linux-kernel - Re: [PATCH] i2c-davinci: Fix use of default platform data if none supplied.

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Date:	Tue, 21 Sep 2010 21:02:58 -0400
From:	Michael Williamson <michael.williamson@...ticallink.com>
To:	Ben Dooks <ben-i2c@...ff.org>
CC:	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	ben-linux@...ff.org, khilman@...prootsystems.com,
	chaithrika@...com, pjohn@...mvista.com, nageswari@...com,
	dirk.behme@...il.com
Subject: Re: [PATCH] i2c-davinci: Fix use of default platform data if none
 supplied.

On 09/21/2010 08:24 PM, Ben Dooks wrote:
> On Sat, Sep 04, 2010 at 12:07:48PM -0400, Michael Williamson wrote:
>> There is a bug in the i2c-davinci device init routine that attempts
>> to use default platform data if none is supplied (e.g., is NULL).
>> This patch fixes the bug.
>>
>> Signed-off-by: Michael Williamson <michael.williamson@...ticallink.com>
>> ---
>>  drivers/i2c/busses/i2c-davinci.c |    8 +++++---
>>  1 files changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/i2c/busses/i2c-davinci.c b/drivers/i2c/busses/i2c-davinci.c
>> index 2222c87..6d4eeeb 100644
>> --- a/drivers/i2c/busses/i2c-davinci.c
>> +++ b/drivers/i2c/busses/i2c-davinci.c
>> @@ -235,10 +235,12 @@ static void i2c_davinci_calc_clk_dividers(struct davinci_i2c_dev *dev)
>>   */
>>  static int i2c_davinci_init(struct davinci_i2c_dev *dev)
>>  {
>> -	struct davinci_i2c_platform_data *pdata = dev->dev->platform_data;
>> +	struct davinci_i2c_platform_data *pdata;
>>  
>> -	if (!pdata)
>> -		pdata = &davinci_i2c_platform_data_default;
>> +	if (!dev->dev->platform_data)
>> +		dev->dev->platform_data = &davinci_i2c_platform_data_default;
>> +
>> +	pdata = dev->dev->platform_data;
> 
> At first glance this looks like a code shift, however is the platform
> data used later in the driver by referencing the device.platform_data
> field?
> 

Yes. The problem call is i2c_davinci_calc_clk_dividers(), which is called 
pretty early in the i2c_davinci_init() routine.  There is a reference to 
pdata->bus_freq without a check to see if pdata is valid.  I think my proposed
fix is not the right approach as you have pointed out.

Perhaps a better patch would be (?):

---
diff --git a/drivers/i2c/busses/i2c-davinci.c b/drivers/i2c/busses/i2c-davinci.c
index 2222c87..15d0cea 100644
--- a/drivers/i2c/busses/i2c-davinci.c
+++ b/drivers/i2c/busses/i2c-davinci.c
@@ -195,6 +195,9 @@ static void i2c_davinci_calc_clk_dividers(struct davinci_i2c_dev *dev)
 	u32 clkl;
 	u32 input_clock = clk_get_rate(dev->clk);
 
+	if (!pdata)
+		pdata = &davinci_i2c_platform_data_default;
+
 	/* NOTE: I2C Clock divider programming info
 	 * As per I2C specs the following formulas provide prescaler
 	 * and low/high divider values
---

> Could you rewrite the header to show this, and if possible note any
> actual oops report that has been seen with this.
> 

I found this because I was told (while proposing a new platform) to use NULL
as platform data as the parameters I was using matched the default data.  I 
didn't get an oops, it just hung (it's pretty early in the board init).  
Our boot strapper/load resets the DDR where the kernel buffer is, so getting
any oops info is a bit time consuming.  The above three lines (and the original
patch) resolved the problem.

I can update the header to indicate a default parameter set if you like.  New patch OK?

-Mike 
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