help with dma_alloc_coherent() + dma_free_coherent()

[Jeff Chua]

I'm trying to understand how this oops in the diva driver and it's just a
simple dma_alloc_coherent() followed by dma_free_coherent(), but it oops.
Why?

Thanks,
Jeff


static u32 *clock_data_addr;
static dma_addr_t clock_data_bus_addr;

        if((clock_data_addr = dma_alloc_coherent(NULL, PAGE_SIZE,
                        &clock_data_bus_addr, GFP_KERNEL)) != 0) {
                printk(KERN_INFO "dma_alloc_coherent ok\n");
                memset (clock_data_addr, 0x00, PAGE_SIZE);
        } else
                printk(KERN_INFO "dma_alloc_coherent bad!!!\n");
        if(clock_data_addr) {
                printk(KERN_INFO "dma_free_coherent!!!\n");
                dma_free_coherent(NULL, PAGE_SIZE, clock_data_addr,
                        clock_data_bus_addr);
                clock_data_addr = NULL;
        }


dma_alloc_coherent ok
dma_free_coherent!!!
BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff813ec01c>] iommu_no_mapping+0xc/0xee
PGD 20e4d8067 PUD 20bda6067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: divadidd(+) i915 drm_kms_helper iwldvm iwlwifi mac80211
cfg80211
CPU 0
Pid: 2823, comm: modprobe Not tainted 3.8.0-rc2 #9 LENOVO 4291JB0/4291JB0
RIP: 0010:[<ffffffff813ec01c>]  [<ffffffff813ec01c>]
iommu_no_mapping+0xc/0xee
RSP: 0018:ffff880211665d68  EFLAGS: 00010246
RAX: 00000000d3801000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000001000 RSI: 00000000d3801000 RDI: 0000000000000000
RBP: 0000000000000001 R08: 0000000000000000 R09: 00000000000045c7
R10: 00000000000045c7 R11: 00000000000045c7 R12: ffffffff81a4d800
R13: ffffffffa013a550 R14: ffff88020be0fa80 R15: 0000000000000001
FS:  00007f9df9786700(0000) GS:ffff88021e200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000002114c1000 CR4: 00000000000407f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
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