[igt-dev] [PATCH i-g-t 3/6] lib: Add function to hash a framebuffer

[Liviu Dudau]

From: Brian Starkey <brian.starkey at arm.com>

To use writeback buffers as a CRC source, we need to be able to hash
them. Implement a simple FVA-1a hashing routine for this purpose.

Doing a bytewise hash on the framebuffer directly can be very slow if
the memory is noncached. By making a copy of each line in the FB first
(which can take advantage of word-access speedup), we can do the hash
on a cached copy, which is much faster (10x speedup on my platform).

Signed-off-by: Brian Starkey <brian.starkey at arm.com>
---
 lib/igt_fb.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/igt_fb.h |  5 +++++
 2 files changed, 70 insertions(+)

diff --git a/lib/igt_fb.c b/lib/igt_fb.c
index 7404ba7c..1501006e 100644
--- a/lib/igt_fb.c
+++ b/lib/igt_fb.c
@@ -1705,3 +1705,68 @@ bool igt_fb_supported_format(uint32_t drm_format)
 
 	return false;
 }
+
+/*
+ * This implements the FNV-1a hashing algorithm instead of CRC, for
+ * simplicity
+ * http://www.isthe.com/chongo/tech/comp/fnv/index.html
+ *
+ * hash = offset_basis
+ * for each octet_of_data to be hashed
+ *         hash = hash xor octet_of_data
+ *         hash = hash * FNV_prime
+ * return hash
+ *
+ * 32 bit offset_basis = 2166136261
+ * 32 bit FNV_prime = 224 + 28 + 0x93 = 16777619
+ */
+int igt_fb_get_crc(struct igt_fb *fb, igt_crc_t *crc)
+{
+#define FNV1a_OFFSET_BIAS 2166136261
+#define FNV1a_PRIME 16777619
+	uint32_t hash;
+	void *map;
+	char *ptr, *line = NULL;
+	int x, y, cpp = igt_drm_format_to_bpp(fb->drm_format) / 8;
+
+	if (fb->is_dumb)
+		map = kmstest_dumb_map_buffer(fb->fd, fb->gem_handle, fb->size,
+					      PROT_READ);
+	else
+		map = gem_mmap__gtt(fb->fd, fb->gem_handle, fb->size,
+				    PROT_READ);
+	ptr = map;
+
+	/*
+	 * Framebuffers are often uncached, which can make byte-wise accesses
+	 * very slow. We copy each line of the FB into a local buffer to speed
+	 * up the hashing.
+	 */
+	line = malloc(fb->stride);
+	if (!line) {
+		munmap(map, fb->size);
+		return -ENOMEM;
+	}
+
+	hash = FNV1a_OFFSET_BIAS;
+
+	for (y = 0; y < fb->height; y++, ptr += fb->stride) {
+
+		memcpy(line, ptr, fb->stride);
+
+		for (x = 0; x < fb->width * cpp; x++) {
+			hash ^= line[x];
+			hash *= FNV1a_PRIME;
+		}
+	}
+
+	crc->n_words = 1;
+	crc->crc[0] = hash;
+
+	free(line);
+	munmap(map, fb->size);
+
+	return 0;
+#undef FNV1a_OFFSET_BIAS
+#undef FNV1a_PRIME
+}
diff --git a/lib/igt_fb.h b/lib/igt_fb.h
index 023b069d..bc615516 100644
--- a/lib/igt_fb.h
+++ b/lib/igt_fb.h
@@ -36,6 +36,8 @@
 
 #include <i915_drm.h>
 
+#include "igt_debugfs.h"
+
 /**
  * igt_fb_t:
  * @fb_id: KMS ID of the framebuffer
@@ -164,5 +166,8 @@ uint32_t igt_drm_format_to_bpp(uint32_t drm_format);
 const char *igt_format_str(uint32_t drm_format);
 bool igt_fb_supported_format(uint32_t drm_format);
 
+/* Get a hash for a framebuffer */
+int igt_fb_get_crc(struct igt_fb *fb, igt_crc_t *crc);
+
 #endif /* __IGT_FB_H__ */
 
-- 
2.16.1