWITCHEROO_OFF);
 	mutex_unlock(&vgasr_mutex);
 	return 0;

include/linux/vga_switcheroo.h

@@ -102,6 +102,9 @@ enum vga_switcheroo_client_id {
  * 	Mandatory. For muxless machines this should be a no-op. Returning 0
  * 	denotes success, anything else failure (in which case the switch is
  * 	aborted)
+ * @switch_ddc: switch DDC lines to given client.
+ * 	Optional. Should return the previous DDC owner on success or a
+ * 	negative int on failure
  * @power_state: cut or reinstate power of given client.
  * 	Optional. The return value is ignored
  * @get_client_id: determine if given pci device is integrated or discrete GPU.
@@ -113,6 +116,7 @@ enum vga_switcheroo_client_id {
 struct vga_switcheroo_handler {
 	int (*init)(void);
 	int (*switchto)(enum vga_switcheroo_client_id id);
+	int (*switch_ddc)(enum vga_switcheroo_client_id id);
 	int (*power_state)(enum vga_switcheroo_client_id id,
 			   enum vga_switcheroo_state state);
 	enum vga_switcheroo_client_id (*get_client_id)(struct pci_dev *pdev);
@@ -156,6 +160,8 @@ int vga_switcheroo_register_handler(const struct vga_switcheroo_handler *handler
 				    enum vga_switcheroo_handler_flags_t handler_flags);
 void vga_switcheroo_unregister_handler(void);
 enum vga_switcheroo_handler_flags_t vga_switcheroo_handler_flags(void);
+int vga_switcheroo_lock_ddc(struct pci_dev *pdev);
+int vga_switcheroo_unlock_ddc(struct pci_dev *pdev);
 
 int vga_switcheroo_process_delayed_switch(void);
 
@@ -179,6 +185,8 @@ static inline int vga_switcheroo_register_audio_client(struct pci_dev *pdev,
 	enum vga_switcheroo_client_id id) { return 0; }
 static inline void vga_switcheroo_unregister_handler(void) {}
 static inline enum vga_switcheroo_handler_flags_t vga_switcheroo_handler_flags(void) { return 0; }
+static inline int vga_switcheroo_lock_ddc(struct pci_dev *pdev) { return -ENODEV; }
+static inline int vga_switcheroo_unlock_ddc(struct pci_dev *pdev) { return -ENODEV; }
 static inline int vga_switcheroo_process_delayed_switch(void) { return 0; }
 static inline enum vga_switcheroo_state vga_switcheroo_get_client_state(struct pci_dev *dev) { return VGA_SWITCHEROO_ON; }