linux-kernel - Re: [PATCH] mtd: bcm47xxsflash: use ioremap

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Date:	Fri, 26 Feb 2016 11:41:39 +0100
From:	Rafał Miłecki <zajec5@...il.com>
To:	"Maciej W. Rozycki" <macro@...tec.com>
Cc:	Brian Norris <computersforpeace@...il.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	Javier Martinez Canillas <javier@....samsung.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Luis de Bethencourt <luisbg@....samsung.com>,
	Jeremy Kerr <jk@...abs.org>,
	Neelesh Gupta <neelegup@...ux.vnet.ibm.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Cyril Bur <cyrilbur@...il.com>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH] mtd: bcm47xxsflash: use ioremap_cachable() instead of KSEG0ADDR()

On 24 January 2016 at 21:17, Maciej W. Rozycki <macro@...tec.com> wrote:
> On Sun, 17 Jan 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.

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).

So what really happens is what my commit message says:
ioremap_cachable -> __ioremap -> remap_area_pages