From mboxrd@z Thu Jan  1 00:00:00 1970
From: s.hauer@pengutronix.de (Sascha Hauer)
Date: Tue, 6 May 2014 22:00:29 +0200
Subject: mx6qsabresd hangs on linux-next
In-Reply-To: <5369081D.5030400@st.com>
References: <CAOMZO5DMEqs=zzNGqgKjK7wTJdiNPgEqxsX=TtqKrnpLMzpbDw@mail.gmail.com>
 <20140506141310.GA2794@dragon>
 <CAOMZO5DH5d_L-deVu=XmBo9ekq+-g704p7cyayLvp=43bnEccQ@mail.gmail.com>
 <CAOMZO5Dx0JMj3dZ7BSgaYSZZ=ZKRqQHEYWj5sb6z_9jc+cpsEw@mail.gmail.com>
 <5369081D.5030400@st.com>
Message-ID: <20140506200029.GD5858@pengutronix.de>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Tue, May 06, 2014 at 06:04:45PM +0200, Maxime Coquelin wrote:
> Hi Fabio,
> 
> On 05/06/2014 05:49 PM, Fabio Estevam wrote:
> >On Tue, May 6, 2014 at 12:36 PM, Fabio Estevam <festevam@gmail.com> wrote:
> >
> >>Indeed, if I revert:
> >>
> >>commit e7489693b3a853ab6dfad52f7e6af553ae8d3f28
> >>Author: Maxime COQUELIN <maxime.coquelin@st.com>
> >>Date:   Wed Jan 29 17:24:08 2014 +0100
> >>
> >>     clk: divider: Optimize clk_divider_bestdiv loop
> >>
> >>     Currently, the for-loop used to try all the different dividers to find the
> >>     one that best fit tries all the values from 1 to max_div,
> >>incrementing by one.
> >>     In case of power-of-two, or table based divider, the loop isn't optimal.
> >>
> >>     Instead of incrementing by one, this patch provides directly the
> >>next divider.
> >>
> >>     Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
> >>     Signed-off-by: Mike Turquette <mturquette@linaro.org>
> >>
> >>Then the board does not hang.
> >
> >Isn't the increment of i missing?
> 
> i is incremented in _next_div():
> 
> +static int _next_div(struct clk_divider *divider, int div)
> +{
> +	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);
> +
> +	return div;
> +}

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)) {
		...
	}

Sascha

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