netdev - Re: [PATCH V1 net-next 1/2] pgtable: Add API to query if write combining is available

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Date:	Wed, 08 Oct 2014 12:24:50 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	moshel@....mellanox.co.il
Cc:	ogerlitz@...lanox.com, jackm@....mellanox.co.il,
	talal@...lanox.com, yevgenyp@...lanox.com, netdev@...r.kernel.org,
	amirv@...lanox.com
Subject: Re: [PATCH V1 net-next 1/2] pgtable: Add API to query if write
 combining is available

From: Moshe Lazer <moshel@....mellanox.co.il>
Date: Wed, 08 Oct 2014 11:44:57 +0300

>> #if defined(__i386__) || defined(__x86_64__)
>> 	if (map->type == _DRM_REGISTERS && !(map->flags & _DRM_WRITE_COMBINING))
>> 		tmp = pgprot_noncached(tmp);
>> 	else
>> 		tmp = pgprot_writecombine(tmp);
>> #elif defined(__powerpc__)
>> 	pgprot_val(tmp) |= _PAGE_NO_CACHE;
>> 	if (map->type == _DRM_REGISTERS)
>> 		pgprot_val(tmp) |= _PAGE_GUARDED;
>> #elif defined(__ia64__)
>> 	if (efi_range_is_wc(vma->vm_start, vma->vm_end -
>> 				    vma->vm_start))
>> 		tmp = pgprot_writecombine(tmp);
>> 	else
>> 		tmp = pgprot_noncached(tmp);
>> #elif defined(__sparc__) || defined(__arm__) || defined(__mips__)
>> 	tmp = pgprot_noncached(tmp);
>> #endif
> The idea was to provide an indication as for whether the arch supports
> write-combining in general.
> If we want to benefit from blue flame operations, we need to map the
> blue flame registers as write combining - otherwise there is no
> benefit. So we would like to know if write combining is supported by
> the system or not.
> 

You completely miss my point.  On a given architectuire it might be
_illegal_ to map certain address ranges as write-combining without
checks like the ones above that ia64 needs.

Therefore your proposed interface is by definition insufficient.
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