linux-kernel - Re: [patch 2/3] s3cmci - call pio

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Message-ID: <48C53107.7090103@weinigel.se>
Date:	Mon, 08 Sep 2008 16:04:55 +0200
From:	Christer Weinigel <christer@...nigel.se>
To:	Ben Dooks <ben-linux@...ff.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/3] s3cmci - call pio_tasklet from IRQ

Ben Dooks wrote:
> On Mon, Sep 08, 2008 at 02:48:50PM +0200, Christer Weinigel wrote:
>> Scheduling a tasklet to perform the pio transfer introduces a bit of
>> extra processing, just call pio_tasklet directly from the interrupt
>> instead.  Writing up to 64 bytes to a FIFO is probably uses less CPU
>> than scheduling a tasklet anyway.
> 
> Hmm, i'd be interested to find out how long these are taking... I might
> try and rig up something to test the time being taken via an SMDK.
> 
> If the fifo read/writes are taking significant amounts of time, then the
> pio tasklet will at least improve the interrupt latencies invloved, as
> iirc we're currently running the main irq handler in IRQ_DISABLED mode
> to stop any problems with re-enternancy.... I'll check this and see what
> is going on.

It should be possible to set a flag and then call the pio_task after the 
spin_unlock_irqrestore instead.  I didn't want to do that to change as 
little of the logic as possible, but it's probably better to do that.

I'm also thinking of changing send_request to do a busy wait for 
commands without data, that will probably need a bit larger changes.

   /Christer
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