Re: [Intel-gfx] [PATCH] drm/i915: Reduce ELD hex dumps a bit

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Feb 15, 2023 at 03:15:41PM +0200, Jani Nikula wrote:
> On Wed, 15 Feb 2023, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> > Do the ELD hexdumps only up to the last differing byte.
> > The rest is typically all zeroes anyway so not much point
> > in dumping it.
> 
> Arguably part of the reason for big dumps is that we use MAX_ELD_BYTES
> for the size instead of drm_eld_size(). Granted, using drm_eld_size()
> brings other complications, so maybe this is better anyway.
> 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_display.c | 23 ++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> > index 3479125fbda6..d73aea9040e0 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > @@ -5348,6 +5348,23 @@ pipe_config_dp_vsc_sdp_mismatch(struct drm_i915_private *dev_priv,
> >  	}
> >  }
> >  
> > +/*
> > + * Like a revese memcmp(), but returns the
> > + * position of the last differing byte.
> > + */
> > +static int
> > +memcmp_pos_reverse(const u8 *a, const u8 *b, size_t len)
> > +{
> > +	int i;
> > +
> > +	for (i = len - 1; i >= 0; i--) {
> > +		if (a[i] != b[i])
> > +			return i;
> > +	}
> > +
> > +	return len;
> > +}
> 
> Maybe make the function return "length of differing prefix" instead?
> 
> If the buffers are identical, you now return len, and the caller dumps
> len + 1, overflowing the buffer. This shouldn't happen, because we've
> checked before that the buffers do differ, but it's a trap for anyone
> who just picks this up for some other use case.
> 
> The fix is simple, return "i + 1" if there's a difference, return 0 at
> the end if there's not, and drop the + 1 from the caller side.
> 
> Renaming the function is the hard part. ;)

memcmp_len_reverse()?

> 
> 
> BR,
> Jani.
> 
> 
> > +
> >  static void
> >  pipe_config_buffer_mismatch(struct drm_i915_private *dev_priv,
> >  			    bool fastset, const char *name,
> > @@ -5357,6 +5374,9 @@ pipe_config_buffer_mismatch(struct drm_i915_private *dev_priv,
> >  		if (!drm_debug_enabled(DRM_UT_KMS))
> >  			return;
> >  
> > +		/* only dump up to the last difference */
> > +		len = memcmp_pos_reverse(a, b, len) + 1;
> > +
> >  		drm_dbg_kms(&dev_priv->drm,
> >  			    "fastset mismatch in %s buffer\n", name);
> >  		print_hex_dump(KERN_DEBUG, "expected: ", DUMP_PREFIX_NONE,
> > @@ -5364,6 +5384,9 @@ pipe_config_buffer_mismatch(struct drm_i915_private *dev_priv,
> >  		print_hex_dump(KERN_DEBUG, "found: ", DUMP_PREFIX_NONE,
> >  			       16, 0, b, len, false);
> >  	} else {
> > +		/* only dump up to the last difference */
> > +		len = memcmp_pos_reverse(a, b, len) + 1;
> > +
> >  		drm_err(&dev_priv->drm, "mismatch in %s buffer\n", name);
> >  		print_hex_dump(KERN_ERR, "expected: ", DUMP_PREFIX_NONE,
> >  			       16, 0, a, len, false);
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center

-- 
Ville Syrjälä
Intel