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SubjectRe: [PATCH E 11/14] OMAP clock: track child clocks
Hello Russell,

On Sat, 14 Feb 2009, Russell King - ARM Linux wrote:

> On Sat, Feb 14, 2009 at 11:23:25AM +0000, Russell King - ARM Linux wrote:
> > There's also a second issue - the comments before omap2_divisor_to_clksel()
> > indicate that this function returns 0xffffffff on error. Unfortunately,
> > this is not so, it actually returns zero on error. Moreover, we test
> > the result of the function against ~0, so we'll never deal with the error
> > case. This really should be fixed so that we return the right value for
> > the error case. (Further comments on this in a follow up.)
>
> The thing I don't like about this is that we have several functions looking
> up clksel_rate entries, some of which return 0 on error and others ~0 on
> error.
>
> This is very prone to mistakes - and is probably why the original problem
> has occurred. I'd much prefer to fix the underlying confusion rather
> than letting it persist, by making all these lookup functions return the
> clksel_rate pointer or NULL.
>
> Not only does that avoid the question about whether the function returns
> 0 or ~0 on error, but it also gets rid of the horrible return-through-pointer
> style of _omap2_clksel_get_src_field() which itself is error prone.
> (The kernel has had a few cases where this kind of thing has lead to
> uninitialized use, so avoiding this where it's easy to do so makes
> sense.)
>
> One final point:
>
> if (parent_div > 0)
> clk->rate /= parent_div;
>
> seems to be impossible to be false - the old code used div == 0 in the
> tables as the end of table sentinel, and if it is encountered,
> _omap2_clksel_get_src_field (and the newer omap2_clksel_lookup_parent)
> causes failure to occur. Also, since parent_div is unsigned, the only
> case where the above statement is false is when div == 0. So, the code
> can be further simplified to:
>
> /* CLKSEL clocks follow their parents' rates, divided by a divisor */
> clk->rate = new_parent->rate / clkr->div;
>
> Agreed?

Agreed on both points -- a few comments on the patch:

> diff -u a/arch/arm/mach-omap2/clock.c b/arch/arm/mach-omap2/clock.c
> --- a/arch/arm/mach-omap2/clock.c
> +++ b/arch/arm/mach-omap2/clock.c
> @@ -596,23 +596,24 @@
> }
>
> /**
> - * omap2_clksel_to_divisor() - turn clksel field value into integer divider
> + * omap2_clksel_lookup_field() - lookup clksel_rate for clksel field value
> * @clk: OMAP struct clk to use
> * @field_val: register field value to find
> *
> * Given a struct clk of a rate-selectable clksel clock, and a register field
> - * value to search for, find the corresponding clock divisor. The register
> + * value to search for, find the corresponding clksel_rate entry. The register
> * field value should be pre-masked and shifted down so the LSB is at bit 0
> - * before calling. Returns 0 on error
> + * before calling. Returns NULL on error.
> */
> -u32 omap2_clksel_to_divisor(struct clk *clk, u32 field_val)
> +static const
> +struct clksel_rate *omap2_clksel_lookup_field(struct clk *clk, u32 field_val)
> {
> const struct clksel *clks;
> const struct clksel_rate *clkr;
>
> clks = omap2_get_clksel_by_parent(clk, clk->parent);
> if (!clks)
> - return 0;
> + return NULL;
>
> for (clkr = clks->rates; clkr->div; clkr++) {
> if ((clkr->flags & cpu_mask) && (clkr->val == field_val))
> @@ -623,22 +624,22 @@
> printk(KERN_ERR "clock: Could not find fieldval %d for "
> "clock %s parent %s\n", field_val, clk->name,
> clk->parent->name);
> - return 0;
> + return NULL;
> }
>
> - return clkr->div;
> + return clkr;
> }
>
> /**
> - * omap2_divisor_to_clksel() - turn clksel integer divisor into a field value
> + * omap2_clksel_lookup_divisor() - lookup clksel_rate for integer divisor
> * @clk: OMAP struct clk to use
> * @div: integer divisor to search for
> *
> - * Given a struct clk of a rate-selectable clksel clock, and a clock divisor,
> - * find the corresponding register field value. The return register value is
> - * the value before left-shifting. Returns ~0 on error
> + * Given a struct clk of a rate-selectable clksel clock and a clock divisor,
> + * find the corresponding clksel_rate entry. Returns NULL on error.
> */
> -u32 omap2_divisor_to_clksel(struct clk *clk, u32 div)
> +static const
> +struct clksel_rate *omap2_clksel_lookup_divisor(struct clk *clk, u32 div)
> {
> const struct clksel *clks;
> const struct clksel_rate *clkr;
> @@ -648,7 +649,7 @@
>
> clks = omap2_get_clksel_by_parent(clk, clk->parent);
> if (!clks)
> - return ~0;
> + return NULL;
>
> for (clkr = clks->rates; clkr->div; clkr++) {
> if ((clkr->flags & cpu_mask) && (clkr->div == div))
> @@ -659,10 +660,10 @@
> printk(KERN_ERR "clock: Could not find divisor %d for "
> "clock %s parent %s\n", div, clk->name,
> clk->parent->name);
> - return ~0;
> + return NULL;
> }
>
> - return clkr->val;
> + return clkr;
> }
>
> /**
> @@ -673,6 +674,7 @@
> */
> u32 omap2_clksel_get_divisor(struct clk *clk)
> {
> + const struct clksel_rate *clkr;
> u32 v;
>
> if (!clk->clksel_mask)
> @@ -681,11 +683,14 @@
> v = __raw_readl(clk->clksel_reg) & clk->clksel_mask;
> v >>= __ffs(clk->clksel_mask);
>
> - return omap2_clksel_to_divisor(clk, v);
> + clkr = omap2_clksel_lookup_field(clk, v);
> +
> + return clkr ? clkr->val : 0;
> }
>
> int omap2_clksel_set_rate(struct clk *clk, unsigned long rate)
> {
> + const struct clksel_rate *clkr;
> u32 v, field_val, validrate, new_div = 0;
>
> if (!clk->clksel_mask)
> @@ -695,17 +700,17 @@
> if (validrate != rate)
> return -EINVAL;
>
> - field_val = omap2_divisor_to_clksel(clk, new_div);
> - if (field_val == ~0)
> + clkr = omap2_clksel_lookup_divisor(clk, new_div);
> + if (!clkr)
> return -EINVAL;
>
> v = __raw_readl(clk->clksel_reg);
> v &= ~clk->clksel_mask;
> - v |= field_val << __ffs(clk->clksel_mask);
> + v |= clkr->val << __ffs(clk->clksel_mask);
> __raw_writel(v, clk->clksel_reg);
> v = __raw_readl(clk->clksel_reg); /* OCP barrier */
>
> - clk->rate = clk->parent->rate / new_div;
> + clk->rate = clk->parent->rate / clkr->div;
>
> _omap2xxx_clk_commit(clk);
>
> @@ -733,18 +738,18 @@
> }
>
> /*
> - * Converts encoded control register address into a full address
> - * On error, the return value (parent_div) will be 0.
> + * Given a struct clk of a rate-selectable clksel clock and a parent clock,
> + * find the default clksel_rate entry. Returns NULL on error.
> */
> -static u32 _omap2_clksel_get_src_field(struct clk *src_clk, struct clk *clk,
> - u32 *field_val)
> +static const
> +struct clksel_rate *omap2_clksel_lookup_parent(struct clk *clk, struct clk *parent)
> {
> const struct clksel *clks;
> const struct clksel_rate *clkr;
>
> - clks = omap2_get_clksel_by_parent(clk, src_clk);
> + clks = omap2_get_clksel_by_parent(clk, parent);
> if (!clks)
> - return 0;
> + return NULL;
>
> for (clkr = clks->rates; clkr->div; clkr++) {
> if (clkr->flags & cpu_mask && clkr->flags & DEFAULT_RATE)
> @@ -755,20 +760,19 @@
> printk(KERN_ERR "clock: Could not find default rate for "
> "clock %s parent %s\n", clk->name,
> src_clk->parent->name);
> - return 0;
> + return NULL;
> }
>
> /* Should never happen. Add a clksel mask to the struct clk. */
> WARN_ON(clk->clksel_mask == 0);
>
> - *field_val = clkr->val;
> -
> - return clkr->div;
> + return NULL;

This should return clkr.

> }
>
> int omap2_clk_set_parent(struct clk *clk, struct clk *new_parent)
> {
> - u32 field_val, v, parent_div;
> + const struct clksel_rate *clkr;
> + u32 v;
>
> if (clk->flags & CONFIG_PARTICIPANT)
> return -EINVAL;
> @@ -776,8 +780,8 @@
> if (!clk->clksel)
> return -EINVAL;
>
> - parent_div = _omap2_clksel_get_src_field(new_parent, clk, &field_val);
> - if (!parent_div)
> + clkr = omap2_clksel_lookup_parent(clk, new_parent);
> + if (!clkr)
> return -EINVAL;
>
> if (clk->usecount > 0)
> @@ -786,7 +790,7 @@
> /* Set new source value (previous dividers if any in effect) */
> v = __raw_readl(clk->clksel_reg);
> v &= ~clk->clksel_mask;
> - v |= field_val << __ffs(clk->clksel_mask);
> + v |= clkr->val << __ffs(clk->clksel_mask);
> __raw_writel(v, clk->clksel_reg);
> v = __raw_readl(clk->clksel_reg); /* OCP barrier */
>
> @@ -800,8 +804,8 @@
> /* CLKSEL clocks follow their parents' rates, divided by a divisor */
> clk->rate = new_parent->rate;
>
> - if (parent_div > 0)
> - clk->rate /= parent_div;
> + if (clkr->div > 0)
> + clk->rate /= clkr->div;

The conditional can be removed, per your second point above.

>
> pr_debug("clock: set parent of %s to %s (new rate %ld)\n",
> clk->name, clk->parent->name, clk->rate);
> diff -u a/arch/arm/mach-omap2/clock.h b/arch/arm/mach-omap2/clock.h
> --- a/arch/arm/mach-omap2/clock.h
> +++ b/arch/arm/mach-omap2/clock.h
> @@ -42,8 +42,6 @@ void omap2_init_clksel_parent(struct clk *clk);
> u32 omap2_clksel_get_divisor(struct clk *clk);
> u32 omap2_clksel_round_rate_div(struct clk *clk, unsigned long target_rate,
> u32 *new_div);
> -u32 omap2_clksel_to_divisor(struct clk *clk, u32 field_val);
> -u32 omap2_divisor_to_clksel(struct clk *clk, u32 div);
> unsigned long omap2_fixed_divisor_recalc(struct clk *clk);
> long omap2_clksel_round_rate(struct clk *clk, unsigned long target_rate);
> int omap2_clksel_set_rate(struct clk *clk, unsigned long rate);
>


- Paul


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