Re: [PATCH] MTD: LPC32xx SLC NAND driver

From: Artem Bityutskiy
Date: Tue May 15 2012 - 03:52:13 EST

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.
> + */
> + 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)

if (!timeout)

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)

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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