Re: do_div vs sector_t (patch)

[Nick Piggin <piggin@cyberone.com.au>]

[-- Attachment #1: Type: text/plain, Size: 1647 bytes --]



Neil Brown wrote:

>On Friday July 11, willy@debian.org wrote:
>
>># define do_div(n,base) ({                              \
>>        uint32_t __base = (base);                       \
>>        uint32_t __rem;                                 \
>>        if (likely(((n) >> 32) == 0)) {                 \
>>
>>so if we call do_div() on a u32, the compiler emits nasal daemons.
>>and we do this -- in the antcipatory scheduler:
>>
>>                if (aic->seek_samples) {
>>                        aic->seek_mean = aic->seek_total + 128;
>>                        do_div(aic->seek_mean, aic->seek_samples);
>>                }
>>
>>seek_mean is a sector_t so sometimes it's 64-bit on a 32-bit platform.
>>so we can't avoid calling do_div().
>>
>>This almost works (the warning is harmless since gcc optimises away the call)
>>
>># define do_div(n,base) ({                                              \
>>        uint32_t __base = (base);                                       \
>>        uint32_t __rem;                                                 \
>>        if ((sizeof(n) < 8) || (likely(((n) >> 32) == 0))) {            \
>>                __rem = (uint32_t)(n) % __base;                         \
>>                (n) = (uint32_t)(n) / __base;                           \
>>        } else                                                          \
>>                __rem = __div64_32(&(n), __base);                       \
>>        __rem;                                                          \
>> })
>>
>>Better ideas?
>>
>
>sector_div, defined in blkdev.h, is probably what you want.
>

Looks right. Following patch alright?


[-- Attachment #2: as-do-div --]
[-- Type: text/plain, Size: 510 bytes --]

--- linux-2.5/drivers/block/as-iosched.c.orig	2003-07-12 10:12:20.000000000 +1000
+++ linux-2.5/drivers/block/as-iosched.c	2003-07-12 10:12:28.000000000 +1000
@@ -837,7 +837,7 @@ static void as_update_iohist(struct as_i
 		aic->seek_total += 256*seek_dist;
 		if (aic->seek_samples) {
 			aic->seek_mean = aic->seek_total + 128;
-			do_div(aic->seek_mean, aic->seek_samples);
+			sector_div(aic->seek_mean, aic->seek_samples);
 		}
 		aic->seek_samples = (aic->seek_samples>>1)
 					+ (aic->seek_samples>>2);