Re: [PATCH v2 1/3] drm/sun4i: hdmi: Check for unset best_parent in sun4i_tmds_determine_rate

[Jonathan Liu]

Hi Maxime,

On 5 January 2018 at 21:03, Maxime Ripard
 wrote:
> On Fri, Jan 05, 2018 at 09:44:39AM +1100, Jonathan Liu wrote:
>> On 5 January 2018 at 06:56, Maxime Ripard
>>  wrote:
>> > On Tue, Dec 26, 2017 at 10:12:25PM +1100, Jonathan Liu wrote:
>> >> We should check if the best match has been set before comparing it.
>> >>
>> >> Fixes: 9c5681011a0c ("drm/sun4i: Add HDMI support")
>> >> Signed-off-by: Jonathan Liu 
>> >> ---
>> >>  drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c | 2 +-
>> >>  1 file changed, 1 insertion(+), 1 deletion(-)
>> >>
>> >> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c 
>> >> b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> >> index dc332ea56f6c..4d235e5ea31c 100644
>> >> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> >> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> >> @@ -102,7 +102,7 @@ static int sun4i_tmds_determine_rate(struct clk_hw 
>> >> *hw,
>> >>   goto out;
>> >>   }
>> >>
>> >> - if (abs(rate - rounded / i) <
>> >> + if (!best_parent || abs(rate - rounded / i) 
>> >> <
>> >
>> > Why is that causing any issue?
>> >
>> > If best_parent is set to 0...
>> >
>> >>   abs(rate - best_parent / best_div)) {
>> >
>> > ... the value returned here is going to be rate, which is going to be
>> > higher than the first part of the comparison meaning ...
>> >
>> >>   best_parent = rounded;
>> >
>> > ... that best_parent is going to be set there.
>>
>> Consider the following:
>> rate = 8350
>> rounded = ideal * 2
>>
>> It is possible that if "rounded = clk_hw_round_rate(parent, ideal)"
>> gives high enough values that the condition "abs(rate - rounded / i) <
>> abs(rate - best_parent / best_div)" is never met.
>>
>> Then you can end up with:
>> req->rate = 0
>> req->best_parent_rate = 0
>> req->best_parent_hw = ...
>>
>> Also, the sun4i_tmds_calc_divider function has a similar check.
>
> Ok. That explanation must be part of your commit log, or at least
> which problem you're trying to address and in which situation it will
> trigger.

Ok. I have added amended the commit message with explanation for v3.

Regards,
Jonathan
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Re: [PATCH v2 1/3] drm/sun4i: hdmi: Check for unset best_parent in sun4i_tmds_determine_rate

[Maxime Ripard]

On Fri, Jan 05, 2018 at 09:44:39AM +1100, Jonathan Liu wrote:
> Hi Maxime,
> 
> On 5 January 2018 at 06:56, Maxime Ripard
>  wrote:
> > On Tue, Dec 26, 2017 at 10:12:25PM +1100, Jonathan Liu wrote:
> >> We should check if the best match has been set before comparing it.
> >>
> >> Fixes: 9c5681011a0c ("drm/sun4i: Add HDMI support")
> >> Signed-off-by: Jonathan Liu 
> >> ---
> >>  drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c 
> >> b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
> >> index dc332ea56f6c..4d235e5ea31c 100644
> >> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
> >> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
> >> @@ -102,7 +102,7 @@ static int sun4i_tmds_determine_rate(struct clk_hw *hw,
> >>   goto out;
> >>   }
> >>
> >> - if (abs(rate - rounded / i) <
> >> + if (!best_parent || abs(rate - rounded / i) <
> >
> > Why is that causing any issue?
> >
> > If best_parent is set to 0...
> >
> >>   abs(rate - best_parent / best_div)) {
> >
> > ... the value returned here is going to be rate, which is going to be
> > higher than the first part of the comparison meaning ...
> >
> >>   best_parent = rounded;
> >
> > ... that best_parent is going to be set there.
> 
> Consider the following:
> rate = 8350
> rounded = ideal * 2
> 
> It is possible that if "rounded = clk_hw_round_rate(parent, ideal)"
> gives high enough values that the condition "abs(rate - rounded / i) <
> abs(rate - best_parent / best_div)" is never met.
> 
> Then you can end up with:
> req->rate = 0
> req->best_parent_rate = 0
> req->best_parent_hw = ...
> 
> Also, the sun4i_tmds_calc_divider function has a similar check.

Ok. That explanation must be part of your commit log, or at least
which problem you're trying to address and in which situation it will
trigger.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com


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Re: [PATCH v2 1/3] drm/sun4i: hdmi: Check for unset best_parent in sun4i_tmds_determine_rate

[Jonathan Liu]

Hi Maxime,

On 5 January 2018 at 06:56, Maxime Ripard
 wrote:
> On Tue, Dec 26, 2017 at 10:12:25PM +1100, Jonathan Liu wrote:
>> We should check if the best match has been set before comparing it.
>>
>> Fixes: 9c5681011a0c ("drm/sun4i: Add HDMI support")
>> Signed-off-by: Jonathan Liu 
>> ---
>>  drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c 
>> b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> index dc332ea56f6c..4d235e5ea31c 100644
>> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> @@ -102,7 +102,7 @@ static int sun4i_tmds_determine_rate(struct clk_hw *hw,
>>   goto out;
>>   }
>>
>> - if (abs(rate - rounded / i) <
>> + if (!best_parent || abs(rate - rounded / i) <
>
> Why is that causing any issue?
>
> If best_parent is set to 0...
>
>>   abs(rate - best_parent / best_div)) {
>
> ... the value returned here is going to be rate, which is going to be
> higher than the first part of the comparison meaning ...
>
>>   best_parent = rounded;
>
> ... that best_parent is going to be set there.

Consider the following:
rate = 8350
rounded = ideal * 2

It is possible that if "rounded = clk_hw_round_rate(parent, ideal)"
gives high enough values that the condition "abs(rate - rounded / i) <
abs(rate - best_parent / best_div)" is never met.

Then you can end up with:
req->rate = 0
req->best_parent_rate = 0
req->best_parent_hw = ...

Also, the sun4i_tmds_calc_divider function has a similar check.

Regards,
Jonathan
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Re: [PATCH v2 1/3] drm/sun4i: hdmi: Check for unset best_parent in sun4i_tmds_determine_rate

[Maxime Ripard]

Hi,

On Tue, Dec 26, 2017 at 10:12:25PM +1100, Jonathan Liu wrote:
> We should check if the best match has been set before comparing it.
> 
> Fixes: 9c5681011a0c ("drm/sun4i: Add HDMI support")
> Signed-off-by: Jonathan Liu 
> ---
>  drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c 
> b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
> index dc332ea56f6c..4d235e5ea31c 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
> @@ -102,7 +102,7 @@ static int sun4i_tmds_determine_rate(struct clk_hw *hw,
>   goto out;
>   }
>  
> - if (abs(rate - rounded / i) <
> + if (!best_parent || abs(rate - rounded / i) <

Why is that causing any issue?

If best_parent is set to 0...

>   abs(rate - best_parent / best_div)) {

... the value returned here is going to be rate, which is going to be
higher than the first part of the comparison meaning ...

>   best_parent = rounded;

... that best_parent is going to be set there.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com


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