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SubjectRe: [PATCH 5/6] drivers/atm: Use DIV_ROUND_UP
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In message <Pine.LNX.4.58.0802141615470.1185@pc-041.diku.dk>,Julia Lawall write
s:
>In each case below, I have followed the original semantics, but in
>drivers/atm/eni.c and drivers/atm/horizon.c, I have some doubts as to
>whether the original semantics is correct. In drivers/atm/eni.c, is the
>division intended to be by div or by div-1? In drivers/atm/horizon.c, it
>seems strange that "case round_down" is implemented by DIV_ROUND_UP, twice.
>The round_down and default (ie round_up) cases seem to be inversed.

i guess it might seem confusing. to round the cell rate down, you need
to round the clock period up for the hardware scheduler. so the up/down
refer to the atm rate, not the clock period which is what appears to be
computed in the following sections.

for example, in the eni driver, but comp_tx() is computing a clock
period. to round the desired cell rate down, the clock period is
rounded up.

>diff -u -p a/drivers/atm/eni.c b/drivers/atm/eni.c
>--- a/drivers/atm/eni.c 2007-07-20 15:28:28.000000000 +0200
>+++ b/drivers/atm/eni.c 2008-02-13 20:50:10.000000000 +0100
>@@ -1270,7 +1270,7 @@ static int comp_tx(struct eni_dev *eni_d
> if (*pre < 3) (*pre)++; /* else fail later */
> div = pre_div[*pre]*-*pcr;
> DPRINTK("max div %d\n",div);
>- *res = (TS_CLOCK+div-1)/div-1;
>+ *res = DIV_ROUND_UP(TS_CLOCK, div)-1;
> }
> if (*res < 0) *res = 0;
> if (*res > MID_SEG_MAX_RATE) *res = MID_SEG_MAX_RATE;
>diff -u -p a/drivers/atm/horizon.c b/drivers/atm/horizon.c
>--- a/drivers/atm/horizon.c 2007-11-08 18:33:26.000000000 +0100
>+++ b/drivers/atm/horizon.c 2008-02-13 20:50:13.000000000 +0100
>@@ -635,7 +635,7 @@ static int make_rate (const hrz_dev * de
> // take care of rounding
> switch (r) {
> case round_down:
>- pre = (br+(c<<div)-1)/(c<<div);
>+ pre = DIV_ROUND_UP(br, c<<div);
> // but p must be non-zero
> if (!pre)
> pre = 1;
>@@ -668,7 +668,7 @@ static int make_rate (const hrz_dev * de
> // take care of rounding
> switch (r) {
> case round_down:
>- pre = (br+(c<<div)-1)/(c<<div);
>+ pre = DIV_ROUND_UP(br, c<<div);
> break;
> case round_nearest:
> pre = (br+(c<<div)/2)/(c<<div);
>@@ -698,7 +698,7 @@ got_it:
> if (bits)
> *bits = (div<<CLOCK_SELECT_SHIFT) | (pre-1);
> if (actual) {
>- *actual = (br + (pre<<div) - 1) / (pre<<div);
>+ *actual = DIV_ROUND_UP(br, pre<<div);
> PRINTD (DBG_QOS, "actual rate: %u", *actual);
> }
> return 0;


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