Hi,

I foward this here because it could interest some people caring about
low latency. I have got no reply from upstream.
It may or not affect preempt-rt kernel.

For info the issue was found with ltt.


Matthieu

PS :
the sdhci driver contains others mdelay that should be killed.

-------- Message original --------
Sujet : Re: sdhci can turn off irq up to 200 ms
Date : Thu, 09 Jul 2009 12:28:01 +0200
De : Matthieu CASTET <matthieu.castet@parrot.com>
Pour : pierre@ossman.eu
Copie à : sdhci-devel@lists.ossman.eu,  "linux-kernel@vger.kernel.org"
<linux-kernel@vger.kernel.org>
Références : <4A4B6184.7000903@parrot.com>

Matthieu CASTET a écrit :
> Hi,
> 
> sdhci code got tasklets (sdhci_tasklet_card and sdhci_tasklet_finish),
> that does :
> {
> spin_lock_irqsave
> 
> if (cond) {
> sdhci_reset
> sdhci_reset
> }
> 
> spin_unlock_irqrestore
> }
> 
> The problem is that sdhci_reset [1] does busy pooling on a register up
> to a timeout of 100 ms.
> That's not low latency friendly.
> 
> On our system, we saw that sdhci_reset take 1 ms. That should be because
> we enter in mdelay, even if the hardware clears the bit faster.
> I wonder why there is an mdelay(1). Using cpu_relax and
> time_is_after_jiffies should make sdhci_reset faster.
> 
In case somebody cares, here a patch that reduce on our hardware
sdhci_reset from 1 ms to 30 us.


Matthieu