Re: [PATCH 09/13] ARM: imx: Remove unused IO_ADDRESS() macros

On Tue, Sep 29, 2020 at 3:11 PM Linus Walleij [off-list ref] wrote:

On Thu, Sep 17, 2020 at 9:16 AM Arnd Bergmann [off-list ref] wrote:

quoted

If all of the devices in here are now also mapped using of_iomap() or
ioremap(), then the iotables are not strictly needed any more, but most
of them are 1MB section sized, which helps slightly reduce TLB usage
compared to non-section ioremap() mappings, so we might want to
keep them anyway, possibly with the constants moved from the header
into the mm-imx??.c files.

Adding Linus to Cc for further thoughts, as he was looking at the early
io mappings recently.

There is early fixmap which I think should replace the iotables
completely if we can, since it is generic kernel code. I was meaning
to look into this "at some point".

The early fixmap seems to be page (0x1000) based so the iotables
might be more efficient if using 1MB sections but I doubt that it is
worth it.

As far as I can tell, the 1MB section maps are the only reason
this exists, as all of them are actually ioremap()ed later on before
use.

I don't know if it makes a difference to real-world performance.
Note that there are only 64 TLB entries on ARM926, or 128 on
ARM1136, so avoiding the lookup/eviction at least theoretically
should be measurably faster.

       Arnd

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