diff --git a/tinygrad/llops/cpu.py b/tinygrad/llops/cpu.py
index f347cb863c..9d283d7021 100644
--- a/tinygrad/llops/cpu.py
+++ b/tinygrad/llops/cpu.py
@@ -4,6 +4,7 @@ from tinygrad.helpers import UnaryOps, BinaryOps, ReduceOps
 class CPUBuffer(np.ndarray):
   def __new__(cls, shape, hostbuf=None):
     if hostbuf is not None:
+      #print(shape, hostbuf.shape)
       return super().__new__(cls, shape, buffer=hostbuf.data)
     else:
       return super().__new__(cls, shape)
@@ -90,6 +91,7 @@ def convdw(x,grad_output,dw,conv_args):
   tx = get_tx(x, conv_args)
   ggg = grad_output.reshape(bs,groups,rcout,oy,ox)
   gdw = dw.reshape((groups,rcout,cin,H,W))
+  gdw[:] = 0
   for g in range(groups):
     #'ikYX,ijYXyx -> kjyx'
     gdw[g] += np.tensordot(ggg[:,g], tx[:,g], ((0,2,3),(0,2,3)))
@@ -97,10 +99,10 @@ def convdw(x,grad_output,dw,conv_args):
 
 def convdx(w,grad_output,dx,conv_args):
   H, W, groups, rcout, cin, oy, ox, iy, ix, ys, xs, bs = conv_args
-  OY,OX = dx.shape[2:4]
   ggg = grad_output.reshape(bs,groups,rcout,oy,ox)
   tw = w.reshape(groups, rcout, cin, H, W)
-  gdx = dx.reshape((bs,groups,cin,OY,OX))
+  gdx = dx.reshape((bs,groups,cin,iy,ix))
+  gdx[:] = 0
   for k in range(oy*ox):
     Y, X = k//ox, k%ox
     iY,iX = Y*ys, X*xs
diff --git a/tinygrad/ops/ops_gpu.py b/tinygrad/ops/ops_gpu.py
index 8720eb3fd8..ea303c4e07 100644
--- a/tinygrad/ops/ops_gpu.py
+++ b/tinygrad/ops/ops_gpu.py
@@ -1,9 +1,16 @@
+import os
 import numpy as np
 from tinygrad.helpers import binary_broadcast, UnaryOps, BinaryOps, ReduceOps
 from ..tensor import Function
-from ..llops.opencl import GPUBuffer as Buffer
-from ..llops.opencl import unary_op, binary_op, reduce_op, perm_axis, inner_slice
-from ..llops.opencl import matmul, conv, convdw, convdx
+
+if int(os.getenv("LLCPU", 0)) == 1:
+  from ..llops.cpu import CPUBuffer as Buffer
+  from ..llops.cpu import unary_op, binary_op, reduce_op, perm_axis, inner_slice
+  from ..llops.cpu import matmul, conv, convdw, convdx
+else:
+  from ..llops.opencl import GPUBuffer as Buffer
+  from ..llops.opencl import unary_op, binary_op, reduce_op, perm_axis, inner_slice
+  from ..llops.opencl import matmul, conv, convdw, convdx
 
 # ************* unary ops *************