linux-kernel - Re: [PATCHv3 4/5] mtd: mxc

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Message-ID: <20100624072732.GQ12115@pengutronix.de>
Date:	Thu, 24 Jun 2010 09:27:32 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	John Ogness <john.ogness@...utronix.de>
Cc:	Ivo Clarysse <ivo.clarysse@...il.com>,
	Sascha Hauer <kernel@...gutronix.de>,
	linux-arm-kernel@...ts.infradead.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv3 4/5] mtd: mxc_nand fixups

On Wed, Jun 23, 2010 at 12:10:08PM +0200, John Ogness wrote:
> On 2010-06-23, Ivo Clarysse <ivo.clarysse@...il.com> wrote:
> > But is it OK to use a regular (non-volatile) variable to communicate
> > between interrupt context and the non-interrupt context ?
> 
> In this case, yes.
> 
> > My original patch for i.MX21 used completions instead:
> >
> > http://lists.infradead.org/pipermail/linux-arm-kernel/2010-April/012694.html
> 
> Ah. It seems you've been through all this before. I wish I had noticed
> that thread before. I will need to check more carefully in the future.
> 
> Yes, your original patch achieves the exact same thing. Whether we use
> wait_event() with a flag or wait_completion() really is the same
> thing. So I guess Sascha can decide what we should do there.
> 
> What I like about your original patch is that only the i.MX21 has the
> cost of constantly enabling/disabling the irq line. It adds 5
> cpu_is_mx21() blocks to the code, but will lead to less work for the CPU
> on non-i.MX21 boards.

Ok, if it's the only way out to have 5 cpu_is_* blocks, then lets go for
it.

BTW I observed that at least on i.MX27 the latencies introduced by
waiting for an interrupt cause a significant performance drop. The
driver gets much faster when we just poll all the time. I don't know how
this affects system performance otherwise, but it may be a possibility
to drop interrupt support at least for i.MX21. I have no idea how long
the longest possible time we'd have to poll is though.

Sascha

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