Re: [PATCH] dma-direct: Fix potential NULL pointer dereference

[Christoph Hellwig]

On Tue, Sep 15, 2020 at 08:03:14AM -0600, Thomas Tai wrote:
> When booting the kernel v5.9-rc4 on a VM, the kernel would panic when
> printing a warning message in swiotlb_map(). It is because dev->dma_mask
> can potentially be a null pointer. Using the dma_get_mask() macro can
> avoid the NULL pointer dereference.

dma_mask must not be zero.  This means drm is calling DMA API functions
on something weird.  This needs to be fixed in the caller.