Hi Maxime, Thanks for your reply. On 9/28/21 4:06 AM, Maxime Ripard wrote: > On Tue, Sep 28, 2021 at 02:46:39AM -0500, Samuel Holland wrote: >> On 9/9/21 3:45 AM, Maxime Ripard wrote: >>> On Fri, Sep 03, 2021 at 10:21:13AM -0500, Samuel Holland wrote: >>>> On 9/3/21 9:50 AM, Maxime Ripard wrote: >>>>> And since we can register all those clocks at device probe time, we >>>>> don't really need to split the driver in two (and especially in two >>>>> different places). The only obstacle to this after your previous series >>>>> is that we don't have of_sunxi_ccu_probe / devm_sunxi_ccu_probe >>>>> functions public, but that can easily be fixed by moving their >>>>> definition to include/linux/clk/sunxi-ng.h >>>> >>>> Where are you thinking the clock definitions would go? We don't export >>>> any of those structures (ccu_mux, ccu_common) or macros >>>> (SUNXI_CCU_GATE_DATA) in a public header either. >>> >>> Ah, right... >>> >>>> Would you want to export those? That seems like a lot of churn. Or would >>>> we put the CCU descriptions in drivers/clk/sunxi-ng and export a >>>> function that the RTC driver can call? (Or some other idea?) >>> >>> I guess we could export it. There's some fairly big headers in >>> include/linux/clk already (tegra and ti), it's not uAPI and we do have >>> reasons to do so, so I guess it's fine. >>> >>> I'd like to avoid having two drivers for the same device if possible, >>> especially in two separate places. This creates some confusion since the >>> general expectation is that there's only one driver per device. There's >>> also the fact that this could lead to subtle bugs since the probe order >>> is the link order (or module loading). >> >> I don't think there can be two "struct device"s for a single OF node. > > That's not what I meant, there's indeed a single of_node for a single > struct device. If we dig a bit into the core framework, the most likely > scenario is that we would register both the RTC and clock driver at > module_init, and with the device already created with its of_node set > during the initial DT parsing. > > We register our platform driver using module_platform_driver, which > expands to calling driver_register() at module_init(), setting the > driver bus to the platform_bus in the process (in > __platform_driver_register()). > > After some sanity check, driver_register() calls bus_add_driver(), which > will call driver_attach() if drivers_autoprobe is set (which is the > default, set into bus_register()). > > driver_attach() will, for each device on the platform bus, call > __driver_attach(). If there's a match between that device and our driver > (which is evaluated by platform_match() in our case), we'll call our > driver probe with that device through driver_probe_device(), > __driver_probe_device() and finally really_probe(). > > However, at no point in time there's any check about whether that device > has already been bound to a driver, nor does it create a new device for > each driver. I would expect this to hit the: if (dev->driver) return -EBUSY; in __driver_probe_device(), or fail the "if (!dev->driver)" check in __driver_attach() for the async case, once the first driver is bound. > So this means that, if you have two drivers that match the > same device (like our clock and RTC drivers), you'll have both probe > being called with the same device, and the probe order will be defined > by the link order. Worse, they would share the same driver_data, with > each driver not being aware of the other. This is incredibly fragile, > and hard to notice since it goes against the usual expectations. > >> So if the CCU part is in drivers/clk/sunxi-ng, the CCU "probe" >> function would have to be called from the RTC driver. > > No, it would be called by the core directly if there's a compatible to > match. > >> Since there has to be cooperation anyway, I don't think there would be >> any ordering problems. > > My initial point was that, with a direct function call, it's both > deterministic and obvious. I believe I did what you are suggesting for v2. From patch 7: --- a/drivers/rtc/rtc-sun6i.c +++ b/drivers/rtc/rtc-sun6i.c @@ -683,6 +684,10 @@ static int sun6i_rtc_probe(struct platform_device *pdev) chip->base = devm_platform_ioremap_resource(pdev, 0); if (IS_ERR(chip->base)) return PTR_ERR(chip->base); + + ret = sun6i_rtc_ccu_probe(&pdev->dev, chip->base); + if (ret) + return ret; } platform_set_drvdata(pdev, chip); >>> And synchronizing access to registers between those two drivers will be >>> hard, while we could just share the same spin lock between the RTC and >>> clock drivers if they are instanciated in the same place. >> >> While the RTC driver currently shares a spinlock between the clock part >> and the RTC part, there isn't actually any overlap in register usage >> between the two. So there doesn't need to be any synchronization. > > I know, but this was more of a social problem than a technical one. Each > contributor and reviewer in the future will have to know or remember > that it's there, and make sure that it's still the case after any change > they make or review. > > This is again a fairly fragile assumption. Yeah, I agree that having a lock that is only sometimes safe to use with certain registers is quite fragile. Would splitting the spinlock in rtc-sun6i.c into "losc_lock" (for the clock provider) and "alarm_lock" (for the RTC driver) make this distinction clear enough? Eventually, I want to split up the struct between the clock provider and RTC driver so it's clear which members belong to whom, and there's no ugly global pointer use. Maybe I should do this first? Regards, Samuel
