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Re: [PATCH] Make c0_compare_int_usable faster

To: ralf@linux-mips.org
Subject: Re: [PATCH] Make c0_compare_int_usable faster
From: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Date: Tue, 23 Oct 2007 21:55:42 +0900 (JST)
Cc: linux-mips@linux-mips.org
In-reply-to: <20071022210400.GB20038@linux-mips.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20071023.011416.61947729.anemo@mba.ocn.ne.jp> <20071022210400.GB20038@linux-mips.org>
Sender: linux-mips-bounce@linux-mips.org
On Mon, 22 Oct 2007 22:04:00 +0100, Ralf Baechle <ralf@linux-mips.org> wrote:
> > Use delta value based on its speed for faster probing.
> 
> Still the same issue, it breaks with Qemu.  You probably don't see this
> if you're testing on a desktop where the TSC is used for timing but on
> a laptop it breaks big time.  I need to increase the shift value to at
> least like 15 to get it to work more or less reliably with Qemu, so a
> somewhat different approach is needed.

OK, Here is the different approach.

---
Subject: [PATCH] Make c0_compare_int_usable faster (take 2)

Try to increase delta value step by step until we can make sure the
counter is not expired.

 arch/mips/kernel/cevt-r4k.c |   14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/mips/kernel/cevt-r4k.c b/arch/mips/kernel/cevt-r4k.c
index 47c8c0e..fa3b9b2 100644
--- a/arch/mips/kernel/cevt-r4k.c
+++ b/arch/mips/kernel/cevt-r4k.c
@@ -179,7 +179,7 @@ static int c0_compare_int_pending(void)
 
 static int c0_compare_int_usable(void)
 {
-       const unsigned int delta = 0x300000;
+       unsigned int delta;
        unsigned int cnt;
 
        /*
@@ -192,9 +192,15 @@ static int c0_compare_int_usable(void)
                        return 0;
        }
 
-       cnt = read_c0_count();
-       cnt += delta;
-       write_c0_compare(cnt);
+       for (delta = 0x10; delta <= 0x400000; delta <<= 1) {
+               cnt = read_c0_count();
+               cnt += delta;
+               write_c0_compare(cnt);
+               irq_disable_hazard();
+               if ((int)(read_c0_count() - cnt) < 0)
+                   break;
+               /* increase delta if the timer was already expired */
+       }
 
        while ((int)(read_c0_count() - cnt) <= 0)
                ;       /* Wait for expiry  */


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