Write-combining

Write-combining

[SA products]

Dear list,

Sorry this maybe off topic-- I am writing a device driver for a kind of
framebuffer device.
This is virtually complete and working, all I wish to add is
write-combining but I can find
very little information on how to do this.  Looking at other device
drivers MTRR crops
up a few times but I is still not clear to me what I have to do.

So far-
I grab the memory region for the framebuffer (1/4Mb) something like so;
......

memio=ioremap(pci_resource_start(dev,MEMIO)&PCI_BASE_ADDRESS_MEM_MASK,MEMIO_SIZE);

......
then try to mtrr it
.....
    err=mtrr_add(memio,MEMIO_SIZE,MTRR_TYPE_WRCOMB,1);
......
which fails because memio is not aligned correctly

my code generates the following messages
slm: init: mtrr option enabled- trying region cc960000
mtrr: base(0xcc960000) is not aligned on a size(0x40000) boundary
slm: init: mtrr: unable to set write combining for slm memory :(

Before I go any further I would like to ask is this a sensible approach
or have I missed
something somewhere?

If this is not the most sensible way please point me towards a sensible
solution-

If this is the correct way of doing things is there
> an easy way to discover the mtrr alignment requirements prior to
mtrr_add?
> Can I align memio properly (how?) or
>   do I start by defining a larger mtrr region below memio and then
masking off the
    unwanted regions either side of my memory region with
mtrr(...MTRR_TYPE_WRBACK..)?
> What happens on none x86 type processors?

any other advice?


Thanks SA

Re: Write-combining

[SA]

Have I missed something here?

Am I being dim?

Or is the usual "instant advice" feature switched off?


The device I am writing the driver for formally didn't support
write-combining and it took
10000 clock ticks to do a partial transfer- now it does support
write-combining and it still
takes 10000 clock ticks to transfer the data- It is performance
critical- How do I switch
write-combining on? It is worth at least a 2x speed up for me?

If it is something obvious please don't be afraid to point it out-----

Thanks SA

Write-combining

[Thiago Robert]

Hello there!

Is there an easy way to be sure about wether the PCI write-combining is 
enabled or not for a given memory region?

I'm writting user-level communication software for a Myrinet network. I 
use Write PIO and I'm aware that WPIO performance is greatly influencied 
by the presence of write combining. Based on the performance I measured 
I can only conclude that Write Combining is enabled but I would like to 
be sure about it (since I'm planning to publish my research).

Is the default behaviour of the Linux kernel to enable write-combining? 
How can I be sure if it is enabled or not?

Thanks in advance.

__________________________
Thiago Robert

Re: Write-combining

[Sean Neakums]

Thiago Robert <robert@inf.ufsc.br> writes:

> Is the default behaviour of the Linux kernel to enable
> write-combining? How can I be sure if it is enabled or not?

My /proc/mtrr lists the following region:

reg03: base=0xf8000000 (3968MB), size=  64MB: write-combining, count=2

which I am guessing is the PCI space, although I'm not certain.

Re: Write-combining

[Thiago Robert]

Is anyone certain about this?

Thanks in advance.

_________________________
Thiago Robert


Sean Neakums wrote:

>Thiago Robert <robert@inf.ufsc.br> writes:
>
>  
>
>>Is the default behaviour of the Linux kernel to enable
>>write-combining? How can I be sure if it is enabled or not?
>>    
>>
>
>My /proc/mtrr lists the following region:
>
>reg03: base=0xf8000000 (3968MB), size=  64MB: write-combining, count=2
>
>which I am guessing is the PCI space, although I'm not certain.
>-
>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>Please read the FAQ at  http://www.tux.org/lkml/
>
>  
>

RE: Write-combining

[Venkatesan, Ganesh]

lspci -vvv for your device should display the device specific BARs. If
you see the corresponding BAR in /proc/mtrr display it means that it is
MTRR mapped device memory.

Thanks,
ganesh 
 

-----Original Message-----
From: linux-kernel-owner@vger.kernel.org
[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Thiago Robert
Sent: Tuesday, May 11, 2004 11:43 AM
To: Sean Neakums
Cc: linux-kernel@vger.kernel.org
Subject: Re: Write-combining

Is anyone certain about this?

Thanks in advance.

_________________________
Thiago Robert


Sean Neakums wrote:

>Thiago Robert <robert@inf.ufsc.br> writes:
>
>  
>
>>Is the default behaviour of the Linux kernel to enable
>>write-combining? How can I be sure if it is enabled or not?
>>    
>>
>
>My /proc/mtrr lists the following region:
>
>reg03: base=0xf8000000 (3968MB), size=  64MB: write-combining, count=2
>
>which I am guessing is the PCI space, although I'm not certain.
>-
>To unsubscribe from this list: send the line "unsubscribe linux-kernel"
in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>Please read the FAQ at  http://www.tux.org/lkml/
>
>  
>

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/