Re: Gigabyte GA-MA69VM-S2 can't do 64bit DMA

[Tejun Heo <htejun@gmail.com>]

Huang, Shane wrote:
> Hi Tejun, 
> 
>> -----Original Message-----
>> From: Tejun Heo [mailto:htejun@gmail.com] 
>>> ahci 0000:00:12.0: version 3.0_
>>> ahci 0000:00:12.0: PCI INT A -> GSI 22 (level, low) -> IRQ 22
>>> ahci 0000:00:12.0: Fujitsu-Siemens AMILO Pa 2510: this 
>> board can't do 64bit
>>> DMA, forcing 32bit
>>> ahci 0000:00:12.0: controller can't do 64bit DMA, forcing 32bit
>>> ahci 0000:00:12.0: AHCI 0001.0100 32 slots 4 ports 3 Gbps 
>> 0xf impl SATA mode
>>> ahci 0000:00:12.0: flags: ncq sntf ilck pm led clo pmp pio slum part
>>> It's correct now.
>> Cool, patch posted with the id string shortened.
> 
> Just some small questions about this topic:
> 
> What is the actual impact of 64-bit DMA not being available?

If IO happens to memory above 4G it will either be bounced (ie. DMA
will be performed to a bounce buffer < 4G which is copied from / to
the original buffer or more likely iommu will kick in.

> How important is it to have this enabled?
>
> In other words, will the performance be improved significantly
> after it is enabled? Do you have any experimental data for it? :-)

There probably won't be any noticeable performance regression if iommu
is working properly and no, I don't have any concrete numbers.

Thanks..

-- 
tejun