Re: Trying to write data to i2c bus

[Corey Minyard <minyard@acm.org>]

On Mon, Feb 19, 2024 at 01:49:44PM +0000, Paz Offer wrote:
> Hi,
> 
> I am new to QEMU development, so please excuse if I my direction here is wrong:
> 
> I am trying to implement an i2c slave device.
> My device should be able to read/write data from its i2c bus.
> 
> I defined my device-state object like so:
> 
>     typedef struct {
>           I2CSlave i2c;
>           void     *my_data;
> 
>     }   MyI2CSlave;
> 
> 
> In my implementation occasionally I may have to send data on the bus, due to an internal event on my side.
> For this I implemented the following code:
> 
>     //  Get bus pointer:
>     BusState *parentBus = qdev_get_parent_bus(DEVICE(&obj->i2c));
>     I2CBus   *i2cBus    = I2C_BUS(parentBus);
>       
>     //  Try to send data on bus:  
>     if (i2c_start_send(i2cBus, address)) {
>         //  error?
>         return;
>     }
>     for (int i = 0; i < size; i++) {
>         i2c_send(i2cBus, data[i]);
>     }
>     i2c_end_transfer(i2cBus);
> 
>       
> The problem is that 'i2c_start_send()' always fails here:
> 
>     if (QLIST_EMPTY(&bus->current_devs)) {
>         return 1;
>     }
> 
> The member 'i2cBus->current_devs.lh_first' is always null.
> 
> I will add that in my QEMU execution I specify the bus 'aspeed.i2c.bus.0' to be used with my device.
> In my 'realize' method I can see that a bus is connected to my device, as 'qdev_get_parent_bus()' does return a valid pointer.
> 
> My question:
> 1. Am I missing some initialization for the bus?
> 2. Is there other way to send data on the i2c bus, assuming it can happen anytime due to an internal event on my device side?

You are missing a lot of stuff, but let's start with basics...

First of all, this is not how i2c generally works.  Generally you have a
bus master, the host, that reads and writes to slave devices on the bus.
The devices on the bus don't asynchronously send data to the host.

That said, you can have multiple bus masters on the bus.  If that's what
you are doing, what are you sending your data to?  You have to have
something that will receive it.

If you have some slave device that has data it's holding for the host,
the host has to fetch it.  You could have an interrupt that comes from
the slave device saying it has data, and there's something called SMBus
alert that can consolidate interrupts (though it's not implemented in
QEMU at the moment).

Are you simulating some real device here?

-corey

> 
> Thanks for any tip,
> Pazo