[PATCH] clk: mux: let clk_mux_val_to_index return U8_MAX in the error case

[PATCH] clk: mux: let clk_mux_val_to_index return U8_MAX in the error case

[Heiner Kallweit]

Currently this function may return a negative errno, but almost no
user checks for the error case. Only imx_clk_gpr_mux_get_parent()
does, but mentions in a comment that they'd prefer a dummy value.
Other users cast the negative errno to u8 instead, what may result
in unwanted results.
Let's deal with this by returning u8 and U8_MAX in the error case.
Then clk_core_get_parent_by_index() can detect that the index is
out of range.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
---
 drivers/clk/clk-mux.c        | 8 ++++----
 include/linux/clk-provider.h | 4 ++--
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/clk/clk-mux.c b/drivers/clk/clk-mux.c
index fa817c317..be0f317ee 100644
--- a/drivers/clk/clk-mux.c
+++ b/drivers/clk/clk-mux.c
@@ -40,8 +40,8 @@ static inline void clk_mux_writel(struct clk_mux *mux, u32 val)
 		writel(val, mux->reg);
 }
 
-int clk_mux_val_to_index(struct clk_hw *hw, const u32 *table, unsigned int flags,
-			 unsigned int val)
+u8 clk_mux_val_to_index(struct clk_hw *hw, const u32 *table, unsigned int flags,
+			unsigned int val)
 {
 	int num_parents = clk_hw_get_num_parents(hw);
 
@@ -51,7 +51,7 @@ int clk_mux_val_to_index(struct clk_hw *hw, const u32 *table, unsigned int flags
 		for (i = 0; i < num_parents; i++)
 			if (table[i] == val)
 				return i;
-		return -EINVAL;
+		return U8_MAX;
 	}
 
 	if (val && (flags & CLK_MUX_INDEX_BIT))
@@ -61,7 +61,7 @@ int clk_mux_val_to_index(struct clk_hw *hw, const u32 *table, unsigned int flags
 		val--;
 
 	if (val >= num_parents)
-		return -EINVAL;
+		return U8_MAX;
 
 	return val;
 }
diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
index faad3cdc1..62fb4118c 100644
--- a/include/linux/clk-provider.h
+++ b/include/linux/clk-provider.h
@@ -1069,8 +1069,8 @@ struct clk *clk_register_mux_table(struct device *dev, const char *name,
 			      NULL, (parent_data), (flags), (reg), (shift),   \
 			      BIT((width)) - 1, (clk_mux_flags), table, (lock))
 
-int clk_mux_val_to_index(struct clk_hw *hw, const u32 *table, unsigned int flags,
-			 unsigned int val);
+u8 clk_mux_val_to_index(struct clk_hw *hw, const u32 *table, unsigned int flags,
+			unsigned int val);
 unsigned int clk_mux_index_to_val(const u32 *table, unsigned int flags, u8 index);
 
 void clk_unregister_mux(struct clk *clk);
-- 
2.40.0

Re: [PATCH] clk: mux: let clk_mux_val_to_index return U8_MAX in the error case

[Stephen Boyd]

Quoting Heiner Kallweit (2023-04-11 23:19:04)
> Currently this function may return a negative errno, but almost no
> user checks for the error case. Only imx_clk_gpr_mux_get_parent()
> does, but mentions in a comment that they'd prefer a dummy value.
> Other users cast the negative errno to u8 instead, what may result
> in unwanted results.
> Let's deal with this by returning u8 and U8_MAX in the error case.
> Then clk_core_get_parent_by_index() can detect that the index is
> out of range.

Is this causing problems for you?

Re: [PATCH] clk: mux: let clk_mux_val_to_index return U8_MAX in the error case

[Heiner Kallweit]

On 13.04.2023 21:10, Stephen Boyd wrote:
> Quoting Heiner Kallweit (2023-04-11 23:19:04)
>> Currently this function may return a negative errno, but almost no
>> user checks for the error case. Only imx_clk_gpr_mux_get_parent()
>> does, but mentions in a comment that they'd prefer a dummy value.
>> Other users cast the negative errno to u8 instead, what may result
>> in unwanted results.
>> Let's deal with this by returning u8 and U8_MAX in the error case.
>> Then clk_core_get_parent_by_index() can detect that the index is
>> out of range.
> 
> Is this causing problems for you?

I'm not directly impacted. I came across this when working on a driver
with the following scenario:

I have a clock muxer where I want to exclude one of the mux parents
by using clk_mux.table. If by chance the boot loader used this mux
parent, then clk_mux_val_to_index() would return an errno.
Also within clk core the return value isn't checked in
clk_mux_get_parent().

Re: [PATCH] clk: mux: let clk_mux_val_to_index return U8_MAX in the error case

[Stephen Boyd]

Quoting Heiner Kallweit (2023-04-13 12:27:27)
> On 13.04.2023 21:10, Stephen Boyd wrote:
> > Quoting Heiner Kallweit (2023-04-11 23:19:04)
> >> Currently this function may return a negative errno, but almost no
> >> user checks for the error case. Only imx_clk_gpr_mux_get_parent()
> >> does, but mentions in a comment that they'd prefer a dummy value.
> >> Other users cast the negative errno to u8 instead, what may result
> >> in unwanted results.
> >> Let's deal with this by returning u8 and U8_MAX in the error case.
> >> Then clk_core_get_parent_by_index() can detect that the index is
> >> out of range.
> > 
> > Is this causing problems for you?
> 
> I'm not directly impacted. I came across this when working on a driver
> with the following scenario:
> 
> I have a clock muxer where I want to exclude one of the mux parents
> by using clk_mux.table. If by chance the boot loader used this mux
> parent, then clk_mux_val_to_index() would return an errno.
> Also within clk core the return value isn't checked in
> clk_mux_get_parent().
> 

Ok. I'd rather go with a total rewrite of the get_parent clk_op[1] and fix
this at the same time.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/clk/linux.git/log/?h=clk-parent-rewrite

Re: [PATCH] clk: mux: let clk_mux_val_to_index return U8_MAX in the error case

[Heiner Kallweit]

On 13.04.2023 22:30, Stephen Boyd wrote:
> Quoting Heiner Kallweit (2023-04-13 12:27:27)
>> On 13.04.2023 21:10, Stephen Boyd wrote:
>>> Quoting Heiner Kallweit (2023-04-11 23:19:04)
>>>> Currently this function may return a negative errno, but almost no
>>>> user checks for the error case. Only imx_clk_gpr_mux_get_parent()
>>>> does, but mentions in a comment that they'd prefer a dummy value.
>>>> Other users cast the negative errno to u8 instead, what may result
>>>> in unwanted results.
>>>> Let's deal with this by returning u8 and U8_MAX in the error case.
>>>> Then clk_core_get_parent_by_index() can detect that the index is
>>>> out of range.
>>>
>>> Is this causing problems for you?
>>
>> I'm not directly impacted. I came across this when working on a driver
>> with the following scenario:
>>
>> I have a clock muxer where I want to exclude one of the mux parents
>> by using clk_mux.table. If by chance the boot loader used this mux
>> parent, then clk_mux_val_to_index() would return an errno.
>> Also within clk core the return value isn't checked in
>> clk_mux_get_parent().
>>
> 
> Ok. I'd rather go with a total rewrite of the get_parent clk_op[1] and fix
> this at the same time.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/clk/linux.git/log/?h=clk-parent-rewrite

This would be even better. Thanks.