linux-kernel - Re: [PATCH] MTD: LPC32xx SLC NAND driver

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Date:	Tue, 15 May 2012 10:55:43 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Roland Stigge <stigge@...com.de>,
	Bastian Hecht <hechtb@...glemail.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Huang Shijie <b32955@...escale.com>,
	Lei Wen <leiwen@...vell.com>
Cc:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	dwmw2@...radead.org, kevin.wells@....com, srinivas.bakki@....com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] MTD: LPC32xx SLC NAND driver

I am CCing few other guys who take care of several drivers which use
similar way of busy-waiting - probably you could change it?

Bastian: drivers/mtd/nand/sh_flctl.c
Lars-Peter: drivers/mtd/nand/jz4740_nand.c
Huang: drivers/mtd/nand/gpmi-nand/gpmi-lib.c
Lei Wen: drivers/mtd/nand/pxa3xx_nand.c

On Sat, 2012-05-12 at 15:29 +0200, Roland Stigge wrote:
> +       /*
> +        * The DMA is finished, but the NAND controller may still have
> +        * buffered data. Wait until all the data is sent.
> +        */
> +       timeout = LPC32XX_DMA_SIMPLE_TIMEOUT;
> +       while ((readl(SLC_STAT(host->io_base)) & SLCSTAT_DMA_FIFO)
> +              && (timeout > 0))
> +               timeout--;
> +       if (!timeout) {
> +               dev_err(mtd->dev.parent, "FIFO held data too long\n");
> +               status = -EIO;
> +       } 

I know the MTD tree is full of this, but this is bad, I think. The
timeout should be time-backed, not CPU-cycles-backed.

I do not know the best way to do this, hopefully someone in the arm list
could suggest, but the following pattern is at least better:

/* Chip reaction time timeout in milliseconds */
#define LPC32XX_DMA_TIMEOUT 100

timeout = loops_per_jiffy * msecs_to_jiffies(LPC32XX_DMA_TIMEOUT);

while ((readl(...)) && timeout-- > 0)
	cpu_relax();

if (!timeout)
	error;

So basically I turned your hard-coded iterations count into a time-based
timeout. I also used cpu_relax() which is commonly used in tight-loops
like this. Here is a piece of documentation about cpu_relax():

"
The right way to perform a busy wait is:

    while (my_variable != what_i_want)
        cpu_relax();

The cpu_relax() call can lower CPU power consumption or yield to a
hyperthreaded twin processor; it also happens to serve as a compiler
barrier, so, once again, volatile is unnecessary.  Of course, busy-
waiting is generally an anti-social act to begin with.
"

-- 
Best Regards,
Artem Bityutskiy

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