linux-kernel - Re: [PATCH] X86: MM: Add PAT Type write-through in combination with mtrr

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Date:	Sun, 27 Oct 2013 18:56:08 +0100
From:	Andreas Werner <wernerandy@....de>
To:	Borislav Petkov <bp@...en8.de>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, dave@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] X86: MM: Add PAT Type write-through in combination with
 mtrr

On Sun, Oct 27, 2013 at 06:31:31PM +0100, Borislav Petkov wrote:
> On Sun, Oct 27, 2013 at 05:51:59PM +0100, Andreas Werner wrote:
> > 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 buffers are not
> > located in the main memory. They are located in the FPGA internal RAM.
> >
> > To transmit or receive a frame, i have to read or write to mmio to get
> > the data.
> >
> > Intel has introduced the instruction "clflush" which can flush a cache
> > line. I want to use the caches for those mmio (eth buffer) to speed
> > up the transmit/receive and to transmit/receive using PCIe bursts
> > (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.
> 
> But allocating a WB region and calling CLFLUSH right after writing
> into it sounds like you want to allocate an UC region, no? Writing
> into it will make sure the data has reached memory and is not in the
> cache, basically what CLFLUSH does but by having it UC, this happens
> automatically.
> 
> So basically what ioremap_nocache does.
> 
> > I found this articel 
> > http://software.intel.com/en-us/forums/topic/393070
> >
> > After that i configured the transmit buffers to be Write-Combining
> > (only write to that adresses) using ioremap_wc, and the receive
> > buffers to be Write-Through (ioremap_cache + mtrr Write-Through + this
> > kernel patch) everything worked as expected.
> 
> Right, but this all sounds like you want to use ioremap_nocache which
> makes your buffers UC.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> Sent from a fat crate under my desk. Formatting is fine.
> --

Maybe you missunderstood me.

My configuration is:

Transmit Buffers WC (only write to that buffer)
i have PICe bursts on my tracer.

Receive Buffers WT (only read to that buffer).
I use clflush_cache_range before reading from that
adresses and i have PCIe bursts on my tracer.

With UC memory there are no PCIe bursts and my bandwidth
is very slow.

Best regards
	Andy
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