Unable to boot mainline on snow chromebook since 3.15

Thierry Reding thierry.reding at gmail.com
Mon Sep 29 05:57:19 PDT 2014

On Wed, Sep 10, 2014 at 03:56:16PM +0100, Grant Likely wrote:
> On Wed, Sep 10, 2014 at 3:31 PM, Mark Brown <broonie at kernel.org> wrote:
> > On Wed, Sep 10, 2014 at 06:06:46AM -0700, Olof Johansson wrote:
> >> On Mon, Sep 8, 2014 at 12:40 PM, Grant Likely <grant.likely at secretlab.ca> wrote:
> >
> >> > Well, lets see... We've got a real user complaining about a platform
> >> > that used to work on mainline, and no longer does. The only loophole
> >> > for ignoring breakage is if there nobody cares that it is broken. That
> >> > currently isn't the case. So even though it's based on a patch that
> >> > has "DO NOT SUBMIT" in large friendly letters on the front cover, it
> >> > doesn't change the situation that mainline has a regression.
> >
> >> Yeah, I'm with you on this Grant, it doesn't matter what the patch is
> >> labelled as.
> >
> >> One way to deal with this could be to add a quirk at boot time --
> >> looking for the simplefb and if found, modifies the regulators to keep
> >> them on. That'd go in the kernel, not in firmware.
> >
> > Well, we should also be fixing simplefb to manage the resources it uses
> > though that doesn't clean up after the broken DTs that are currently
> > deployed.
> >
> > As well as the regulators we'll also need to fix the clocks.  If we're
> > going to start adding these fixups perhaps we want to consider having a
> > wrapper stage that deals with rewriting DTs prior to trying to use them?
> > I'm not sure if it makes much difference but there's overlap with other
> > tools like the ATAGs conversion wrapper and building separately would
> > let the fixup code run early without directly going into the early init
> > code (which seems a bit scary).
> >
> >> Much better would have been if the DRM changes worked when they
> >> landed, so that the migration form simplefb to drm was invisible to
> >> the user. Or at least, to get them working ASAP since they're still
> >> broken. :(
> >
> > As far as I can tell the problem here is coming from the decision to
> > have simplefb use resources without knowing about them - can we agree
> > that this is a bad idea?
> No, I don't think we can... there is a certain amount of "firmware got
> things working for us, and we're going to use it for a while" that is
> absolutely reasonable. simplefb is a good example, but there are
> certainly others.
> I /do/ think it would be better for the simplefb data to get embedded
> or linked into the node of the graphics controller so that it can be
> torn down appropriately, and we need a rule for how long boot-state
> can be considered valid so that a proper driver can either reserve the
> resources for a given SoC, or do a full handoff from the simplefb.
> Even without that though, we need to be able to handle the case of an
> anonymous simplefb node with no regulator information. If that means
> the default simplefb behaviour is to inhibit runtime pm on all
> resources until a real driver show up, then that might just be what we
> need to do.
> Two things should probably be changed from the current setup. 1)
> simplefb shouldn't be a platform driver. It is a boot thing that
> handles initial state from the graphics chip. By implementing it as a
> platform driver, it prevents the real driver from binding to the real
> device if the simplefb data embedded into it. 2) make sure that an SoC
> driver can protect the needed resources before they are automatically
> disabled. Either by putting them in an earlier initcall, or handling
> it in the subsystem code. I don't know enough about the regulator and
> clock runtime PM to know what the best way to do this is.

I posted a patch[0] earlier to do this for the clock framework in "that
other thread". The idea is that shim drivers for these types of firmware
devices can tell the various subsystems that they might need resources
that aren't explicitly requested. The current implementation simply uses
the existing infrastructure already present for the clk_ignore_unused
command-line argument and allows drivers to declare this requirement. It
also allows these drivers to retire the request once they've properly
handed off to the real driver.

Something similar could be done other frameworks.

One of the objections to that in the other thread is that it won't
prevent clocks from being disabled if some other driver was using those
same clocks and doing a clk_enable()/clk_disable() on them. But quite
frankly I don't think that's something we need to worry about.

Though there are two cases: one is to use simplefb as a means to have
early boot messages on a graphical display (and optionally hand off to a
real driver). The other is to use simplefb as the only framebuffer
driver until a proper driver has been implemented. The latter would have
the disadvantage of not allowing unused resources from being garbage
collected at all. Then again, I don't think power consumption is going
to be a very big issue on hardware where no proper display driver is


[0]: http://lists.infradead.org/pipermail/linux-arm-kernel/2014-September/291295.html
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