Hi Daniel, 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 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 :-) That's clear, but I'm still not sure why we need a bus for this :-) I'm not very enthousiastic about that. Some of our problems come from the fact we need to coordinate many devices, adding new ones hardly seem to be a solution to that. Granted, the components framework doesn't work nicely, and is in dire need of love (and documentation), or possibly better a complete replacement. I'll try to review the series this week and see if alternatives would be possible. -- Regards, Laurent Pinchart
