linux-kernel - Re: sdhci can turn off irq up to 200 ms

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Date:	Mon, 7 Sep 2009 12:47:41 -0700 (PDT)
From:	Daniel J Blueman <daniel.blueman@...il.com>
To:	Matthieu CASTET <matthieu.castet@...rot.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: sdhci can turn off irq up to 200 ms

On Jul 9, 11:30 am, Matthieu CASTET <matthieu.cas...@...rot.com>
wrote:
> 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.

On my Core2Duo, cpu_relax (implementing rep;nop) takes 3.2ns, but a
(synchronous) read over the PCI bus takes 0.5-1us, so it's hard to say
how much benefit the cpu_relax call will give, or am I missing
something?

If the code is reading from a memory location, or perhaps writing to
non-writethrough memory, it's a different story.

Daniel
--
Daniel J Blueman
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