Hi Alan, > > On Tue, Jan 16, 2024 at 05:44:47AM +0000, Xu Yang wrote: > > Hi Alan, > > > > > > > > Those of us unfamiliar with this code need you to explain a lot more > > > about what's going on. > > > > > > On Mon, Jan 15, 2024 at 03:02:06AM +0000, Xu Yang wrote: > > > > Taking below diagram as example: > > > > > > > > ci_hdrc.0 register usb get tcpm_port > > > > (driver: ci_hdrc) ---------> role <---- (driver: tcpm) > > > > ^ ^ switch | ^ > > > > | | | | > > > > +1| | +1 | |+1 > > > > | +-------------------------------------+ | > > > > | | > > > > 4c200000.usb 1-0050 > > > > (driver: ci_hdrc_imx) (driver: tcpci) > > > > > > > > 1. Driver ci_hdrc_imx and tcpci are built as module at least. > > > > 2. When module ci_hdrc_imx is loaded, it will register ci_hdrc.0 device > > > > and try to get ci_hdrc module's reference. > > > > > > This is very confusing. Normally, a device is registered by the parent > > > module and its driver belongs in the child module. When the child > > > module is loaded it automatically gets a reference to the parent module, > > > because it calls functions that are defined in the parent. I don't know > > > of any cases where a parent module takes a reference to one of its > > > children -- this would make it impossible to unload the child module! > > > > > > In your diagram I can't tell whether ci_hdrc is the parent module and > > > ci_hdrc_imx is the child, or vice versa. I'll guess that ci_hdrc_imx is > > > the child, since it the one which gets a reference to the other. But > > > now we have the ci_hdrc.0 device being registered by the child module > > > and its driver belonging to the parent module, which is backward! > > > > > > Very difficult to understand. Please explain more fully. > > > > I checked again and let me correct the words. > > > > 2. When module ci_hdrc_imx is loaded, it will register ci_hdrc.0 device. > > At the same time, the reference of module ci_hdrc is added by 1 > > automatically due to ci_hdrc_imx calls some functions in module ci_hdrc. > > ci_hdrc will register usb-role-switch device. > > > > Therefore, module ci_hdrc_imx depends on module ci_hdrc. Device ci_hdrc.0 > > is a child of 4c200000.usb. > > And ci_hdrc_imx is a child module of ci_hdrc. Got it. > > > > > ci_hdrc will register > > > > usb-role-switch device. > > > > 3. When module tcpci is loaded, it will register tcpm port device and try > > > > to get tcpm module's reference. The tcpm module will get usb-role-switch > > > > which is registered by ci_hdrc. > > > > > > What do you mean by "will get"? Do you mean that tcpm will become the > > > driver for the usb_role_switch device? Or do you mean that it simply > > > calls get_device(&usb_role_switch)? > > > > > > If the latter is the case, how does the tcpm driver learn the address of > > > usb_role_switch in the first place? > > > > Via > > port->role_sw = usb_role_switch_get(port->dev) > > or > > port->role_sw = fwnode_usb_role_switch_get(tcpc->fwnode). > > > > The usb controller will register usb-role-swtich device to the global list > > of usb_role class. The fwnode of usb-role-swtich device is also set to usb > > controller's fwnode. Initially, a fwnode graph between usb controller of > > node and tcpm connector node had already been established. These two > > functions will find usb-role-swtich device based on this fwnode graph > > and fwnode matching. > > If usb_role_switch_get() gives away references to the usb_role_switch > device, it should have a way to take those references back. But I guess > it doesn't. > > > After usb-role-switce device is found, these two > > functions will call: try_module_get(sw->dev.parent->driver->owner). > > You mean usb_role_switch_get() and fwnode_usb_role_switch_get() do this? Yes. > > > Here sw->dev.parent is device ci_hdrc.0. sw->dev.parent->driver is ci_hdrc. > > > > > > > > > In current design, tcpm will also try to > > > > get ci_hdrc module's reference after get usb-role-switch. > > > > > > This might be a bug. There should not be any need for the tcpm driver > > > to take a reference to the ci_hdrc module. But there should be a way > > > for the ci_hdrc driver to notify tcpm when the usb_role_switch device is > > > about to be unregistered. If tcpm is usb_role_switch's driver then this > > > notification happens automatically, by means of the .remove() callback. > > > > I'm not the designer of usb_role class driver. Not sure if this is needed to get > > module reference of its parent device's driver. Maybe need @heikki's input. > > > > @heikki.krogerus, can you give some explanations? > > Yes, please, some additional explanation would help. > > > > > 4. Due to no modules depend on ci_hdrc_imx, ci_hdrc_imx can be manually > > > > unloaded. Then device ci_hdrc.0 will be removed by ci_hdrc_imx and > > > > device usb-role-switch is also unregistered. > > > > > > At this point, tcpm should learn that it has to drop all its references > > > to usb_role_swich. Since the module which registered usb_role_switch > > > isn't tcpm's ancestor, tcpm must not keep _any_ references to the device > > > after it is unregistered. > > > > Yes, I also think so. > > > > > > > > Well, strictly speaking that's not true. By misusing the driver model, > > > tcpm could keep a reference to the ci_hdrc module until it was finished > > > using usb_role_switch. Is that what you are trying to do? > > > > No, I'm trying to get module reference of ci_hdrc_imx too. Then, > > ci_hdrc_imx can't be unloaded before tcpci module unloaded. > > You shouldn't do this. Users should be able to unload ci_hdrc_imx > whenever they want, even if tcpci is still loaded. Okay. Understand. > > > > > 5. Then, if I try to unload module tcpci, "NULL pointer dereference" > > > > will be shown due to below code: > > > > > > > > module_put(sw->dev.parent->driver->owner); > > I forgot to ask: What function makes this call? Is it part of the > usb_role class driver? usb_role_switch_put() do this. Yes, it's a function of usb_role class driver. > > > > > parent->driver is NULL at this time. > > > > > > What is dev at this point? And what is dev.parent? And what did > > > dev.parent->driver used to be before it was set to NULL? > > > > Here sw->dev is usb-role-switch device. sw->dev.parent is ci_hdrc.0 device. > > sw->dev.parent->driver was ci_hdrc. > > Which is now gone, right. I understand. > > Let's see what Heikki has to say. > > However, assuming he wants to continue misusing the driver model in this > way, what you should do is add a new field to sw, where you will store > sw->dev.parent->driver.owner at the time of the try_module_get() call > (but only if the call succeeds!). Then when the module_put() call runs, > have it use the value stored in this new field instead of dereferencing > sw->dev.parent->driver.owner. It sounds like a better solution. Thanks for the suggestion! Best Regards, Xu Yang
