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

From: "Rafał Miłecki" <zajec5@gmail.com>
To: "Maciej W. Rozycki" <macro@imgtec.com>
Cc: Brian Norris <computersforpeace@gmail.com>,
	"linux-mtd@lists.infradead.org" <linux-mtd@lists.infradead.org>,
	Javier Martinez Canillas <javier@osg.samsung.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Fengguang Wu <fengguang.wu@intel.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Luis de Bethencourt <luisbg@osg.samsung.com>,
	Jeremy Kerr <jk@ozlabs.org>,
	Neelesh Gupta <neelegup@linux.vnet.ibm.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Cyril Bur <cyrilbur@gmail.com>,
	Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH] mtd: bcm47xxsflash: use ioremap_cachable() instead of KSEG0ADDR()
Date: Fri, 26 Feb 2016 17:31:53 +0100	[thread overview]
Message-ID: <CACna6rx5ajqRq3m4nLr8jRy+-RjCuTsSYCnjNhj6Q0F0LJtQYQ@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.00.1602261411591.15885@tp.orcam.me.uk>

On 26 February 2016 at 15:18, Maciej W. Rozycki <macro@imgtec.com> wrote:
> On Fri, 26 Feb 2016, Rafał Miłecki wrote:
>
>> >> KSEG0ADDR was translating 0x1c000000 into 0x9c000000. With
>> >> ioremap_cachable we use MIPS's __ioremap (and remap_area_pages). This
>> >> results in different address (e.g. 0xc0080000) but it still should be
>> >> cached as expected and it was successfully tested with BCM47186B0.
>> >
>> >  This is due to this piece:
>> >
>> >         /*
>> >          * Map uncached objects in the low 512mb of address space using KSEG1,
>> >          * otherwise map using page tables.
>> >          */
>> >         if (IS_LOW512(phys_addr) && IS_LOW512(last_addr) &&
>> >             flags == _CACHE_UNCACHED)
>> >                 return (void __iomem *) CKSEG1ADDR(phys_addr);
>> >
>> > special-casing uncached mapping only (replicated in 2 places).  I think
>> > there will really be no harm from returning a KSEG0 mapping for calls
>> > requesting a caching mode equal to `_page_cachable_default', which --
>> > depending on the cache architecture -- will have been either hardwired or
>> > prearranged via Config.K0.  I think there's really no need to put pressure
>> > on the TLB, which may be small, in cases where a fixed mapping will do.
>>
>> No, it isn't hitting condition you pointed. We call ioremap_cachable
>> which uses _page_cachable_default as a flag. This flag
>> (_page_cachable_default) isn't equal to the _CACHE_UNCACHED.
>
>  That's exactly what I wrote: code I quoted is "special-casing uncached
> mapping only" -- which as you have correctly observed does not apply after
> your change anymore.  Which is why previously you got an address in the
> unmapped KSEG1 segment and now you get an address in the mapped KSEG2
> rather than the unmapped KSEG0 segment.
>
>> Moreover code you pointed uses CKSEG1ADDR which would result in
>> setting bit KSEG1 (0xa0000000). As I pointed in the commit message
>> address it ORed with KSEG2 (0xc0000000).
>
>  It's not merely ORed, it's actually mapped via the TLB.
>
>  I hope this makes things clear.

Ah, sorry, I missed the point of your explanation. Now it's clear, thanks!

-- 
Rafał