panfrost: Add notes about the tiler allocations

This explains how the polygon list is allocated, updating the headers
appropiately to sync the terminology.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

src/gallium/drivers/panfrost/pan_tiler.c

@@ -0,0 +1,86 @@
+/*
+ * Copyright (C) 2019 Collabora
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * Authors:
+ *   Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
+ */
+
+/* Mali GPUs are tiled-mode renderers, rather than immediate-mode.
+ * Conceptually, the screen is divided into 16x16 tiles. Vertex shaders run.
+ * Then, a fixed-function hardware block (the tiler) consumes the gl_Position
+ * results. For each triangle specified, it marks each containing tile as
+ * containing that triangle. This set of "triangles per tile" form the "polygon
+ * list". Finally, the rasterization unit consumes the polygon list to invoke
+ * the fragment shader.
+ *
+ * In practice, it's a bit more complicated than this. 16x16 is the logical
+ * tile size, but Midgard features "hierarchical tiling", where power-of-two
+ * multiples of the base tile size can be used: hierarchy level 0 (16x16),
+ * level 1 (32x32), level 2 (64x64), per public information about Midgard's
+ * tiling. In fact, tiling goes up to 2048x2048 (!), although in practice
+ * 128x128 is the largest usually used (though higher modes are enabled).  The
+ * idea behind hierarchical tiling is to use low tiling levels for small
+ * triangles and high levels for large triangles, to minimize memory bandwidth
+ * and repeated fragment shader invocations (the former issue inherent to
+ * immediate-mode rendering and the latter common in traditional tilers).
+ *
+ * The tiler itself works by reading varyings in and writing a polygon list
+ * out. Unfortunately (for us), both of these buffers are managed in main
+ * memory; although they ideally will be cached, it is the drivers'
+ * responsibility to allocate these buffers. Varying buffe allocation is
+ * handled elsewhere, as it is not tiler specific; the real issue is allocating
+ * the polygon list.
+ *
+ * This is hard, because from the driver's perspective, we have no information
+ * about what geometry will actually look like on screen; that information is
+ * only gained from running the vertex shader. (Theoretically, we could run the
+ * vertex shaders in software as a prepass, or in hardware with transform
+ * feedback as a prepass, but either idea is ludicrous on so many levels).
+ *
+ * Instead, Mali uses a bit of a hybrid approach, splitting the polygon list
+ * into three distinct pieces. First, the driver statically determines which
+ * tile hierarchy levels to use (more on that later). At this point, we know the
+ * framebuffer dimensions and all the possible tilings of the framebuffer, so
+ * we know exactly how many tiles exist across all hierarchy levels. The first
+ * piece of the polygon list is the header, which is exactly 8 bytes per tile,
+ * plus padding and a small 64-byte prologue. (If that doesn't remind you of
+ * AFBC, it should. See pan_afbc.c for some fun parallels). The next part is
+ * the polygon list body, which seems to contain 512 bytes per tile, again
+ * across every level of the hierarchy. These two parts form the polygon list
+ * buffer. This buffer has a statically determinable size, approximately equal
+ * to the # of tiles across all hierarchy levels * (8 bytes + 512 bytes), plus
+ * alignment / minimum restrictions / etc.
+ *
+ * The third piece is the easy one (for us): the tiler heap. In essence, the
+ * tiler heap is a gigantic slab that's as big as could possibly be necessary
+ * in the worst case imaginable. Just... a gigantic allocation that we give a
+ * start and end pointer to. What's the catch? The tiler heap is lazily
+ * allocated; that is, a huge amount of memory is _reserved_, but only a tiny
+ * bit is actually allocated upfront. The GPU just keeps using the
+ * unallocated-but-reserved portions as it goes along, generating page faults
+ * if it goes beyond the allocation, and then the kernel is instructed to
+ * expand the allocation on page fault (known in the vendor kernel as growable
+ * memory). This is quite a bit of bookkeeping of its own, but that task is
+ * pushed to kernel space and we can mostly ignore it here, just remembering to
+ * set the GROWABLE flag so the kernel actually uses this path rather than
+ * allocating a gigantic amount up front and burning a hole in RAM.
+ */