Problem with DMA on x86_64 with 3 GB RAM

[Larry Finger <Larry.Finger@lwfinger.net>]

I am trying to debug a bcm43xx DMA problem on an x86_64 system with 3 GB RAM. Depending on the 
particular chip and its implementation, dma transfers may use 64-, 32-, or 30-bit addressing, with 
the problem interface using 30-bit addressing. From test prints, the correct mask (0x3FFFFFFF) is 
supplied to pci_set_dma_mask and pci_set_consistent_dma_mask. Neither call returns an error. In 
addition, several x86_64 systems with more than 1 GB RAM have worked with the current code.

If the system is booted with mem=1024M on the command line, it operates normally; however, it gets a 
kernel NULL pointer dereference and panics when booted with either 2 or 3 GB RAM.

The config parameters in the processor section are as follows:

CONFIG_X86_64=y
CONFIG_64BIT=y
CONFIG_X86=y
CONFIG_ZONE_DMA32=y
CONFIG_LOCKDEP_SUPPORT=y
CONFIG_STACKTRACE_SUPPORT=y
CONFIG_SEMAPHORE_SLEEPERS=y
CONFIG_MMU=y
CONFIG_RWSEM_GENERIC_SPINLOCK=y
CONFIG_GENERIC_HWEIGHT=y
CONFIG_GENERIC_CALIBRATE_DELAY=y
CONFIG_X86_CMPXCHG=y
CONFIG_EARLY_PRINTK=y
CONFIG_GENERIC_ISA_DMA=y
CONFIG_GENERIC_IOMAP=y
CONFIG_ARCH_MAY_HAVE_PC_FDC=y
CONFIG_ARCH_POPULATES_NODE_MAP=y
CONFIG_DMI=y
CONFIG_AUDIT_ARCH=y
CONFIG_DEFCONFIG_LIST="/lib/modules/$UNAME_RELEASE/.config"

I would appreciate any tips on debugging this problem. Are there any quantities that should be 
dumped, etc?

Thanks,

Larry