Messages in this thread | | | Date | Sun, 24 Jan 2016 10:44:06 +0100 | Subject | Re: [PATCH] mtd: bcm47xxsflash: use devm_ioremap_nocache() instead of KSEG0ADDR() | From | Rafał Miłecki <> |
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On 23 January 2016 at 22:49, Brian Norris <computersforpeace@gmail.com> wrote: > 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?
Now you pointed this difference between reads and writes I sounds worrying indeed. I'm not aware of ever hitting this problem but maybe I just didn't use flash in a way triggering it?
I'm looking for a way to test it. Using user space I could try doing something like: echo foo > a.txt cat a.txt echo bar > a.txt cat a.txt
I guess even more reliable test would to be test in in kernel space. I guess I could modify bcm47xxsflash_write to read flash region that is going to be modified: before modification and after. Both reads using KSEG0ADDR. Then compare if the second read matches was was written.
Does my idea for tests make sense?
-- Rafał
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