linux-kernel - Re: Posted Write onto a PCI end point device using Linux

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Message-ID: <AANLkTikx314joTYUBeEequWCbiv7rAdSeZaUv1zn=Ags@mail.gmail.com>
Date:	Wed, 8 Dec 2010 09:23:04 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Sanka Piyaratna <cesanka@...oo.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Posted Write onto a PCI end point device using Linux

On Wed, Dec 8, 2010 at 12:05 AM, Sanka Piyaratna <cesanka@...oo.com> wrote:
> Thanks for that Robert. When you say DMA, this has to be implemented at the
> endpoint device doesn't it? There is no DMA mode on the CPU for doing this is
> there?

No, there's not (on typical hardware, anyway).

>
>  Sanka Piyaratna
>
>
>
>
> ________________________________
> From: Robert Hancock <hancockrwd@...il.com>
> To: Sanka Piyaratna <cesanka@...oo.com>
> Cc: linux-kernel@...r.kernel.org
> Sent: Wed, 8 December, 2010 3:25:47 PM
> Subject: Re: Posted Write onto a PCI end point device using Linux
>
> On Tue, Dec 7, 2010 at 9:57 PM, Sanka Piyaratna <cesanka@...oo.com> wrote:
>> Hi Robert,
>> Could you please tell me how I can initiate a posted write towards a PCIe
>> endpoint with larger than 32 bit payload using Linux?
>> Thanks and regards,
>
> You can try writeq to get a 64-bit payload, but I don't think it's
> available on all platforms. In particular with a 32-bit CPU, it may
> not be possible at all. If you need to send bigger payloads then you
> need to use DMA.
>
>>
>> Sanka Piyaratna
>>
>> ________________________________
>> From: Robert Hancock <hancockrwd@...il.com>
>> To: Sanka Piyaratna <cesanka@...oo.com>
>> Cc: linux-kernel@...r.kernel.org
>> Sent: Wed, 8 December, 2010 12:16:54 PM
>> Subject: Re: Posted Write onto a PCI end point device using Linux
>>
>> On 12/07/2010 05:29 AM, Sanka Piyaratna wrote:
>>> Hi,
>>>
>>>
>>> I am wondering whether it is possible to issue a posted write using the
>>> host
>>> computer's root complex towards an PCI express end point. I have done this
>>> using
>>>
>>> DMA in the past. However, if posted writes are possible that would save a
>>> lot of
>>>
>>> time and effort and would be quicker too.
>>
>> Writes from the CPU to PCI memory are always posted. However, the posted
>> write buffer size is usually quite limited and so this probably will
>> still cause CPU overhead when writing any significant amount of data
>> this way.
>>
>>
>
>
>
>
>
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