Re: [PATCH] mtd: bcm47xxsflash: use devm_ioremap_nocache() instead of KSEG0ADDR()

[Maciej W. Rozycki]

On Fri, 8 Jan 2016, Brian Norris wrote:

> > > > This removes some (implicit) MIPS dependencies and makes the code more
> > > > portable, whether we need it or not :)
> > > 
> > > So now we have following forwardtrace:
> > > devm_ioremap_nocache
> > > ioremap_nocache
> > > __ioremap_mode
> > > __ioremap
> > > CKSEG1ADDR
> 
> I just noticed that ioremap() and ioremap_nocache() are the same on
> MIPS. So I could just do devm_ioremap_resource() and save myself a few
> lines...

 My bad, I wrote from memory and didn't double check what the caching mode 
for plain `ioremap' is.  You need to use `ioremap_cache' for a cached 
mapping.  Unfortunately the MIPS port is missing this generic interface 
and defines `ioremap_cachable' instead.  I've just posted an obvious fix 
for this problem.

 Ralf, can you pick this fix for 4.5?  It qualifies as obvious I believe, 
similar to a recent `ioremap_uc' addition.

> > > It results in different address than KSEG0ADDR:
> > > [    1.339752] [bcm47xxsflash_bcma_probe] KSEG0ADDR(BCMA_SOC_FLASH2):9c000000
> > > [    1.346848] [bcm47xxsflash_bcma_probe] devm_ioremap_nocache:bc000000
> > > 
> > > But it still works as expected! :)
> > > [    1.609426] 6 bcm47xxpart partitions found on MTD device bcm47xxsflash
> > > [    1.616169] Creating 6 MTD partitions on "bcm47xxsflash":
> > 
> >  It is a functional change though and I think the change from a cached to 
> > uncached mapping (i.e. from `ioremap' to `ioremap_nocache') has to be a 
> > separate patch, so that both changes can be reviewed independently.
> 
> As I noted before sending my patch, I don't think this driver should
> have been using KSEG0 anyway; it should have been KSEG1, right? I can
> note that in the patch description, but I don't really see why it needs
> to be a separate patch.

 You did mention that, but didn't actually justify why an uncached mapping 
is required here.

 This code is in a function called `bcm47xxsflash_read' and reads from an 
MMIO region, presumably flash memory which behaves like ordinary memory on 
reads (i.e. no side effects).  Therefore using a cached mapping will in 
most cases result in much better performance as the CPU will load 
(prefetch) data in cacheline-sized quantities rather than hitting the 
external bus every time with a word-sized quantity transferred only.

 Switching to an uncached mapping would invalidate this optimisation and 
is independent from a build error fix.  Therefore it requires a separate 
patch and review.  Perhaps `memremap' should be used instead (possibly 
with the MEMREMAP_WT policy), but this is exactly why a separate review is 
required for that part.

  Maciej