On 2020-09-15 10:07 a.m., Christoph Hellwig wrote:
> 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.
>
Thanks, Christoph for your comment. The caller already fixed the null
pointer in the latest v5.9-rc5. I am thinking that if we had used the
dma_get_mask(), the kernel couldn't panic and could properly print out
the warning message.
Thomas