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

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Date:	Sun, 6 Sep 2009 15:25:37 +0200
From:	Pierre Ossman <pierre@...man.eu>
To:	Matthieu CASTET <matthieu.castet@...rot.com>
Cc:	sdhci-devel@...ts.ossman.eu,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: sdhci can turn off irq up to 200 ms

On Thu, 9 Jul 2009 12:28:01 +0200
Matthieu CASTET <matthieu.castet@...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.
> 

I seem to recall having problems with jiffies not updating with those
locks held (or perhaps it was when inside the isr).

What arch have you been testing this on?

Rgds
-- 
     -- Pierre Ossman

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