diff --git a/llms/mlx_lm/models/mamba2.py b/llms/mlx_lm/models/mamba2.py
index 7c044dba..521d3e8d 100644
--- a/llms/mlx_lm/models/mamba2.py
+++ b/llms/mlx_lm/models/mamba2.py
@@ -42,6 +42,10 @@ class ModelArgs(BaseModelArgs):
             self.time_step_rank = math.ceil(self.hidden_size / 16)
 
 
+def segsum(x):
+    return mx.cumsum(x, axis=-1).reshape(*x.shape[:-1], 1, x.shape[-1])
+
+
 class DepthWiseConv1d(nn.Module):
     def __init__(self, channels, kernel_size, bias=True, padding=0):
         super().__init__()
@@ -133,19 +137,6 @@ def ssd_forward_attn(
     return y, next_state
 
 
-def segsum(x):
-    # x shape: [b, h, l]
-    b, h, l = x.shape
-    indices = mx.arange(l)
-    mask = indices[:, None] >= indices[None, :]  # [l, l] lower triangular mask
-    # Expand x for broadcasting
-    x_expanded = x.reshape(b, h, l, 1)  # [b, h, l, 1]
-    # Apply mask and sum
-    masked_x = x_expanded * mask.reshape(1, 1, l, l)  # [b, h, l, l]
-    x_segsum = mx.sum(masked_x, axis=2, keepdims=True)  # [b, h, 1, l]
-    return x_segsum
-
-
 class Mamba2Block(nn.Module):
     def __init__(self, args: ModelArgs):
         super().__init__()