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

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Date:	Sun, 26 Oct 2014 17:00:25 +0200
From:	Moshe Lazer <moshel@....mellanox.co.il>
To:	David Miller <davem@...emloft.net>
CC:	ogerlitz@...lanox.com, jackm@....mellanox.co.il,
	talal@...lanox.com, yevgenyp@...lanox.com, netdev@...r.kernel.org,
	amirv@...lanox.com, moshel@...lanox.com
Subject: Re: [PATCH V1 net-next 1/2] pgtable: Add API to query if write combining
 is available


On 10/12/2014 12:54 PM, Moshe Lazer wrote:
>
> On 10/8/2014 7:24 PM, David Miller wrote:
>> 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.
> Thanks Dave, I'll try to clarify my point.
> For me the writecombine_available() is a way to know if the 
> pgprot_writecombine() is effective or just cover call to the 
> pgprot_noncached().
> I want to use the writecombine_available() regardless to the mapping 
> address.
> For example in mlx4 query_device I want to indicate that blue-flame is 
> not supported if  `writecombine_available() == false`.
> In this case we don't have the mapping address yet.
>
> Later on if an arch has write-combining (writecombine_available() ==  
> true) when we try to map the blue-flame registers (in mlx4_ib_mmap):
>
>     vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>
>     if (io_remap_pfn_range(vma, vma->vm_start,
>         to_mucontext(context)->uar.pfn +
>         dev->dev->caps.num_uars,
>         PAGE_SIZE, vma->vm_page_prot))
>             return -EAGAIN;
>
> I can be sure that pgprot_writecombine() is not a cover for 
> pgprot_noncached().
> The address checks that you mentioned should be part of 
> io_remap_pfn_range, this function should fail if the vma->vm_start is 
> not compatible to the vma->vm_page_prot.
> Please let me know if I misunderstood something.
>
Dave, is this clears my point or I still missing something?

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