Re: Analog Digital Converter - ADS1256 extreme newbie questions

[Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org>]

On Fri, Sep 10, 2010 at 10:11:35AM -0300, Flávio Alberto Lopes Soares wrote:
> Hello all,
> I'm working 9 years as a Linux C/C++ programmer but I spent 99,9% of this
> time working in user space programming, now I need to write a driver to use
> the ADS1256 Analog-Digital converter (
> http://focus.ti.com/docs/prod/folders/print/ads1256.html ) in an ARM board
> using S3C2440 processor, my lecture start point
> was Documentation/spi/spi-sumary from 2.6.32.2 kernel tree version and
> drivers/misc/eeprom/at25.c code (this is an eeprom driver but I want look
> the SPI general view), reading the text and the code I found some conceptual
> doubts, my initial idea was create a character device driver and attach the
> device read operation with a ADS1256 start lecture and pass the ADC channel
> data to this device, this concept is correct ? There's some standard
> interface to do this if my idea is wrong ?
> 
> Looking in at25.c in at25_probe(struct spi_device *spi) function this
> "struct spi_device *spi" parameter represents the S3C2440 SPI controller and
> came from SPI core subsystem ? If yes how the SPI controller knows that this
> is an eeprom device in the snippet bellow ?
> ...
> const struct spi_eeprom *chip;
> ...
> /* Chip description */
> chip = spi->dev.platform_data;
> 
> who provides this platform_data structure and who knows it must be a struct
> spi_eeprom object ?

Hi Flávio.

The platform_data structure is provided by the code that registers the
spi_device; usually this is some form of machine specific platform
code in an arch/*/ subdirectory.  The driver knows that platform_data
points to the drivers structure because the code that registered the
device must be responsible to provide the right kind of platform_data
pointer for the driver that will be bound to it.

For an example, look at arch/arm/mach-omap2/board-3430sdp.c which has
an spi_board_info structure which passes both the requested driver
(.modalias = "ads7846") and the data structure required by the ads7846
driver (.platform_data = "&tsc2046_config").

The matching driver is drivers/input/touchscreen/ads7846.c

You'll notice that board-3430sdp.c and ads7846.c both use the
ads7846_platform_data structure.

g.

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