linux-kernel - Re: [PATCH v3] ARM: early fixmap support for earlycon

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Date:	Sat, 06 Jun 2015 15:29:02 +0200
From:	Stefan Agner <stefan@...er.ch>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Mark Salter <msalter@...hat.com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v3] ARM: early fixmap support for earlycon

On 2015-06-06 14:48, Russell King - ARM Linux wrote:
> On Sat, Jun 06, 2015 at 02:31:28PM +0200, Stefan Agner wrote:
<snip>
>> --- a/arch/arm/include/asm/fixmap.h
>> +++ b/arch/arm/include/asm/fixmap.h
>> @@ -6,9 +6,13 @@
>>  #define FIXADDR_TOP		(FIXADDR_END - PAGE_SIZE)
>>
>>  #include <asm/kmap_types.h>
>> +#include <asm/pgtable.h>
>>
>>  enum fixed_addresses {
>> -	FIX_KMAP_BEGIN,
>> +	FIX_EARLYCON_MEM_BASE,
>> +	__end_of_permanent_fixed_addresses,
>> +
>> +	FIX_KMAP_BEGIN = __end_of_permanent_fixed_addresses,
>>  	FIX_KMAP_END = FIX_KMAP_BEGIN + (KM_TYPE_NR * NR_CPUS) - 1,
>>
>>  	/* Support writing RO kernel text via kprobes, jump labels, etc. */
>> @@ -18,7 +22,14 @@ enum fixed_addresses {
>>  	__end_of_fixed_addresses
>>  };
>>
>> +#define FIXMAP_PAGE_COMMON	(L_PTE_YOUNG | L_PTE_PRESENT | L_PTE_XN | L_PTE_DIRTY)
>> +
>> +#define FIXMAP_PAGE_NORMAL	(FIXMAP_PAGE_COMMON | L_PTE_MT_WRITEBACK)
>> +#define FIXMAP_PAGE_IO		(FIXMAP_PAGE_COMMON | L_PTE_MT_DEV_SHARED | L_PTE_SHARED)
>> +#define FIXMAP_PAGE_NOCACHE	FIXMAP_PAGE_IO
> 
> I'm really not happy with this.  What are the expected semantics of the
> set_fixmap_nocache() and set_fixmap_offset_nocache() ?  Are they there
> for mapping a device, or are they there for mapping _memory_ ?
> 
> I would prefer that FIXMAP_PAGE_NOCACHE is _not_ provided until the
> semantics of that can be clarified, as there is a difference between the
> two on ARM.

set_fixmap_nocache is used for PCIe and IO APIC stuff, so actually IO
devices. However, as far as I understand the x86 page table definitions
in pgtable_types.h, IO devices are not explicitly uncached on x86. The
define set_fixmap_nocache make sure IO devices are mapped uncached...
Maybe set_fixmap_io_nocache instead of set_fixmap_nocache would be more
appropriate...? 

Maybe the initial authors of the common fixmap.h have a more clear
understanding of the expected semantics, Mark?

Since the focus currently is on set_fixmap_io, we could define
FIXMAP_PAGE_IO only for the time being...?

Agreed to the rest, will apply the review in v4.

--
Stefan

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