PCI/GPU: implement VGA arbitration on Linux

Background:
Graphic devices are accessed through ranges in I/O or memory space. While most
modern devices allow relocation of such ranges, some "Legacy" VGA devices
implemented on PCI will typically have the same "hard-decoded" addresses as
they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
Section 7, Legacy Devices.

The Resource Access Control (RAC) module inside the X server currently does
the task of arbitration when more than one legacy device co-exists on the same
machine. But the problem happens when these devices are trying to be accessed
by different userspace clients (e.g. two server in parallel). Their address
assignments conflict. Therefore an arbitration scheme _outside_ of the X
server is needed to control the sharing of these resources. This document
introduces the operation of the VGA arbiter implemented for Linux kernel.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Tiago Vignatti <tiago.vignatti@nokia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/Makefile

@@ -1 +1 @@
-obj-y			+= drm/
+obj-y			+= drm/ vga/

drivers/gpu/vga/Kconfig

@@ -0,0 +1,10 @@
+config VGA_ARB
+	bool "VGA Arbitration" if EMBEDDED
+	default y
+	depends on PCI
+	help
+	  Some "legacy" VGA devices implemented on PCI typically have the same
+	  hard-decoded addresses as they did on ISA. When multiple PCI devices
+	  are accessed at same time they need some kind of coordination. Please
+	  see Documentation/vgaarbiter.txt for more details. Select this to
+	  enable VGA arbiter.

drivers/gpu/vga/Makefile

@@ -0,0 +1 @@
+obj-$(CONFIG_VGA_ARB)  += vgaarb.o

drivers/pci/pci.c

@@ -2520,6 +2520,50 @@ int pci_resource_bar(struct pci_dev *dev, int resno, enum pci_bar_type *type)
 	return 0;
 }
 
+/**
+ * pci_set_vga_state - set VGA decode state on device and parents if requested
+ * @dev the PCI device
+ * @decode - true = enable decoding, false = disable decoding
+ * @command_bits PCI_COMMAND_IO and/or PCI_COMMAND_MEMORY
+ * @change_bridge - traverse ancestors and change bridges
+ */
+int pci_set_vga_state(struct pci_dev *dev, bool decode,
+		      unsigned int command_bits, bool change_bridge)
+{
+	struct pci_bus *bus;
+	struct pci_dev *bridge;
+	u16 cmd;
+
+	WARN_ON(command_bits & ~(PCI_COMMAND_IO|PCI_COMMAND_MEMORY));
+
+	pci_read_config_word(dev, PCI_COMMAND, &cmd);
+	if (decode == true)
+		cmd |= command_bits;
+	else
+		cmd &= ~command_bits;
+	pci_write_config_word(dev, PCI_COMMAND, cmd);
+
+	if (change_bridge == false)
+		return 0;
+
+	bus = dev->bus;
+	while (bus) {
+		bridge = bus->self;
+		if (bridge) {
+			pci_read_config_word(bridge, PCI_BRIDGE_CONTROL,
+					     &cmd);
+			if (decode == true)
+				cmd |= PCI_BRIDGE_CTL_VGA;
+			else
+				cmd &= ~PCI_BRIDGE_CTL_VGA;
+			pci_write_config_word(bridge, PCI_BRIDGE_CONTROL,
+					      cmd);
+		}
+		bus = bus->parent;
+	}
+	return 0;
+}
+
 #define RESOURCE_ALIGNMENT_PARAM_SIZE COMMAND_LINE_SIZE
 static char resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
 spinlock_t resource_alignment_lock = SPIN_LOCK_UNLOCKED;

drivers/video/Kconfig

@@ -7,6 +7,8 @@ menu "Graphics support"
 
 source "drivers/char/agp/Kconfig"
 
+source "drivers/gpu/vga/Kconfig"
+
 source "drivers/gpu/drm/Kconfig"
 
 config VGASTATE

include/linux/pci.h

@@ -806,6 +806,8 @@ int pci_cfg_space_size_ext(struct pci_dev *dev);
 int pci_cfg_space_size(struct pci_dev *dev);
 unsigned char pci_bus_max_busnr(struct pci_bus *bus);
 
+int pci_set_vga_state(struct pci_dev *pdev, bool decode,
+		      unsigned int command_bits, bool change_bridge);
 /* kmem_cache style wrapper around pci_alloc_consistent() */
 
 #include <linux/dmapool.h>

include/linux/vgaarb.h

@@ -0,0 +1,195 @@
+/*
+ * vgaarb.c
+ *
+ * (C) Copyright 2005 Benjamin Herrenschmidt <benh@kernel.crashing.org>
+ * (C) Copyright 2007 Paulo R. Zanoni <przanoni@gmail.com>
+ * (C) Copyright 2007, 2009 Tiago Vignatti <vignatti@freedesktop.org>
+ */
+
+#ifndef LINUX_VGA_H
+
+#include <asm/vga.h>
+
+/* Legacy VGA regions */
+#define VGA_RSRC_NONE	       0x00
+#define VGA_RSRC_LEGACY_IO     0x01
+#define VGA_RSRC_LEGACY_MEM    0x02
+#define VGA_RSRC_LEGACY_MASK   (VGA_RSRC_LEGACY_IO | VGA_RSRC_LEGACY_MEM)
+/* Non-legacy access */
+#define VGA_RSRC_NORMAL_IO     0x04
+#define VGA_RSRC_NORMAL_MEM    0x08
+
+/* Passing that instead of a pci_dev to use the system "default"
+ * device, that is the one used by vgacon. Archs will probably
+ * have to provide their own vga_default_device();
+ */
+#define VGA_DEFAULT_DEVICE     (NULL)
+
+/* For use by clients */
+
+/**
+ *     vga_set_legacy_decoding
+ *
+ *     @pdev: pci device of the VGA card
+ *     @decodes: bit mask of what legacy regions the card decodes
+ *
+ *     Indicates to the arbiter if the card decodes legacy VGA IOs,
+ *     legacy VGA Memory, both, or none. All cards default to both,
+ *     the card driver (fbdev for example) should tell the arbiter
+ *     if it has disabled legacy decoding, so the card can be left
+ *     out of the arbitration process (and can be safe to take
+ *     interrupts at any time.
+ */
+extern void vga_set_legacy_decoding(struct pci_dev *pdev,
+									unsigned int decodes);
+
+/**
+ *     vga_get         - acquire & locks VGA resources
+ *
+ *     pdev: pci device of the VGA card or NULL for the system default
+ *     rsrc: bit mask of resources to acquire and lock
+ *     interruptible: blocking should be interruptible by signals ?
+ *
+ *     This function acquires VGA resources for the given
+ *     card and mark those resources locked. If the resource requested
+ *     are "normal" (and not legacy) resources, the arbiter will first check
+ *     wether the card is doing legacy decoding for that type of resource. If
+ *     yes, the lock is "converted" into a legacy resource lock.
+ *     The arbiter will first look for all VGA cards that might conflict
+ *     and disable their IOs and/or Memory access, inlcuding VGA forwarding
+ *     on P2P bridges if necessary, so that the requested resources can
+ *     be used. Then, the card is marked as locking these resources and
+ *     the IO and/or Memory accesse are enabled on the card (including
+ *     VGA forwarding on parent P2P bridges if any).
+ *     This function will block if some conflicting card is already locking
+ *     one of the required resources (or any resource on a different bus
+ *     segment, since P2P bridges don't differenciate VGA memory and IO
+ *     afaik). You can indicate wether this blocking should be interruptible
+ *     by a signal (for userland interface) or not.
+ *     Must not be called at interrupt time or in atomic context.
+ *     If the card already owns the resources, the function succeeds.
+ *     Nested calls are supported (a per-resource counter is maintained)
+ */
+
+extern int vga_get(struct pci_dev *pdev, unsigned int rsrc,
+											int interruptible);
+
+/**
+ *     vga_get_interruptible
+ *
+ *     Shortcut to vga_get
+ */
+
+static inline int vga_get_interruptible(struct pci_dev *pdev,
+										unsigned int rsrc)
+{
+       return vga_get(pdev, rsrc, 1);
+}
+
+/**
+ *     vga_get_interruptible
+ *
+ *     Shortcut to vga_get
+ */
+
+static inline int vga_get_uninterruptible(struct pci_dev *pdev,
+											unsigned int rsrc)
+{
+       return vga_get(pdev, rsrc, 0);
+}
+
+/**
+ *     vga_tryget      - try to acquire & lock legacy VGA resources
+ *
+ *     @pdev: pci devivce of VGA card or NULL for system default
+ *     @rsrc: bit mask of resources to acquire and lock
+ *
+ *     This function performs the same operation as vga_get(), but
+ *     will return an error (-EBUSY) instead of blocking if the resources
+ *     are already locked by another card. It can be called in any context
+ */
+
+extern int vga_tryget(struct pci_dev *pdev, unsigned int rsrc);
+
+/**
+ *     vga_put         - release lock on legacy VGA resources
+ *
+ *     @pdev: pci device of VGA card or NULL for system default
+ *     @rsrc: but mask of resource to release
+ *
+ *     This function releases resources previously locked by vga_get()
+ *     or vga_tryget(). The resources aren't disabled right away, so
+ *     that a subsequence vga_get() on the same card will succeed
+ *     immediately. Resources have a counter, so locks are only
+ *     released if the counter reaches 0.
+ */
+
+extern void vga_put(struct pci_dev *pdev, unsigned int rsrc);
+
+
+/**
+ *     vga_default_device
+ *
+ *     This can be defined by the platform. The default implementation
+ *     is rather dumb and will probably only work properly on single
+ *     vga card setups and/or x86 platforms.
+ *
+ *     If your VGA default device is not PCI, you'll have to return
+ *     NULL here. In this case, I assume it will not conflict with
+ *     any PCI card. If this is not true, I'll have to define two archs
+ *     hooks for enabling/disabling the VGA default device if that is
+ *     possible. This may be a problem with real _ISA_ VGA cards, in
+ *     addition to a PCI one. I don't know at this point how to deal
+ *     with that card. Can theirs IOs be disabled at all ? If not, then
+ *     I suppose it's a matter of having the proper arch hook telling
+ *     us about it, so we basically never allow anybody to succeed a
+ *     vga_get()...
+ */
+
+#ifndef __ARCH_HAS_VGA_DEFAULT_DEVICE
+extern struct pci_dev *vga_default_device(void);
+#endif
+
+/**
+ *     vga_conflicts
+ *
+ *     Architectures should define this if they have several
+ *     independant PCI domains that can afford concurrent VGA
+ *     decoding
+ */
+
+#ifndef __ARCH_HAS_VGA_CONFLICT
+static inline int vga_conflicts(struct pci_dev *p1, struct pci_dev *p2)
+{
+       return 1;
+}
+#endif
+
+/*
+ * Register a client with the VGA arbitration logic
+ * return value: number of VGA devices in system.
+ *
+ * Clients have two callback mechanisms they can use.
+ * irq enable/disable callback -
+ *    If a client can't disable its GPUs VGA resources, then we
+ *    need to be able to ask it to turn off its irqs when we
+ *    turn off its mem and io decoding.
+ * set_vga_decode
+ *    If a client can disable its GPU VGA resource, it will
+ *    get a callback from this to set the encode/decode state
+ *
+ * Clients with disable abilities should check the return value
+ * of this function and if the VGA device count is > 1, should
+ * disable VGA decoding resources.
+ *
+ * Rationale: we cannot disable VGA decode resources unconditionally
+ * some single GPU laptops seem to require ACPI or BIOS access to the
+ * VGA registers to control things like backlights etc.
+ * Hopefully newer multi-GPU laptops do something saner, and desktops
+ * won't have any special ACPI for this.
+ */
+int vga_client_register(struct pci_dev *pdev, void *cookie,
+			void (*irq_set_state)(void *cookie, bool state),
+			unsigned int (*set_vga_decode)(void *cookie, bool state));
+
+#endif /* LINUX_VGA_H */