Execute in place.

[Charles Manning]

David

Is there any discussion leading up to XIP? ie. Why? ... and why not!

A few questions/comments inline.

On Wednesday 04 June 2003 20:34, David Woodhouse wrote:
> I've done some work on kernel XIP. The basic principle of operation is
> that we copy any code which may need to run while the flash chips are in
> anything but 'read' mode into RAM, and we disable interrupts while the
> chips are busy. During erases, we poll not only for erase completion but
> also for pending IRQs. If an IRQ is pending, we suspend the erase
> operation, re-enable IRQs and call cond_resched().

It scares me that this adds a potentially huge interrupt latency. Some NOR 
devices need a good 20usec or so to get into erase suspend state. There are 
many situations (eg ARM FIQs) where this would not be a nice thing to do.  
Thus, mixing XIP with say a file system which might cause erases at 
(relatively) arbitrary times needs to be done with great caution. Men with 
red flags needed up front.

>
> The flash driver code is fairly simple, although it wants a little bit
> of cleanup and I want to be convinced that the arch-specific bits are
> done right before committing it.
>
> The arch-specific parts are implemented for XScale only so far, but
> should be relatively simple to do for new architectures. You need to
> ensure that parts of kernel code can be loaded into RAM instead of ROM,
> when marked with the '__xipram' attribute, that the udelay() function is
> in that section, and also your software TLB handlers if you have them --
> or indeed anything else which may be needed in the critical sections of
> the flash chip driver. I've reintroduced the get_unaligned() macro to
> potentially-unaligned data loads in the critical region, so alignment
> fixups do _not_ need to be in RAM.
>
> You also need to provide suitable xip_cli(), xip_sti(), and
> xip_irqpending() functions or macros in include/linux/mtd/xip.h. I
> suspect that wants moving to include/asm-$(ARCH)/xip.h or similar.

The fundamental implementation seems quite simple and elegant.IMHO.

-- Charles