drm/i915: kerneldoc for tiling IOCTL and swizzle functions

Chris rightfully suggested that documenting fences without documenting
the BO tiling tracking doesn't make much sense, so fix that.

The important bit to stress here (since it lead to some confusion) is
the GEM doesn't really care about tiling. Except for a few select cases
where the kernel needs to manage something that userspace can't take
care of: Namely the limited number of fences and fixing up swizzling,
although we still fail at the later.

v2: Move the low-level tiling/swizzling functions and kerneldoc to
i915_gem_fence.c and leave only the userspace interface here.
Suggested by Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

2 files changed, 66 insertions, 46 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_fence.c b/drivers/gpu/drm/i915/i915_gem_fence.c
index c643260a90c5..af1f8c461060 100644
--- a/drivers/gpu/drm/i915/i915_gem_fence.c
+++ b/drivers/gpu/drm/i915/i915_gem_fence.c
@@ -497,8 +497,7 @@ void i915_gem_restore_fences(struct drm_device *dev)
 }
 
 /**
- *
- * Support for managing tiling state of buffer objects.
+ * DOC: tiling swizzling details
  *
  * The idea behind tiling is to increase cache hit rates by rearranging
  * pixel data so that a group of pixel accesses are in the same cacheline.
@@ -546,6 +545,9 @@ void i915_gem_restore_fences(struct drm_device *dev)
  */
 
 /**
+ * i915_gem_detect_bit_6_swizzle - detect bit 6 swizzling pattern
+ * @dev: DRM device
+ *
  * Detects bit 6 swizzling of address lookup between IGD access and CPU
  * access through main memory.
  */
@@ -692,7 +694,7 @@ i915_gem_detect_bit_6_swizzle(struct drm_device *dev)
 	dev_priv->mm.bit_6_swizzle_y = swizzle_y;
 }
 
-/**
+/*
  * Swap every 64 bytes of this page around, to account for it having a new
  * bit 17 of its physical address and therefore being interpreted differently
  * by the GPU.
@@ -715,6 +717,18 @@ i915_gem_swizzle_page(struct page *page)
 	kunmap(page);
 }
 
+/**
+ * i915_gem_object_do_bit_17_swizzle - fixup bit 17 swizzling
+ * @obj: i915 GEM buffer object
+ *
+ * This function fixes up the swizzling in case any page frame number for this
+ * object has changed in bit 17 since that state has been saved with
+ * i915_gem_object_save_bit_17_swizzle().
+ *
+ * This is called when pinning backing storage again, since the kernel is free
+ * to move unpinned backing storage around (either by directly moving pages or
+ * by swapping them out and back in again).
+ */
 void
 i915_gem_object_do_bit_17_swizzle(struct drm_i915_gem_object *obj)
 {
@@ -737,6 +751,14 @@ i915_gem_object_do_bit_17_swizzle(struct drm_i915_gem_object *obj)
 	}
 }
 
+/**
+ * i915_gem_object_save_bit_17_swizzle - save bit 17 swizzling
+ * @obj: i915 GEM buffer object
+ *
+ * This function saves the bit 17 of each page frame number so that swizzling
+ * can be fixed up later on with i915_gem_object_do_bit_17_swizzle(). This must
+ * be called before the backing storage can be unpinned.
+ */
 void
 i915_gem_object_save_bit_17_swizzle(struct drm_i915_gem_object *obj)
 {
diff --git a/drivers/gpu/drm/i915/i915_gem_tiling.c b/drivers/gpu/drm/i915/i915_gem_tiling.c
index fa7a8d7a24e0..ac3eb566c9d2 100644
--- a/drivers/gpu/drm/i915/i915_gem_tiling.c
+++ b/drivers/gpu/drm/i915/i915_gem_tiling.c
@@ -31,53 +31,31 @@
 #include <drm/i915_drm.h>
 #include "i915_drv.h"
 
-/** @file i915_gem_tiling.c
- *
- * Support for managing tiling state of buffer objects.
- *
- * The idea behind tiling is to increase cache hit rates by rearranging
- * pixel data so that a group of pixel accesses are in the same cacheline.
- * Performance improvement from doing this on the back/depth buffer are on
- * the order of 30%.
- *
- * Intel architectures make this somewhat more complicated, though, by
- * adjustments made to addressing of data when the memory is in interleaved
- * mode (matched pairs of DIMMS) to improve memory bandwidth.
- * For interleaved memory, the CPU sends every sequential 64 bytes
- * to an alternate memory channel so it can get the bandwidth from both.
- *
- * The GPU also rearranges its accesses for increased bandwidth to interleaved
- * memory, and it matches what the CPU does for non-tiled.  However, when tiled
- * it does it a little differently, since one walks addresses not just in the
- * X direction but also Y.  So, along with alternating channels when bit
- * 6 of the address flips, it also alternates when other bits flip --  Bits 9
- * (every 512 bytes, an X tile scanline) and 10 (every two X tile scanlines)
- * are common to both the 915 and 965-class hardware.
- *
- * The CPU also sometimes XORs in higher bits as well, to improve
- * bandwidth doing strided access like we do so frequently in graphics.  This
- * is called "Channel XOR Randomization" in the MCH documentation.  The result
- * is that the CPU is XORing in either bit 11 or bit 17 to bit 6 of its address
- * decode.
+/**
+ * DOC: buffer object tiling
  *
- * All of this bit 6 XORing has an effect on our memory management,
- * as we need to make sure that the 3d driver can correctly address object
- * contents.
+ * i915_gem_set_tiling() and i915_gem_get_tiling() is the userspace interface to
+ * declare fence register requirements.
  *
- * If we don't have interleaved memory, all tiling is safe and no swizzling is
- * required.
+ * In principle GEM doesn't care at all about the internal data layout of an
+ * object, and hence it also doesn't care about tiling or swizzling. There's two
+ * exceptions:
  *
- * When bit 17 is XORed in, we simply refuse to tile at all.  Bit
- * 17 is not just a page offset, so as we page an objet out and back in,
- * individual pages in it will have different bit 17 addresses, resulting in
- * each 64 bytes being swapped with its neighbor!
+ * - For X and Y tiling the hardware provides detilers for CPU access, so called
+ *   fences. Since there's only a limited amount of them the kernel must manage
+ *   these, and therefore userspace must tell the kernel the object tiling if it
+ *   wants to use fences for detiling.
+ * - On gen3 and gen4 platforms have a swizzling pattern for tiled objects which
+ *   depends upon the physical page frame number. When swapping such objects the
+ *   page frame number might change and the kernel must be able to fix this up
+ *   and hence now the tiling. Note that on a subset of platforms with
+ *   asymmetric memory channel population the swizzling pattern changes in an
+ *   unknown way, and for those the kernel simply forbids swapping completely.
  *
- * Otherwise, if interleaved, we have to tell the 3d driver what the address
- * swizzling it needs to do is, since it's writing with the CPU to the pages
- * (bit 6 and potentially bit 11 XORed in), and the GPU is reading from the
- * pages (bit 6, 9, and 10 XORed in), resulting in a cumulative bit swizzling
- * required by the CPU of XORing in bit 6, 9, 10, and potentially 11, in order
- * to match what the GPU expects.
+ * Since neither of this applies for new tiling layouts on modern platforms like
+ * W, Ys and Yf tiling GEM only allows object tiling to be set to X or Y tiled.
+ * Anything else can be handled in userspace entirely without the kernel's
+ * invovlement.
  */
 
 /* Check pitch constriants for all chips & tiling formats */
@@ -166,8 +144,18 @@ i915_gem_object_fence_ok(struct drm_i915_gem_object *obj, int tiling_mode)
 }
 
 /**
+ * i915_gem_set_tiling - IOCTL handler to set tiling mode
+ * @dev: DRM device
+ * @data: data pointer for the ioctl
+ * @file: DRM file for the ioctl call
+ *
  * Sets the tiling mode of an object, returning the required swizzling of
  * bit 6 of addresses in the object.
+ *
+ * Called by the user via ioctl.
+ *
+ * Returns:
+ * Zero on success, negative errno on failure.
  */
 int
 i915_gem_set_tiling(struct drm_device *dev, void *data,
@@ -285,7 +273,17 @@ err:
 }
 
 /**
+ * i915_gem_get_tiling - IOCTL handler to get tiling mode
+ * @dev: DRM device
+ * @data: data pointer for the ioctl
+ * @file: DRM file for the ioctl call
+ *
  * Returns the current tiling mode and required bit 6 swizzling for the object.
+ *
+ * Called by the user via ioctl.
+ *
+ * Returns:
+ * Zero on success, negative errno on failure.
  */
 int
 i915_gem_get_tiling(struct drm_device *dev, void *data,