On Mon, Jan 31, 2022 at 05:34:26PM +0100, Greg Kroah-Hartman wrote: > On Mon, Jan 31, 2022 at 04:15:09PM +0100, Daniel Vetter wrote: > > On Mon, Jan 31, 2022 at 2:48 PM Greg Kroah-Hartman > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Thu, Jan 27, 2022 at 12:01:08PM -0800, Stephen Boyd wrote: > > > > The component framework only provides 'bind' and 'unbind' callbacks to > > > > tell the host driver that it is time to assemble the aggregate driver > > > > now that all the components have probed. The component framework doesn't > > > > attempt to resolve runtime PM or suspend/resume ordering, and explicitly > > > > mentions this in the code. This lack of support leads to some pretty > > > > gnarly usages of the 'prepare' and 'complete' power management hooks in > > > > drivers that host the aggregate device, and it fully breaks down when > > > > faced with ordering shutdown between the various components, the > > > > aggregate driver, and the host driver that registers the whole thing. > > > > > > > > In a concrete example, the MSM display driver at drivers/gpu/drm/msm is > > > > using 'prepare' and 'complete' to call the drm helpers > > > > drm_mode_config_helper_suspend() and drm_mode_config_helper_resume() > > > > respectively, so that it can move the aggregate driver suspend/resume > > > > callbacks to be before and after the components that make up the drm > > > > device call any suspend/resume hooks they have. This only works as long > > > > as the component devices don't do anything in their own 'prepare' and > > > > 'complete' callbacks. If they did, then the ordering would be incorrect > > > > and we would be doing something in the component drivers before the > > > > aggregate driver could do anything. Yuck! > > > > > > > > Similarly, when trying to add shutdown support to the MSM driver we run > > > > across a problem where we're trying to shutdown the drm device via > > > > drm_atomic_helper_shutdown(), but some of the devices in the encoder > > > > chain have already been shutdown. This time, the component devices > > > > aren't the problem (although they could be if they did anything in their > > > > shutdown callbacks), but there's a DSI to eDP bridge in the encoder > > > > chain that has already been shutdown before the driver hosting the > > > > aggregate device runs shutdown. The ordering of driver probe is like > > > > this: > > > > > > > > 1. msm_pdev_probe() (host driver) > > > > 2. DSI bridge > > > > 3. aggregate bind > > > > > > > > When it comes to shutdown we have this order: > > > > > > > > 1. DSI bridge > > > > 2. msm_pdev_shutdown() (host driver) > > > > > > > > and so the bridge is already off, but we want to communicate to it to > > > > turn things off on the display during msm_pdev_shutdown(). Double yuck! > > > > Unfortunately, this time we can't split shutdown into multiple phases > > > > and swap msm_pdev_shutdown() with the DSI bridge. > > > > > > > > Let's make the component_master_ops into an actual device driver that has > > > > probe/remove/shutdown functions. The driver will only be bound to the > > > > aggregate device once all component drivers have called component_add() > > > > to indicate they're ready to assemble the aggregate driver. This allows > > > > us to attach shutdown logic (and in the future runtime PM logic) to the > > > > aggregate driver so that it runs the hooks in the correct order. > > > > > > I know I asked before, but I can not remember the answer. > > > > > > This really looks like it is turning into the aux bus code. Why can't > > > you just use that instead here for this type of thing? You are creating > > > another bus and drivers for that bus that are "fake" which is great, but > > > that's what the aux bus code was supposed to help out with, so we > > > wouldn't have to write more of these. > > > > > > So, if this really is different, can you document it here so I remember > > > next time you resend this patch series? > > > > aux takes a device and splits it into a lot of sub-devices, each with > > their own driver. > > > > This takes a pile of devices, and turns it into a single logical > > device with a single driver. > > > > So aux is 1:N, component is N:1. > > > > And yes you asked this already, I typed this up already :-) > > Ok, thanks. But then why is a bus needed if there's a single driver? > I guess a bus for that driver? So one bus, one driver, and one device? Maybe? I have honestly no idea how this should be best modelled in the linux device model. > I think we need better documentation here... https://dri.freedesktop.org/docs/drm/driver-api/component.html?highlight=component_del#component-helper-for-aggregate-drivers There's a kerneldoc overview for component, but it's for driver authors that want to use component to glue different hw pieces into a logical driver, so it skips over these internals. And I'm honestly not sure how we want to leak implementation internals like the bus/driver/device structure ot users of component.c. -Daniel > > thanks, > > greg k-h -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
