Re: hung bootup with "drm/radeon/kms: move radeon KMS on/off switchout of staging."

[Ingo Molnar]

* Alex Deucher <alexdeucher@xxxxxxxxx> wrote:

> On Thu, Feb 4, 2010 at 1:12 PM, Ingo Molnar <mingo@xxxxxxx> wrote:
> >
> > * Matthew Garrett <mjg59@xxxxxxxxxxxxx> wrote:
> >
> >> On Thu, Feb 04, 2010 at 06:54:45PM +0100, Ingo Molnar wrote:
> >>
> >> > But you could claim that it's not a regression because 1) technically the
> >> > code got introduced in drivers/staging/, and staging drivers are not on
> >> > the regression list 2) the Kconfig value is default-off so it can only
> >> > harm those who got lured by a new Kconfig value popping up in -rc7 in a
> >> > well working driver they already have enabled.
> >> >
> >> > So the moving of driver functionality from drivers/staging/ to drivers/
> >> > is a grey area it appears. Wouldnt it have been better to do this in the
> >> > next merge window, as all other drivers do? It's not new hardware
> >> > enablement either, it's feature enablement for an existing driver.
> >>
> >> The reason the option was in staging (as has been mentioned before) was
> >> because the ABI wasn't felt to be stable enough. Upstream is now willing to
> >> commit to that stability, so now seems as good a time to move it as any.
> >> There's no code change and there's no default configuration change, so I
> >> really can't see any way that it can be classed as a regression.
> >
> > But that argument in essence renders the regression policy meaningless for
> > such code: just about any new driver feature under the sun could be shaped as
> > a Kconfig option, introduced via a drivers/staging Kconfig entry, and then
> > activated via a twoliner commit in a later -rc.
> >
> > IMHO the point of tracking regressions is to reduce the bugginess of the
> > kernel and thus to help users, not to give ground for legalistic arguments.
> >
> > There _are_ common-sense exceptions from the regression rules, such as the
> > introduction of a new piece of hardware that was previously unsupported
> > (hence there's no expectation of stability) - but the tweaking of an
> > existing, widely used driver (even if the new opion is default-off) hardly
> > seems to qualify for that.
> >
> 
> This is a completely new driver.  It's only part of the existing drm for 
> compatibility reasons.  It requires an entirely different graphics stack 
> above it and works very differently from the old drm stack.

Will the user know? IMHO what matters in the end is user expectation.

Lets walk through what a current kernel tester of the drm/radeon driver sees 
when he types 'make oldconfig' after installing the (to-be-released) .33-rc7 
kernel. Firstly, the user with a brand-new distro already has this enabled:

  CONFIG_DRM_RADEON=y

and knows the driver, and it performs adequately. Then in -rc7 he gets a new 
option:

  ATI Radeon (DRM_RADEON) [Y/n/?] y
    Enable modesetting on radeon by default (DRM_RADEON_KMS) [N/y/?] (NEW) 

The user might easily go: "Hey this is a driver i already have, and there's a 
new sub-option for this well-working driver. Sure, enable it, these kernel 
folks know what they are doing and i rarely see any crashes past -rc2 
kernels."

Does this new option tell him what you just told me, that:

 > This is a completely new driver.  It's only part of the existing drm for 
 > compatibility reasons.  It requires an entirely different graphics stack 
 > above it and works very differently from the old drm stack.

?

it doesnt. Even if he types '?', it tells:

  CONFIG_DRM_RADEON_KMS:

  Choose this option if you want kernel modesetting enabled by default,
  and you have a new enough userspace to support this. Running old
  userspaces with this enabled will cause pain.

The user will likely go "cool I have a fresh distro with recent Xorg, lets 
try it".

If this is really a brand new driver essentially fresh out of 
drivers/staging/ in -rc7 you should be abundantly clear about that in the 
Kconfig help text - that it's a brand new driver and that it might crash on 
bootup, etc.

Thanks,

	Ingo

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