问题
1) Is it possible to access a physical address which is not defined in /proc/iomem?
2) If the physical address range of a device does not appear in /proc/iomem, does it mean that the device has not been utilized/initialized yet?
回答1:
1) Is it possible to access a physical address which is not defined in /proc/iomem?
Yes.
Assuming an ARM processor which memory maps all directly-connected periperals, the driver could perform an ioremap() operation to map the physical memory to virtual memory for access.
But a properly written driver would first call request_mem_region() to ensure that it can use (and lay claim to) that physical address space.
The information in /proc/iomem comes from drivers calling request_mem_region().
2) If the physical address range of a device does not appear in /proc/iomem, does it mean that the device has not been utilized/initialized yet?
You would have to inspect the driver code to determine how well written the driver is.
Is there a request_mem_region() before the ioremap()?
Check the system log using the dmesg command; perhaps driver initialization failed.
Assuming that this is a statically-linked driver rather than a loadable module, then as each kernel device driver has its init() routine called you can get trace output by having added the option "initcall_debug" on the kernel command line. If you are using U-Boot, then this option should be added to the "bootargs" variable (which is used for the kernel command line).
来源：https://stackoverflow.com/questions/39217907/expose-information-to-proc-iomem