reading i2c eeprom early in boot

reading i2c eeprom early in boot

[Karl Hiramoto]

Hi,

My goal is to assign a MAC address stored in a I2C EEPROM to a ethernet device on an ARM  SOC so I can boot via NFS.  Reading and writing this address  works with the at24 driver after its loaded.

Currently the i2c __init call is after the ethernet devices __init, so this makes using the I2C drivers not possible, as nothing is instantiated.

So to me it looks like i have two options.

1.  Reorder the init calls.   I'd prefer not doing this for fear of  breaking something else.

2.  The I2C bus on my board is just using GPIO pins.     I could bit bang these pins before the i2c drivers are loaded, in the arch initcalls.   Are there any functions in the kernel to do this easily?  Like optimally some libary function in the kernel to do something like:

i2c_gpio_read(int sda_pin, int scl_pin, int i2c_address, int len, void *data)

This library function may need to  have to have a spinlock or mutex to avoid conflicts on these pins.



Is there  a better solution?

--
karl

Re: reading i2c eeprom early in boot

[Jean Delvare]

Hi Karl,

On Thu, 24 Jun 2010 12:19:07 +0200, Karl Hiramoto wrote:
> My goal is to assign a MAC address stored in a I2C EEPROM to a ethernet device on an ARM  SOC so I can boot via NFS.  Reading and writing this address  works with the at24 driver after its loaded.
> 
> Currently the i2c __init call is after the ethernet devices __init, so this makes using the I2C drivers not possible, as nothing is instantiated.
> 
> So to me it looks like i have two options.
> 
> 1.  Reorder the init calls.   I'd prefer not doing this for fear of  breaking something else.
> 
> 2.  The I2C bus on my board is just using GPIO pins.     I could bit bang these pins before the i2c drivers are loaded, in the arch initcalls.   Are there any functions in the kernel to do this easily?  Like optimally some libary function in the kernel to do something like:
> 
> i2c_gpio_read(int sda_pin, int scl_pin, int i2c_address, int len, void *data)
> 
> This library function may need to  have to have a spinlock or mutex to avoid conflicts on these pins.

I don't think anything like this exists, and I sincerely hope it will
never exist. It would inevitably duplicate code that already exists,
and add confusion all around, and bugs.

> Is there  a better solution?

If parts of i2c need to be initialized earlier, then let's just do
this. That being said, i2c core initialization happens at
postcore_initcall() level since kernel 2.6.28, and many i2c bus drivers
for embedded platforms use subsys_initcall(). i2c-gpio has just need
changed to do exactly this:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b8680784875b26a2876df5959673a496d56af673
Isn't it enough to solve your problem?

-- 
Jean Delvare