Message-ID: <trinity-899f1812-2208-4b01-ab7e-05b7e4304ac6-1377281890307@3capp-gmx-bs52>
Date:	Fri, 23 Aug 2013 20:18:10 +0200 (CEST)
From:	"Andreas Werner" <wernerandy@....de>
To:	"Andy Lutomirski" <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Re: Re: question about ioremap_cache and PAT

Hi,

why are you curious?

I have never heard about movntdqa. Have you ever tried it?
May be it is a good idea to try i out.

I think i will commit the patch to the kernel and see what happens :-)

Best regards.
>> On Fri, Aug 23, 2013 at 9:59 AM, Andreas Werner <wernerandy@....de>
>> wrote:
>>>
>>>
>>> Hi,
>>> thank you for your answer.
>>>
>>> So we are two persons for now who need WT :-)
>>>
>>> Im currently working on an ethernet driver for our own ETH core.
>>> The problem is that one requirement is to not use DMA to transmit or
>>> receive the data.
>>> This means the that the ethernet buffer are not located in the main
>>> memory. They are located in
>>> the FPGA.
>>>
>>> To transmit or receive a frame, i have to read or write to mmio to get
>>> the data.
>>>
>>> Intel has introduced the command "clflush" which can flush a cache
>>> line.
>>> I wanted to activate the caches for those mmio (eth buffer) to speed up
>>> the transmit or receive.
>>> After that the transfer over PCIe uses burst read/write.
>>>
>>> The problem was if i set the buffer to Write-Back and call clflush on
>>> those mmio-addresses, the system crashed without any output.
>>> I found this articel
>>> http://software.intel.com/en-us/forums/topic/393070.[http://software.intel.com/en-us/forums/topic/393070]
>>>
>>> After that i configured the transmit buffer to be Write-Combining (only
>>> write to that adresses) using ioremap_wc, and
>>> the receive buffer to be Write-Through (ioremap_cache + mtrr Write-Back
>>> + my Kernel Hack :-)) everything worked fine.
>>> The other configuration Register on the FPGA are just mapped with
>>> ioremap.
>>
>> I'm curious: have you tried movntdqa on UC memory for this?
>> (Certainly WP or WT is easier.)
>>
>> In any case, I hope to have patches to support WP and WT without using
>> PAT reasonably soon.
>>
>>>
>>> On PCIe Tracer i can see the burst read/write on my device.
>>>
>>> Is it possible to get hits into the Kernel?
>>>
>>> My modification in arch/x86/mm/pat.c:
>>>
>>> --- pat.c.orig 2013-02-03 01:18:49.491879407 +0100
>>> +++ pat.c 2013-02-03 01:19:19.053509836 +0100
>>> @@ -149,10 +149,16 @@ static unsigned long pat_x_mtrr_type(u64
>>>    u8 mtrr_type;
>>>
>>>    mtrr_type = mtrr_type_lookup(start, end);
>>> -  if (mtrr_type != MTRR_TYPE_WRBACK)
>>> +
>>> +  if (mtrr_type == MTRR_TYPE_WRTHROUGH) {
>>> +   return _PAGE_CACHE_WB;
>>> +  }
>>> +  else if( mtrr_type == MTRR_TYPE_WRBACK )
>>> +   return _PAGE_CACHE_WB;
>>> +  else
>>>     return _PAGE_CACHE_UC_MINUS;
>>> -
>>> -  return _PAGE_CACHE_WB;
>>> +
>>>   }
>>>
>>>   return req_type;
>>>
>>
>> That seems more or less reasonable to me.  If you want it included,
>> send it to x86@...nel.org (cc lkml) and see what they say.
>>
>> It would be prettier if you combined the conditions into a single
>> if/else, though.
>>
>>>
>>> Best regards.
>>>
>>>
>>> Gesendet: Montag, 12. August 2013 um 19:53 Uhr
>>> Von: "Andy Lutomirski" <luto@...capital.net>
>>> An: "Andreas Werner" <wernerandy@....de>
>>> Cc: linux-kernel@...r.kernel.org
>>> Betreff: Re: question about ioremap_cache and PAT
>>> On 08/11/2013 09:50 AM, Andreas Werner wrote:
>>>> Hi i have a question about ioremap_cache and the resulting PAT
>>>> attribute on X86 system. If I configure the mtrr to Write-Through for
>>>> an adress range, and call ioremap_cache to map the mmio, the resulting
>>>> PAT attribute is set to UC.
>>>> If I check the Intel document IA-32 SDM vol 3a, the resulting PAT
>>>> attribute should be WB.
>>>>
>>>> I found the function pat_x_mtrr_type in arch/x86/mm/pat.c where the
>>>> resulting attribute is returned. There will be always UC return expect
>>>> if the MTRR is set to WB.
>>>>
>>>> Why is there only WB or UC returned? In the Intel document there are a
>>>> lot of combinations "allowed".
>>>>
>>>> I need a Attribute of WT, so what i did is to modify the
>>>> pat_x_mtrr_type function to return also WB if the MTRR is set to WT.
>>>>
>>>> Is this a solution to solve that or whats the reasion why the kernel
>>>> doesn´t support this combination?
>>>
>>> The kernel doesn't support it because I'm apparently the only person
>>> who
>>> ever wanted it and I haven't implemented it yet.
>>>
>>> This stuff is handled in hardware, so modifying the kernel's idea of
>>> what hardware does won't do much. Also, the kernel using MTRRs is on
>>> its (very slow) way out. You could probably hack something up, but I
>>> can almost guarantee that hpa, etc won't accept the patches.
>>>
>>> That being said, I'm planning to support WT directly using PAT in the
>>> near future. This will work on most recent cpus (there are errata that
>>> will prevent use of the high PAT entries on some cpus).
>>>
>>> What do you need WT for? I want it for NVDIMMs, and all I need to get
>>> started now is a heatsink*, so I'll hopefully start implementing this
>>> stuff in the next week or so.
>>>
>>> --Andy
>>>
>>> * Damnit, Intel, it's not 2003 any more. You already figured out that
>>> heatsinks want screw holes. But why couldn't you make sure that all
>>> so-called "LGA 2011" sockets have the screw holes in the same place?
>>>
>>>
>>>>
>>>> Best regards
>>>>
>>>> B
>>>> B
>>>> B
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>>>> A
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>>>> B
>>>> Best regards
>>>>
>>>
>>
>>
>>
>> --
>> Andy Lutomirski
>> AMA Capital Management, LLC
>>
> 
> 
> 
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