[lkml]   [2012]   [May]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: [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)

    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
    [unhandled content-type:application/pgp-signature]
     \ /
      Last update: 2012-05-15 10:41    [W:0.033 / U:33.652 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site