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

1 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/vga_switcheroo.h b/include/linux/vga_switcheroo.h
index a745f4f0..b39a5f3 100644
--- a/include/linux/vga_switcheroo.h
+++ b/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; }