Re: netpoll/netconsole minor tweaks

[Jesper Juhl]

On Thu, 18 Sep 2003, Alan Cox wrote:

> On Iau, 2003-09-18 at 17:48, Chris Wright wrote:
> > > <pedant>
> > > should be cpu_relax();
> > > </pedant>
> >
> > Hrm, there's many spots that aren't using it.  What's the benefit, less
> > power consumption?  Is it worth a patch to convert other things over?
>
> It speeds up hyperthreading CPUs by letting them know that this
> paticular thread is waiting for stuff (sched_yield for silicon)
>

Does that mean that it would be benneficial to do something like this in
for example eepro100.c ??

diff -up linux-2.6.0-test5-orig/drivers/net/eepro100.c
linux-2.6.0-test5/drivers/net/eepro100.c
--- linux-2.6.0-test5-orig/drivers/net/eepro100.c       2003-09-08 21:50:09.000000000 +0200
+++ linux-2.6.0-test5/drivers/net/eepro100.c    2003-09-19 01:03:19.000000000 +0200
@@ -913,9 +913,11 @@ static void do_slow_command(struct net_d

        for (wait = 0; wait <= 100; wait++)
                if (inb(cmd_ioaddr) == 0) return;
-       for (; wait <= 20000; wait++)
+       for (; wait <= 20000; wait++) {
                if (inb(cmd_ioaddr) == 0) return;
                else udelay(1);
+               cpu_relax();
+       }
        printk(KERN_ERR "Command %4.4x was not accepted after %d polls!"
               "  Current status %8.8x.\n",
               cmd, wait, inl(dev->base_addr + SCBStatus));



or maybe even take it to the extreme like


diff -up linux-2.6.0-test5-orig/drivers/net/eepro100.c
linux-2.6.0-test5/drivers/net/eepro100.c
--- linux-2.6.0-test5-orig/drivers/net/eepro100.c       2003-09-08 21:50:09.000000000 +0200
+++ linux-2.6.0-test5/drivers/net/eepro100.c    2003-09-19 01:07:50.000000000 +0200
@@ -902,20 +902,25 @@ static void do_slow_command(struct net_d
 {
        long cmd_ioaddr = dev->base_addr + SCBCmd;
        int wait = 0;
-       do
+       do {
                if (inb(cmd_ioaddr) == 0) break;
-       while(++wait <= 200);
+               cpu_relax();
+       } while(++wait <= 200);
        if (wait > 100)
                printk(KERN_ERR "Command %4.4x never accepted (%d
polls)!\n",
                       inb(cmd_ioaddr), wait);

        outb(cmd, cmd_ioaddr);

-       for (wait = 0; wait <= 100; wait++)
+       for (wait = 0; wait <= 100; wait++) {
                if (inb(cmd_ioaddr) == 0) return;
-       for (; wait <= 20000; wait++)
+               cpu_relax();
+       }
+       for (; wait <= 20000; wait++) {
                if (inb(cmd_ioaddr) == 0) return;
                else udelay(1);
+               cpu_relax();
+       }
        printk(KERN_ERR "Command %4.4x was not accepted after %d polls!"
               "  Current status %8.8x.\n",
               cmd, wait, inl(dev->base_addr + SCBStatus));


How short/long waits are we talking about before it is benneficial to be
calling cpu_relax() ?


Kind regards,

Jesper Juhl <jju@xxxxxx>
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