Re: [PATCH] Define wc_wmb, a write barrier for PCI write combining

[Jesse Barnes <jbarnes@virtuousgeek.org>]

On Tuesday, February 28, 2006 9:44 am, Jesse Barnes wrote:
> Something like this would be really handy.  Check out fbmem.c:fb_mmap
> for a bad example of what can happen w/o it.
>
> In fact, I think it might make sense to export WC functionality via an
> mmap flag (on an advisory basis since the platform may not support it
> or there may be aliasing issues that prevent it); having an arch
> independent routine to request it would make that addition easy to do
> in generic code.  (In particular I wanted this for the sysfs PCI
> interface. Userspace apps can map PCI resources there and it would be
> nice if they could map them with WC semantics if requested.)

Oh, forgot to mention fallback semantics.  Instead of almost every driver 
doing:
	if (!(iocookie = ioremap_writecombine(addr, size)))
		iocookie = ioremap(addr, size); /* fallback to uncached */

maybe it would be best to have something like

	iocookie = ioremap_wc_or_uc(addr, size);

that tries write combine first but silently falls back to UC if the 
former is impossible (or a new ioremap with flags or priority args or 
whatever).  OTOH some drivers may want to be notified if the WC mapping 
fails?  Just a thought...

Jesse