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

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Date:	Mon, 7 Sep 2009 11:56:10 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	Pierre Ossman <pierre@...man.eu>
CC:	"sdhci-devel@...ts.ossman.eu" <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

Pierre Ossman a écrit :
> 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?
It have been tested on arm.

Matthieu
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