Re: [PATCH] clk: clk-divider: Fix infinite loop for table divider

From: Maxime Coquelin
Date: Wed May 07 2014 - 04:38:26 EST


Hi Fabio,

On 05/06/2014 10:26 PM, Fabio Estevam wrote:
From: Fabio Estevam <fabio.estevam@xxxxxxxxxxxxx>

Since commit e7489693b3 (clk: divider: Optimize clk_divider_bestdiv loop) we
notice a system hang on mx6qboard.

The cause for the hang is well explained by Sascha Hauer [1]:

"This cannot work. _round_up_table is implemented like this:

static int _round_up_table(const struct clk_div_table *table, int div)
{
const struct clk_div_table *clkt;
int up = _get_table_maxdiv(table);

for (clkt = table; clkt->div; clkt++) {
if (clkt->div == div)
return clkt->div;
...
}
...
}

Here when a table entry matches the input div this function will return
exactly the input div. This means _next_div() will always return the
same value and clk_divider_bestdiv() has an infinite loop:

for (i = 1; i <= maxdiv; i = _next_div(divider, i)) {
...
}
"
[1] http://marc.info/?l=linux-arm-kernel&m=139940658726253&w=2

Remove the 'return _round_up_table' so that the system can work normally again.

Signed-off-by: Fabio Estevam <fabio.estevam@xxxxxxxxxxxxx>
---
drivers/clk/clk-divider.c | 2 --
1 file changed, 2 deletions(-)

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index b3c8396..fc3e344 100644
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -246,8 +246,6 @@ static int _next_div(struct clk_divider *divider, int div)

if (divider->flags & CLK_DIVIDER_POWER_OF_TWO)
return __roundup_pow_of_two(div);
- if (divider->table)
- return _round_up_table(divider->table, div);

Couldn't we initialize "up" variable to INT_MAX in _round_up_table() as I just proposed in mail thread reporting the regression?

See http://marc.info/?l=linux-arm-kernel&m=139945172905130&w=2

Regards,
Maxime


return div;
}

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