[PATCH] MTD: LPC32xx SLC NAND driver

? 2012?05?15? 15:55, Artem Bityutskiy ??:

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:

quoted

+       /*
+        * The DMA is finished, but the NAND controller may still have
+        * buffered data. Wait until all the data is sent.

When all the data is sent, is there an interrupt for this?


Best Regards
Huang Shijie

quoted

+        */
+       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.
"