[PATCH v4 06/15] mei: pxp: support matching with a gfx discrete card

[Winkler, Tomas]

> >
> > > -----Original Message-----
> > > From: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> > > Sent: Friday, September 09, 2022 09:16
> > > To: Ceraolo Spurio, Daniele <daniele.ceraolospurio at intel.com>
> > > Cc: intel-gfx at lists.freedesktop.org;
> > > dri-devel at lists.freedesktop.org; Winkler, Tomas
> > > <tomas.winkler at intel.com>; Lubart, Vitaly <vitaly.lubart at intel.com>;
> > > Teres Alexis, Alan Previn <alan.previn.teres.alexis at intel.com>
> > > Subject: Re: [PATCH v4 06/15] mei: pxp: support matching with a gfx
> > > discrete card
> > >
> > > On Thu, Sep 08, 2022 at 05:16:03PM -0700, Daniele Ceraolo Spurio wrote:
> > > > From: Tomas Winkler <tomas.winkler at intel.com>
> > > >
> > > > With on-boards graphics card, both i915 and MEI are in the same
> > > > device hierarchy with the same parent, while for discrete gfx card
> > > > the MEI is its child device.
> > > > Adjust the match function for that scenario by matching MEI parent
> > > > device with i915.
> > > >
> > > > V2:
> > > >  1. More detailed commit message
> > > >  2. Check for dev is not null before it is accessed.
> > > >
> > > > Signed-off-by: Tomas Winkler <tomas.winkler at intel.com>
> > > > Signed-off-by: Daniele Ceraolo Spurio
> > > > <daniele.ceraolospurio at intel.com>
> > > > Cc: Vitaly Lubart <vitaly.lubart at intel.com>
> > > > Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> > > > Reviewed-by: Alan Previn <alan.previn.teres.alexis at intel.com>
> > > > ---
> > > >  drivers/misc/mei/pxp/mei_pxp.c | 13 ++++++++++---
> > > >  1 file changed, 10 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/misc/mei/pxp/mei_pxp.c
> > > > b/drivers/misc/mei/pxp/mei_pxp.c index 17c5d201603f..afc047627800
> > > > 100644
> > > > --- a/drivers/misc/mei/pxp/mei_pxp.c
> > > > +++ b/drivers/misc/mei/pxp/mei_pxp.c
> > > > @@ -159,17 +159,24 @@ static int mei_pxp_component_match(struct
> > > device
> > > > *dev, int subcomponent,  {
> > > >  	struct device *base = data;
> > > >
> > > > +	if (!dev)
> > > > +		return 0;
> > >
> > > How can that happen?
> > >
> > > > +
> > > >  	if (!dev->driver || strcmp(dev->driver->name, "i915") ||
> > >
> > > That's crazy to assume, but whatever :(
> > Explained here:
> > https://lore.kernel.org/all/20220418175932.1809770-2-
> wonchung at google.c
> > om/
> 
> Still crazy :(
> 
> >
> > >
> > > >  	    subcomponent != I915_COMPONENT_PXP)
> > > >  		return 0;
> > > >
> > > >  	base = base->parent;
> > > > -	if (!base)
> > > > +	if (!base) /* mei device */
> > >
> > > Why does this mean that?
> > >
> > > Where is that documented?
> > >
> > > >  		return 0;
> > > >
> > > > -	base = base->parent;
> > > > -	dev = dev->parent;
> > > > +	base = base->parent; /* pci device */
> > >
> > > Again, why is this the case?
> > >
> > > > +	/* for dgfx */
> > > > +	if (base && dev == base)
> > > > +		return 1;
> > > >
> > > > +	/* for pch */
> > > > +	dev = dev->parent;
> > >
> > > You are digging through a random device tree and assuming that you
> "know"
> > > what the topology is going to be, that feels very very fragile and
> > > ripe for problems going forward.
> >
> > I don't think it is random.
> 
> Today it is one specific way, but how do you know this always will be this
> way?
> 
> > > How do you ensure that this really is they way the tree is for ALL
> systems?
> >
> > Yes we take the topology assumption in PCI hierarchy.
> > There is a case where both GFX and MEI are in PCH and you cannot stick
> additional PCI extender or anything else there.
> > And case where MEI is child on a standalone graphics card this is set
> > in software so topology is not going to change unless we rewrite
> everything.  Be happy to hear your insights.
> 
> This is ripe to break in the future if someone makes a differently structured
> device as there is nothing forcing the current layout to always be this way by
> hardware designers.
> 
> The goal of the driver model is to NOT have these types of hard-coded
> topology assumptions because, supprise, assumptions like this have always
> come back to bite people in the end.
> 
> This is your driver, so that's fine, but really this feels very very wrong and you
> will have a hard time guaranteeing that this will always be this way for the
> next 20+ years of hardware designs.  So why not do it properly from the
> beginning and pass in the correct pointers to different places?
> 
> There is a very good reason that the driver model/core does not make it easy
> to determine what type of device a 'struct device *' is, because you shouldn't
> have to rely on that type of thing ever.  You are taking it one step further and
> just assuming that you know what the type is here, with no real way to
> ensure that this is the case.
> 
> In short, this feels like a very bad design as it is very fragile.

I believe I understand your concern but I would need to invent another 
addressing scheme to associate hw components that are already 
addressable by let say PCI hierarchy.  We've changed two subsystems for this work
components and aux bus already.  So let's have some fun in the future.

Thanks
Tomas