linux-kernel - Re: [ARM]Fixmap mapping region is not enough for system of 14+ CPUs.

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Date:	Wed, 12 Mar 2014 13:08:42 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Liu hua <sdu.liu@...wei.com>
cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>, msalter@...hat.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Wang Nan <wangnan0@...wei.com>, peifeiyue@...wei.com,
	wangkefeng.wang@...wei.com, Li Zefan <lizefan@...wei.com>
Subject: Re: [ARM]Fixmap mapping region is not enough for system of 14+
 CPUs.

On Wed, 12 Mar 2014, Liu hua wrote:

> Hi Russell, Will or Nicolas,
> 
> (In this mail, we only discuss ARM 32-bit linux.)
> 
> As we know, the region (0xfff00000-0xfffdffff) is reserved as fixmap
> mapping region.
> 
> The function "kmap_atomic" maps highmem pages to this region referring
> to CPUID and per-cpu variable "__kmap_atomic_idx" via
> 
>   idx = type + KM_TYPE_NR * smp_processor_id();
>   vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
> 
> Size of region used by one cpu is 0x10000 (KM_TYPE_NR << PAGE_SHIFT).
> And the total size of the fixmap mapping region is 0xe0000.
> (only support 14 CPUs).
> 
> So in a system of more than 14 CPUs, this region is not large enough.
> should we change the memory layout on ARM Linux to support 14+ cpu system ?
> Or can we do anything else to support that ?

How many CPUs do you have?

What about the following patch?  If this doesn't work for you then more 
intrusive changes will be needed.

diff --git a/arch/arm/include/asm/fixmap.h b/arch/arm/include/asm/fixmap.h
index 68ea615c2a..254f2df08d 100644
--- a/arch/arm/include/asm/fixmap.h
+++ b/arch/arm/include/asm/fixmap.h
@@ -14,7 +14,17 @@
  */
 
 #define FIXADDR_START		0xfff00000UL
+
+#if !defined(CONFIG_HAVE_TCM) && !defined(CONFIG_CPU_XSCALE)
+/*
+ * If no TCM nor on on a XScale then enlarge the fixmap area to
+ * accommodate up to 30 CPUs.
+ */
+#define FIXADDR_END		0xffff0000UL
+#else
 #define FIXADDR_END		0xfffe0000UL
+#endif
+
 #define FIXADDR_TOP		(FIXADDR_END - PAGE_SIZE)
 
 enum fixed_addresses {


Nicolas
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