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
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