On Sun, Jan 15, 2017 at 12:17:40PM +0200, Ran Shalit wrote:

Hello,

I've seen some modules where it is tested if virtual address is 0, and
if yes, it is acted as if it is an error.
But can't virtual address be zero too ?

For example, in the following module:
https://github.com/khilman/omap-test-dmatest/blob/master/main.c
it is checked the returned value of dma_alloc_coherent as following:

dma_test[i].src_buf = (unsigned long) dma_alloc_coherent(NULL,
buf_size (dma_addr_t *)&dma_test[i].src_buf_phys, GFP_KERNEL |
GFP_DMA);
if (!dma_test[i].src_buf) {
   printk("dmatest_init(): get_zeroed_page() failed.\n");
   r = -ENOMEM;
   goto cleanup;
}

As you can see , when  dma_alloc_coherent returns 0 , it is acted as
if it is an error.

Yes, if dma_alloc_coherent returns NULL, an error happened and the
memory could not be allocated.

In-kernel memory pointers will never be NULL if they are valid.

Hope this helps,

greg k-h