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

[Jani Nikula <jani.nikula@linux.intel.com>]

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. ;)


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