Re: [PATCH] drm/vmwgfx: Fix scatterlist unmapping

From: Thomas Hellstrom
Date: Wed Apr 25 2018 - 09:21:42 EST

Hi, Robin,

Thanks for the patch. It was some time since I put together that code, but I remember hitting something similar to

Even if it's clear from the documentation that orig_nents should be used.


On 04/13/2018 05:14 PM, Robin Murphy wrote:
dma_unmap_sg() should be called with the same number of entries
originally passed to dma_map_sg(), not the number it returned, which may
be fewer. Admittedly this driver probably never runs on non-coherent
architectures where getting that wrong could lead to data loss, but it's
always good to be correct, and it's trivially easy to fix by just
restoring the SG table state before the call instead of afterwards.

Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>

Found by inspection while poking around TTM users.

drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c b/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c
index 21111fd091f9..971223d39469 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c
@@ -369,9 +369,9 @@ static void vmw_ttm_unmap_from_dma(struct vmw_ttm_tt *vmw_tt)
struct device *dev = vmw_tt->dev_priv->dev->dev;
+ vmw_tt->sgt.nents = vmw_tt->sgt.orig_nents;
dma_unmap_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.nents,
- vmw_tt->sgt.nents = vmw_tt->sgt.orig_nents;