Messages in this thread | | | Date | Wed, 23 Jan 2013 10:57:45 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH] clk: Add axi-clkgen driver |
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On Wed, Jan 23, 2013 at 11:49:16AM +0100, Lars-Peter Clausen wrote: > On 01/23/2013 11:27 AM, Russell King - ARM Linux wrote: > > On Wed, Jan 23, 2013 at 11:00:56AM +0100, Lars-Peter Clausen wrote: > >> I think I read somewhere at some point that ioread{8,16,32} is preferred > >> over write{b,h,l} in new code. > > > > But... there's *no* point using ioread*() if you don't only use the > > ioremap() interface. > > > > ioread*() is there to allow PC IO and PC MMIO accesses through one > > accessor depending on the cookie. ioremap() only ever returns cookies > > for MMIO accesses. > > So your point is, if you know that it is MMIO memory only ever use readl and > friends?
My point is... if you're going to stick with using ioremap(), then don't bother with the potential overhead from ioread*() - you don't gain anything but you have a few extra CPU cycles per access. You might as well just stick with read*() which aren't going anywhere anytime soon.
Sure, if you need stuff to work on both IO and MMIO accesses, then use ioread*() but you'd also need the driver to: - accept IORESOURCE_IO resource types. - request these resources in the IO resource tree rather than the MMIO resource tree. - use ioport_map() to "map" these resource types.
So, I don't mind seeing ioread*() in a driver _if_ it also uses IO resources and handles them correctly, but it's pointless to use this accessor in any driver which doesn't.
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