Re: [PATCH v6 02/35] component: Introduce the aggregate bus_type

[Daniel Vetter]

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