[bug report] drm/prime: replace NULL with error value in drm_prime_pages_to_sg

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello YoungJun Cho,

The patch 7e3d88f9cce3: "drm/prime: replace NULL with error value in
drm_prime_pages_to_sg" from Jun 24, 2013, leads to the following
static checker warning:

	drivers/gpu/drm/drm_prime.c:317 drm_gem_map_dma_buf()
	warn: 'sgt' can also be NULL

drivers/gpu/drm/drm_prime.c
   307          /*
   308           * two mappings with different directions for the same attachment are
   309           * not allowed
   310           */
   311          if (WARN_ON(prime_attach->dir != DMA_NONE))
   312                  return ERR_PTR(-EBUSY);
   313  
   314          sgt = obj->dev->driver->gem_prime_get_sg_table(obj);

The problematic functions here are drm_gem_cma_prime_get_sg_table() and
xen_drm_front_gem_get_sg_table().  My preference would be to update
those functions to return error pointers, but I'm not familiar with the
code or able to test it.

But this static checker test seems pretty good so I'm likely to publish
it soon and then this sort of bug will normally be caught quickly.

   315  
   316          if (!IS_ERR(sgt)) {
   317                  if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
   318                                        DMA_ATTR_SKIP_CPU_SYNC)) {
   319                          sg_free_table(sgt);
   320                          kfree(sgt);
   321                          sgt = ERR_PTR(-ENOMEM);
   322                  } else {
   323                          prime_attach->sgt = sgt;
   324                          prime_attach->dir = dir;
   325                  }
   326          }
   327  
   328          return sgt;

regards,
dan carpenter
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