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

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Date:	Thu, 9 Jul 2009 12:28:01 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	<pierre@...man.eu>
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

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

View attachment "sdhci.diff" of type "text/x-diff" (764 bytes)