[RFC/NOT FOR MERGING 2/3] serial: omap: remove hwmod dependency

[Paul Walmsley]

Hi,

On Thu, 14 Feb 2013, Tony Lindgren wrote:

> I don't think so as hwmod should only touch the sysconfig space
> when no driver has claimed it.

hwmod does need to touch the SYSCONFIG register during pm_runtime suspend 
and resume operations, and during device enable and disable operations.  
Here's the relevant code:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=arch/arm/mach-omap2/omap_hwmod.c;h=4653efb87a2721ea20a9fe06a30a6c204d6d2282;hb=HEAD#l2157

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=arch/arm/mach-omap2/omap_hwmod.c;h=4653efb87a2721ea20a9fe06a30a6c204d6d2282;hb=HEAD#l2195

Drivers shouldn't touch their IP block's SYSCONFIG registers.  They don't 
have anything specifically to do with the underlying device IP block, but 
with the SoC integration, even though they live in the device's 
memory-mapped address range.  It's unfortunate that TI didn't allocate a 
separate, small address space for these registers, translated by the bus, 
similar to the PCI-E Enhanced Configuration Access Mechanism:

http://wiki.osdev.org/PCI_Express#Extended_Configuration_Space

... but that's unfortunately the reality of the OMAP hardware.

Regarding ioremap(), it seems reasonable for drivers to call ioremap(), as 
long as the implementation of ioremap() can be overridden by the device's 
bus.  PCI device drivers already do this -- albeit in a PCI-specific way 
-- with pci_ioremap_bar():

http://git.kernel.org/?p=linux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git&a=search&h=HEAD&st=grep&s=pci_ioremap_bar

So instead of something bus-specific like that, a better way would be to 
use something like:

va = dev->bus->ioremap( ... );
va = dev->bus->iounmap( ... );

Any driver using such a mechanism would be intrinsically generic. 
platform_bus could simply set its bus->ioremap to the existing ioremap() 
function, while buses like a omap_hwmod_bus could use a function that 
returned the address range that omap_hwmod_bus has already ioremap()ped.

regards,

- Paul