Re: [igt-dev] [PATCH V6 i-g-t 3/6] lib: Add function to hash a framebuffer

Ser, Simon
2019-07-10T15:30:02Z

Mostly LGTM, here are a few nits.

On Wed, 2019-06-12 at 23:17 -0300, Brian Starkey wrote:
> 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: use igt_memcpy_from_wc() instead of plain memcpy, as suggested by
> Chris Wilson
>
> Signed-off-by: Brian Starkey <brian.starkey@arm.com>
> [rebased and updated to the most recent API]
> Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
> ---
> lib/igt_fb.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> lib/igt_fb.h | 3 +++
> 2 files changed, 69 insertions(+)
>
> diff --git a/lib/igt_fb.c b/lib/igt_fb.c
> index 9d4f905e..d07dae39 100644
> --- a/lib/igt_fb.c
> +++ b/lib/igt_fb.c
> @@ -3256,6 +3256,72 @@ 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

I'd just use plain uint32_t variables for those, but no big deal.

> + 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);

We could return -EINVAL in case fb->num_planes != 1.

> + 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;

Nit: no need for this, can assign the result of mmap directly to ptr.

> +
> + /*
> + * 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, stride);

Nit: no need to copy the whole stride actually, we can just copy
fb->width * cpp since we're only going to read that.

> +
> + 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 adefebe1..a2741c05 100644
> --- a/lib/igt_fb.h
> +++ b/lib/igt_fb.h
> @@ -37,6 +37,7 @@
> #include <i915_drm.h>
>
> #include "igt_color_encoding.h"
> +#include "igt_debugfs.h"
>
> /*
> * Internal format to denote a buffer compatible with pixman's
> @@ -194,5 +195,7 @@ 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__ */
>
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