Re: [PATCH 5/5] hw/i2c: Document the I2C qdev helpers

[Markus Armbruster]

Philippe Mathieu-Daudé <f4bug@amsat.org> writes:

> In commit d88c42ff2c we added new prototype but neglected to
> add their documentation. Fix that.
>
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
>  include/hw/i2c/i2c.h | 48 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 48 insertions(+)
>
> diff --git a/include/hw/i2c/i2c.h b/include/hw/i2c/i2c.h
> index c533058998..fcc61e509b 100644
> --- a/include/hw/i2c/i2c.h
> +++ b/include/hw/i2c/i2c.h
> @@ -79,8 +79,56 @@ int i2c_send_recv(I2CBus *bus, uint8_t *data, bool send);
>  int i2c_send(I2CBus *bus, uint8_t data);
>  uint8_t i2c_recv(I2CBus *bus);
>  
> +/**
> + * Create an I2C slave device on the heap.
> + * @name: a device type name
> + * @addr: I2C address of the slave when put on a bus
> + *
> + * This only initializes the device state structure and allows
> + * properties to be set. Type @name must exist. The device still
> + * needs to be realized. See qdev-core.h.
> + */
>  I2CSlave *i2c_slave_new(const char *name, uint8_t addr);
> +
> +/**
> + * Create an I2C slave device on the heap.

Suggest "Create and realize ..."

> + * @bus: I2C bus to put it on
> + * @name: I2C slave device type name
> + * @addr: I2C address of the slave when put on a bus
> + *
> + * Create the device state structure, initialize it, put it on the
> + * specified @bus, and drop the reference to it (the device is realized).
> + * Any error aborts the process.

Stick to imperative mood: Abort on error.

Do we need the sentence?  Doc comments of object_new(), qdev_new() and
i2c_slave_new() don't have it, they document *preconditions* instead,
using "must", and rely on the tacit understanding that a function may
abort or crash when its documented preconditions aren't met.  Matter of
taste, I guess.

> + */
>  I2CSlave *i2c_slave_create_simple(I2CBus *bus, const char *name, uint8_t 
> addr);
> +
> +/**
> + * i2c_slave_realize_and_unref: realize and unref an I2C slave device

Either consistently waste space for repeating the function name at the
beginning of its doc comment, or consistently don't :)

qdev_realize_and_unref()'s doc comment says "and drop a reference"
instead of "unref", because "unref" is not a word.

> + * @dev: I2C slave device to realize
> + * @bus: I2C bus to put it on
> + * @addr: I2C address of the slave on the bus
> + * @errp: error pointer

$ git-grep -h "@errp:" | sort -u
 *  @errp: pointer to Error*, to store an error if it happens
 * @errp:   error object
 * @errp: Error object
 * @errp: Error object which may be set by job_complete(); this is not
 * @errp: Error object.
 * @errp: If an error occurs, a pointer to an area to store the error
 * @errp: Output pointer for error information. Can be NULL.
 * @errp: Pointer for reporting an #Error.
 * @errp: Populated with an error in failure cases
 * @errp: a pointer to an Error that is filled if getting/setting fails.
 * @errp: a pointer to return the #Error object if an error occurs.
 * @errp: an error indicator
 * @errp: error
 * @errp: error object
 * @errp: error object handle
 * @errp: handle to an error object
 * @errp: if an error occurs, a pointer to an area to store the error
 * @errp: indirect pointer to Error to be set
 * @errp: location to store error
 * @errp: location to store error information
 * @errp: location to store error information.
 * @errp: location to store error, will be set only for exception
 * @errp: pointer to Error*, to store an error if it happens.
 * @errp: pointer to NULL initialized error object
 * @errp: pointer to a NULL initialized error object
 * @errp: pointer to a NULL-initialized error object
 * @errp: pointer to an error
 * @errp: pointer to error object
 * @errp: pointer to initialized error object
 * @errp: pointer to uninitialized error object

Aside: gotta love these two.

 * @errp: returns an error if this function fails
 * @errp: set *errp if the check failed, with reason
 * @errp: set in case of an error
 * @errp: set on error
 * @errp: unused
 * @errp: where to put errors

Plenty of choice, recommend not to invent another one :)

> + *
> + * Call 'realize' on @dev, put it on the specified @bus, and drop the
> + * reference to it. Errors are reported via @errp and by returning
> + * false.

Recommend to use a separate paragraph for the return value.  Since your
comment style resembles GTK-Doc style[*], you may just as well use it
for the return value, like this:

      Returns: %true on success, %false on failure.

By convention, it goes after the function description.

> + *
> + * This function is useful if you have created @dev via qdev_new(),
> + * i2c_slave_new() or i2c_slave_try_new() (which take a reference to
> + * the device it returns to you), so that you can set properties on it
> + * before realizing it. If you don't need to set properties then
> + * i2c_slave_create_simple() is probably better (as it does the create,
> + * init and realize in one step).
> + *
> + * If you are embedding the I2C slave into another QOM device and
> + * initialized it via some variant on object_initialize_child() then
> + * do not use this function, because that family of functions arrange
> + * for the only reference to the child device to be held by the parent
> + * via the child<> property, and so the reference-count-drop done here
> + * would be incorrect.  (Instead you would want i2c_slave_realize(),
> + * which doesn't currently exist but would be trivial to create if we
> + * had any code that wanted it.)
> + */

The advice on use is more elaborate qdev_realize_and_unref()'s.  That
one simply shows intended use.  I doubt we need more.  But as the person
who wrote qdev_realize_and_unref(), I'm singularly unqualified judging
the need ;)

>  bool i2c_slave_realize_and_unref(I2CSlave *dev, I2CBus *bus, Error **errp);
>  
>  /* lm832x.c */


[*] A style I dislike, but it's common in QEMU, so you're certainly
entitled to use it.