Revert "drm/i915: Don't save/restore hardware status page address register"

This reverts commit a7a75c8f.

There are two different variations on how Intel hardware addresses the
"Hardware Status Page". One as a location in physical memory and the
other as an offset into the virtual memory of the GPU, used in more
recent chipsets. (The HWS itself is a cacheable region of memory which
the GPU can write to without requiring CPU synchronisation, used for
updating various details of hardware state, such as the position of
the GPU head in the ringbuffer, the last breadcrumb seqno, etc).

These two types of addresses were updated in different locations of code
- one inline with the ringbuffer initialisation, and the other during
device initialisation. (The HWS page is logically associated with
the rings, and there is one HWS page per ring.) During resume, only the
ringbuffers were being re-initialised along with the virtual HWS page,
leaving the older physical address HWS untouched. This then caused a
hang on the older gen3/4 (915GM, 945GM, 965GM) the first time we tried
to synchronise the GPU as the breadcrumbs were never being updated.
Reported-and-tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Jan Niehusmann <jan@gondor.com>
Reported-and-tested-by: Justin P. Mattock <justinmattock@gmail.com>
Reported-and-tested-by: Michael "brot" Groh <brot@minad.de>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -383,6 +383,7 @@ typedef struct drm_i915_private {
 	u32 saveDSPACNTR;
 	u32 saveDSPBCNTR;
 	u32 saveDSPARB;
+	u32 saveHWS;
 	u32 savePIPEACONF;
 	u32 savePIPEBCONF;
 	u32 savePIPEASRC;

drivers/gpu/drm/i915/i915_suspend.c

@@ -796,6 +796,9 @@ int i915_save_state(struct drm_device *dev)
 
 	pci_read_config_byte(dev->pdev, LBB, &dev_priv->saveLBB);
 
+	/* Hardware status page */
+	dev_priv->saveHWS = I915_READ(HWS_PGA);
+
 	i915_save_display(dev);
 
 	/* Interrupt state */
@@ -842,6 +845,9 @@ int i915_restore_state(struct drm_device *dev)
 
 	pci_write_config_byte(dev->pdev, LBB, dev_priv->saveLBB);
 
+	/* Hardware status page */
+	I915_WRITE(HWS_PGA, dev_priv->saveHWS);
+
 	i915_restore_display(dev);
 
 	/* Interrupt state */