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

["Ser, Simon" <simon.ser@intel.com>]

On Wed, 2019-07-10 at 15:30 +0000, Ser, Simon wrote:
> 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.

Let's not waste cycles. With this ^ fixed, this patch is:

Reviewed-by: Simon Ser <simon.ser@intel.com>

Other nits are optional.

> > +	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__ */
> >  
> > _______________________________________________
> > igt-dev mailing list
> > igt-dev@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/igt-dev
> _______________________________________________
> igt-dev mailing list
> igt-dev@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/igt-dev
_______________________________________________
igt-dev mailing list
igt-dev@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/igt-dev