On Sun, 29 May 2011, Russell King - ARM Linux wrote:

quoted

That would return memory that is not guaranteed to be within the first 
16MB of address space, so a GFP_DMA allocation would succeed with memory 
not from ZONE_DMA.

Err, no. GFP_DMA returns memory in a zone which the platform has setup.
There's nothing specific about it being "16MB" or any other size; the
arch can chose what size that is.

Sorry, was talking from the x86 perspective, which probably doesn't make 
much sense since we're talking about an arm driver :)

quoted

That's an invalid configuration, so users, including 
you, should at least edit their .config by hand to enable CONFIG_ZONE_DMA 
as a workaround.

Again, no.  This change has caused a load of previously working drivers
to suddenly start failing without _any_ explanation why or even warning
about the change.  It needs to start off as a WARN_ON() so that stuff
can be fixed, and then changed to a hard error.

I haven't seen a "load" of error reports where this is causing an issue, 
maybe it is much more popular on arm?

This also isn't a hard error, admins should be able to enable 
CONFIG_ZONE_DMA and rebuild so that the driver being loaded can get the 
type of memory it is requesting.  Just putting a WARN_ON() doesn't provide 
any incentive to ever get this stuff fixed.