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From: Brian Norris <computersforpeace@gmail.com>
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Subject: Re: [PATCH] mtd: bcm47xxsflash: use devm_ioremap_nocache() instead
 of KSEG0ADDR()
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On Sat, Jan 16, 2016 at 01:38:11AM +0100, Rafał Miłecki wrote:
> So I wanted to stick to the cached mapping, [...]

I mentioned this earlier on, but I don't feel like I've gotten a clear
answer. Is a cached mapping actually safe here? From the looks of it,
the memory mapping is a read-only memory-mapped flash, and flash writes
/ erasures are done through a different bus (register writes vis BCMA
bus). So if we have a cached mapping of that memory, it doens't
naturally synchronize with any write/erase operations. Doesn't this mean
you might get stale data if you do a sequence of read / erase / read,
for instance, since the 2nd read will return cached data from the 1st
read?

IIUC, this could be solved by:
(a) using an uncached mapping or
(b) explicitly invalidating the relevant region after doing flash writes
or erasures

But I wonder why you haven't seen any problems if you've been using
KSEG0 (cached) this whole time. Maybe just luck? Or you don't actually
write to the flash that much?

Brian