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

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Date:	Tue, 15 May 2012 16:15:22 +0800
From:	Huang Shijie <b32955@...escale.com>
To:	<dedekind1@...il.com>
CC:	Roland Stigge <stigge@...com.de>,
	Bastian Hecht <hechtb@...glemail.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Lei Wen <leiwen@...vell.com>, <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

于 2012年05月15日 15:55, Artem Bityutskiy 写道:
> 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.
When all the data is sent, is there an interrupt for this?


Best Regards
Huang Shijie

>> +        */
>> +       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.
> "
>


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