[dv/push_pull] Make sure that ack_o is always reset

[glaserf]

This PR addresses a rare issue where ack_o of the push_pull_interface could get stuck high when asserted at the exact same time that the interface is reset. For the valid/ready flavor of the interface, a similar mechanism is already there.

[marnovandermaas]

I think this looks reasonable to me. Just for my understanding can you point me to the equivalent race check for the valid/ready interface? Also, what task is the one that is racing against the drive task?

[martin-velay]

It sounds like a good catch, thanks for the fix!

[glaserf]

I think this looks reasonable to me. Just for my understanding can you point me to the equivalent race check for the valid/ready interface? Also, what task is the one that is racing against the drive task?

Sure:

opentitan/hw/dv/sv/push_pull_agent/push_pull_driver_lib.sv

Lines 136 to 139 in e65c354

	// In case there is race condition between the logic above and reset_signals task.
	// We always set the valid_int again to 0 to make sure the data comes out of reset is not
	// valid.
	if (cfg.in_reset) `CB.valid_int <= '0;

Maybe we can update the comment there, I think it says the same, just in a bit different words.

As for the task, I am not sure where the "reset_task" in question resides, maybe someone from the DV team can comment?

[martin-velay]

This reset_signals is defined in the dv_base_driver and implemented in each child.

I cannot see the implementation for req_int in pull_host_driver, if we follow the same logic I guess we should do the same for this variable too.

And at some point we plan to improve the DV base library to be more "resetable" and probably change slightly the logic.

[marnovandermaas]

Thanks for the explanation, looks good to me.

[glaserf]

Thanks for your feedback. I now added handling of this case to all four flavors of this protocol and reworded the pre-existing comment.

@rswarbrick can you please have a look and get it merged, if you don't see any issues?

[martin-velay]

It looks good to me, thanks for the latest changes!