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authorLinus Walleij <linus.walleij@linaro.org>2016-02-08 09:14:37 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-02-25 11:57:48 -0800
commitfa4aa48526e852a550613acffc81cacc5233848e (patch)
tree013bfe06534e37754a9ce0249c3644c8b7e95e91
parent57cd1f0a2031e82ac1cc13e56f12b7d979ee5403 (diff)
downloadlwn-fa4aa48526e852a550613acffc81cacc5233848e.tar.gz
lwn-fa4aa48526e852a550613acffc81cacc5233848e.zip
ARM: 8517/1: ICST: avoid arithmetic overflow in icst_hz()
commit 5070fb14a0154f075c8b418e5bc58a620ae85a45 upstream. When trying to set the ICST 307 clock to 25174000 Hz I ran into this arithmetic error: the icst_hz_to_vco() correctly figure out DIVIDE=2, RDW=100 and VDW=99 yielding a frequency of 25174000 Hz out of the VCO. (I replicated the icst_hz() function in a spreadsheet to verify this.) However, when I called icst_hz() on these VCO settings it would instead return 4122709 Hz. This causes an error in the common clock driver for ICST as the common clock framework will call .round_rate() on the clock which will utilize icst_hz_to_vco() followed by icst_hz() suggesting the erroneous frequency, and then the clock gets set to this. The error did not manifest in the old clock framework since this high frequency was only used by the CLCD, which calls clk_set_rate() without first calling clk_round_rate() and since the old clock framework would not call clk_round_rate() before setting the frequency, the correct values propagated into the VCO. After some experimenting I figured out that it was due to a simple arithmetic overflow: the divisor for 24Mhz reference frequency as reference becomes 24000000*2*(99+8)=0x132212400 and the "1" in bit 32 overflows and is lost. But introducing an explicit 64-by-32 bit do_div() and casting the divisor into (u64) we get the right frequency back, and the right frequency gets set. Tested on the ARM Versatile. Cc: linux-clk@vger.kernel.org Cc: Pawel Moll <pawel.moll@arm.com> Signed-off-by: Linus Walleij <linus.walleij@linaro.org> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--arch/arm/common/icst.c8
1 files changed, 6 insertions, 2 deletions
diff --git a/arch/arm/common/icst.c b/arch/arm/common/icst.c
index 3b3e58b7ba74..d7ed252708c5 100644
--- a/arch/arm/common/icst.c
+++ b/arch/arm/common/icst.c
@@ -16,7 +16,7 @@
*/
#include <linux/module.h>
#include <linux/kernel.h>
-
+#include <asm/div64.h>
#include <asm/hardware/icst.h>
/*
@@ -29,7 +29,11 @@ EXPORT_SYMBOL(icst525_s2div);
unsigned long icst_hz(const struct icst_params *p, struct icst_vco vco)
{
- return p->ref * 2 * (vco.v + 8) / ((vco.r + 2) * p->s2div[vco.s]);
+ u64 dividend = p->ref * 2 * (u64)(vco.v + 8);
+ u32 divisor = (vco.r + 2) * p->s2div[vco.s];
+
+ do_div(dividend, divisor);
+ return (unsigned long)dividend;
}
EXPORT_SYMBOL(icst_hz);