Question regarding usage of pdev->id and platform_data

[Grant Likely]

Hi Thomas,

On Wed, Feb 23, 2011 at 10:48:37PM +0530, Thomas Abraham wrote:
> Hi,
> 
> I am adding support for device tree based probe for the s5pv310 serial
> driver for a platform that has 4 instances of the uart port. I have
> few questions on this and appreciate any help for the following
> questions.
> 
> 1. The driver is based on the usage of pdev->id in several parts of
> the driver. But the platform_device created using the
> of_platform_bus_probe function assigns -1 to pdev->id. Is the use of
> pdev->id not advisable or is it okay if the driver assigns a pdev->id
> value during the probe.

No, the driver should *not* write a value to pdev->id at probe time.
Doing so breaks the device model.  Instead, any enumeration that the
driver cares about should be stored in the driver's private data
structure.  I would recommend modifying the driver to copy pdev->id
into its private structure.  If it is -1, then dynamically assign an
id.

> 
> 2. The driver uses pdev->dev.platform_data even after the probe is
> complete. I read in one of the posts from Grant Likely that drivers
> should not assign pdev->dev.platfrom_data.

Correct.

> Is it okay if the driver
> parse the platform data related information from the device node and
> assign it to pdev->dev.platform_data.

No.  The driver *must not* modify or assign platform_data.  That
pointer provides information to the driver, but there are memory
allocation lifecycle issues if it tries to store something there.
(Okay, I it *can* be done if the driver very carefully cleaned up
after itself; but I strongly recommend against it; that isn't what
that pointer is for)  Modifying it also causes issues with drivers
that can accept either platform_data or device tree data.

Instead, the driver should store all data it needs in its private data
structure.  That's exactly why drivers have private data structures.

g.