LddAccessingDevices < Training2

%SECTION0{name=LddAcessingDevices}% Accessing devices

   * we must take care not to conflict with other drivers

   * Linux kernel only sees virtual addresses -> we cannot simply
     access physical memory.

   * we must take that compiler optimization / cpu reordering doesn't bite us, but forget about =volatile=! (Documentation/volatile-considered-harmfull) 

   * especially powerpc people: beware of big endian registers!


---++API:

*avoid conflicts with other drivers:*

<verbatim>
struct resource *request_mem_region(unsigned long start, unsigned long len, char *name);
void release_mem_region(unsigned long start, unsigned long len);
</verbatim>

this shows up in =/proc/iomem=:

<verbatim>
-bash-3.2# cat /proc/iomem 
90000000-97ffffff : /plb/pciex@0a0000000
98000000-9fffffff : /plb/pciex@0c0000000
ef600200-ef600207 : serial
ef600300-ef600307 : serial
ef600400-ef600411 : ibm_iic
ef600500-ef600511 : ibm_iic
ef6c0000-ef6cffff : dwc_otg.0
  ef6c0000-ef6cffff : dwc_otg
f8000000-f8001fff : ndfc-nand.0
fc000000-ffffffff : fc000000.nor_flash
</verbatim>

*exclusive variants*

   * protect against =/dev/mem=

<verbatim>
request_mem_region_exclusive(start, n, name);
</verbatim>

   * control with =iomem=relaxed=

*map physical address into kernel address space:*

<verbatim>
void *ioremap(unsigned long phys_addr, unsigned long size);
void iounmap(void *addr);
</verbatim>


*avoid compiler optimization problems:*

<verbatim>
void barrier();
</verbatim>

*avoid compiler _and_ hardware reordering problems:*

<verbatim>
mb(void)
rmb(void)
wmb(void)
</verbatim>

*be portable _and_ avoid above problems:*

<verbatim>
void io(write|read)(8|16|32)[be] functions
</verbatim>

=be= versions for big endian registers!
5.6. Interfacing real hardware		1. Denx Training Topics		5.6.2. Interrupt Handling
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