[igt-dev] [PATCH v7 i-g-t 3/4] lib: Add function to hash a framebuffer

[Brian Starkey]

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).

V6: Simon Sir
 - Replace #define by plain uint32_t variables
 - Return -EINVAL in case fb->num_planes != 1
 - Directly assign the mmap result to ptr
 - No need to copy the whole stride, just copy fb->width * cpp since
we're only going to read that

v5: use igt_memcpy_from_wc() instead of plain memcpy, as suggested by
    Chris Wilson

Signed-off-by: Brian Starkey <brian.starkey at arm.com>
[rebased and updated to the most recent API]
Signed-off-by: Liviu Dudau <liviu.dudau at arm.com>
[rebased and updated the patch to address feedback]
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
Reviewed-by: Simon Ser <simon.ser at intel.com>
---
 lib/igt_fb.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/igt_fb.h |  2 ++
 2 files changed, 70 insertions(+)

diff --git a/lib/igt_fb.c b/lib/igt_fb.c
index 6b674c1b..64d52634 100644
--- a/lib/igt_fb.c
+++ b/lib/igt_fb.c
@@ -3491,6 +3491,74 @@ 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)
+{
+	uint32_t FNV1a_OFFSET_BIAS = 2166136261;
+	uint32_t 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;
+	uint32_t stride = calc_plane_stride(fb, 0);
+
+	if (fb->num_planes != 1)
+		return -EINVAL;
+
+	if (fb->is_dumb)
+		ptr = kmstest_dumb_map_buffer(fb->fd, fb->gem_handle, fb->size,
+					      PROT_READ);
+	else
+		ptr = gem_mmap__gtt(fb->fd, fb->gem_handle, fb->size,
+				    PROT_READ);
+
+	/*
+	 * 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(stride);
+	if (!line) {
+		munmap(map, fb->size);
+		return -ENOMEM;
+	}
+
+	hash = FNV1a_OFFSET_BIAS;
+
+	for (y = 0; y < fb->height; y++, ptr += stride) {
+
+		igt_memcpy_from_wc(line, ptr, fb->width * cpp);
+
+		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
+}
+
 /**
  * igt_format_is_yuv:
  * @drm_format: drm fourcc
diff --git a/lib/igt_fb.h b/lib/igt_fb.h
index 69132b41..d2394638 100644
--- a/lib/igt_fb.h
+++ b/lib/igt_fb.h
@@ -202,5 +202,7 @@ int igt_format_plane_bpp(uint32_t drm_format, int plane);
 void igt_format_array_fill(uint32_t **formats_array, unsigned int *count,
 			   bool allow_yuv);
 
+int igt_fb_get_crc(struct igt_fb *fb, igt_crc_t *crc);
+
 #endif /* __IGT_FB_H__ */
 
-- 
2.23.0
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