s3c2410fb: fix clockrate calculation

s3c2410fb: fix clockrate calculation

[Ben Dooks]

[-- Attachment #1: s3c2410-fb-fix-rounding.patch --]
[-- Type: text/plain, Size: 1545 bytes --]

In the final part of the calculation for the tft display clockrate we
divide the output pf s3c2410fb_calc_pixclk() by 2 which leaves us with
a rounding error if the result is odd.

Change to using DIV_ROUND_UP() to ensure that we always choose a higher
divisor and thus a lower frequency.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>

---
 drivers/video/s3c2410fb.c |    4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)
 
Index: b/drivers/video/s3c2410fb.c
===================================================================
--- a/drivers/video/s3c2410fb.c	2009-08-20 08:45:41.000000000 +0100
+++ b/drivers/video/s3c2410fb.c	2009-08-20 08:45:42.000000000 +0100
@@ -369,7 +369,9 @@ static void s3c2410fb_activate_var(struc
 	void __iomem *regs = fbi->io;
 	int type = fbi->regs.lcdcon1 & S3C2410_LCDCON1_TFT;
 	struct fb_var_screeninfo *var = &info->var;
-	int clkdiv = s3c2410fb_calc_pixclk(fbi, var->pixclock) / 2;
+	int clkdiv;
+
+	clkdiv = DIV_ROUND_UP(s3c2410fb_calc_pixclk(fbi, var->pixclock), 2);
 
 	printk(KERN_INFO "%s: pixclock=%d, clkdiv=%d\n",
 	       __func__, var->pixclock, clkdiv);

-- 
Ben (ben@fluff.org, http://www.fluff.org/)

  'a smiley only costs 4 bytes'

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Re: s3c2410fb: fix clockrate calculation

[Andrew Morton]

On Thu, 20 Aug 2009 22:50:47 +0100
Ben Dooks <ben@simtec.co.uk> wrote:

> In the final part of the calculation for the tft display clockrate we
> divide the output pf s3c2410fb_calc_pixclk() by 2 which leaves us with
> a rounding error if the result is odd.
> 
> Change to using DIV_ROUND_UP() to ensure that we always choose a higher
> divisor and thus a lower frequency.
> 
> Signed-off-by: Ben Dooks <ben@simtec.co.uk>
> 
> ---
>  drivers/video/s3c2410fb.c |    4 +++-
>  2 files changed, 5 insertions(+), 2 deletions(-)
>  
> Index: b/drivers/video/s3c2410fb.c
> ===================================================================
> --- a/drivers/video/s3c2410fb.c	2009-08-20 08:45:41.000000000 +0100
> +++ b/drivers/video/s3c2410fb.c	2009-08-20 08:45:42.000000000 +0100
> @@ -369,7 +369,9 @@ static void s3c2410fb_activate_var(struc
>  	void __iomem *regs = fbi->io;
>  	int type = fbi->regs.lcdcon1 & S3C2410_LCDCON1_TFT;
>  	struct fb_var_screeninfo *var = &info->var;
> -	int clkdiv = s3c2410fb_calc_pixclk(fbi, var->pixclock) / 2;
> +	int clkdiv;
> +
> +	clkdiv = DIV_ROUND_UP(s3c2410fb_calc_pixclk(fbi, var->pixclock), 2);
>  
>  	printk(KERN_INFO "%s: pixclock=%d, clkdiv=%d\n",
>  	       __func__, var->pixclock, clkdiv);

The changelog forgot to tell us what the impact of this bug is, so I
cannot work out whether we need this fix in 2.6.32, 2.6.31, 2.6.30.x, ....


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Re: s3c2410fb: fix clockrate calculation

[Ben Dooks]

Andrew Morton wrote:
> On Thu, 20 Aug 2009 22:50:47 +0100
> Ben Dooks <ben@simtec.co.uk> wrote:
> 
>> In the final part of the calculation for the tft display clockrate we
>> divide the output pf s3c2410fb_calc_pixclk() by 2 which leaves us with
>> a rounding error if the result is odd.
>>
>> Change to using DIV_ROUND_UP() to ensure that we always choose a higher
>> divisor and thus a lower frequency.
>>
>> Signed-off-by: Ben Dooks <ben@simtec.co.uk>
>>
>> ---
>>  drivers/video/s3c2410fb.c |    4 +++-
>>  2 files changed, 5 insertions(+), 2 deletions(-)
>>  
>> Index: b/drivers/video/s3c2410fb.c
>> ===================================================================
>> --- a/drivers/video/s3c2410fb.c	2009-08-20 08:45:41.000000000 +0100
>> +++ b/drivers/video/s3c2410fb.c	2009-08-20 08:45:42.000000000 +0100
>> @@ -369,7 +369,9 @@ static void s3c2410fb_activate_var(struc
>>  	void __iomem *regs = fbi->io;
>>  	int type = fbi->regs.lcdcon1 & S3C2410_LCDCON1_TFT;
>>  	struct fb_var_screeninfo *var = &info->var;
>> -	int clkdiv = s3c2410fb_calc_pixclk(fbi, var->pixclock) / 2;
>> +	int clkdiv;
>> +
>> +	clkdiv = DIV_ROUND_UP(s3c2410fb_calc_pixclk(fbi, var->pixclock), 2);
>>  
>>  	printk(KERN_INFO "%s: pixclock=%d, clkdiv=%d\n",
>>  	       __func__, var->pixclock, clkdiv);
> 
> The changelog forgot to tell us what the impact of this bug is, so I
> cannot work out whether we need this fix in 2.6.32, 2.6.31, 2.6.30.x, ....

Sorry, found this whilst working on a new machine and the pix clock being
too fast for the display. The machine is not yet merged into the mainline
so this not an important fix.

-- 
Ben Dooks, Software Engineer, Simtec Electronics

http://www.simtec.co.uk/

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