linux-kernel - Re: [PATCH v1 2/2] drm/tegra: Support disabled CONFIG

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Date:   Thu, 14 Dec 2017 22:41:24 +0100
From:   Lucas Stach <dev@...xeye.de>
To:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] drm/tegra: Support disabled CONFIG_PM

Am Montag, den 11.12.2017, 18:26 +0300 schrieb Dmitry Osipenko:
> On 11.12.2017 17:27, Thierry Reding wrote:
> > On Mon, Dec 11, 2017 at 04:53:56PM +0300, Dmitry Osipenko wrote:
> > > On 11.12.2017 13:13, Thierry Reding wrote:
> > > > On Mon, Dec 11, 2017 at 02:19:44AM +0300, Dmitry Osipenko
> > > > wrote:
> > > > > Add manual HW power management to drivers probe/remove in
> > > > > order to
> > > > > not fail in a case of runtime power management being disabled
> > > > > in kernel
> > > > > config.
> > > > > 
> > > > > Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> > > > > ---
> > > > >  drivers/gpu/drm/tegra/dc.c   | 164
> > > > > +++++++++++++++++++++++++++----------------
> > > > >  drivers/gpu/drm/tegra/dsi.c  | 138 +++++++++++++++++++++--
> > > > > -------------
> > > > >  drivers/gpu/drm/tegra/hdmi.c |  90 ++++++++++++++++--------
> > > > >  drivers/gpu/drm/tegra/sor.c  | 103 +++++++++++++++++------
> > > > > ----
> > > > >  4 files changed, 310 insertions(+), 185 deletions(-)
> > > > 
> > > > I think that's the wrong way around. We unconditionally select
> > > > PM on
> > > > 64-bit ARM already, and I think we should do the same on 32-bit 
> > > > ARM.
> > > > There's really no excuse not to enable runtime PM these days.
> > > 
> > > What is the rational behind enabling PM unconditionally? It is
> > > actually a very
> > > useful debug feature when there is something wrong with the PM.
> > > It looks like
> > > Tegra DRM driver is the only driver on Tegra that doesn't work
> > > properly with PM
> > > being disabled. Please, let's just fix it.
> > 
> > What's useful about disabling PM? The problem with allowing !PM is
> > that
> > it adds one more combination that needs to be build- and runtime
> > tested.
> 
> As I already stated, disabling PM is very useful for debugging when
> system hangs
> unexpectedly. I found it very helpful several times.

This assumes that the bootloader/firmware left the power domains
powered up. Without PM_GENERIC_DOMAINS, which depends on CONFIG_PM the
kernel has no means to control the state of the power domains. Probe
deferral based on the power domain will also not work, so driver may
probe and try to access power-gated devices, leading to system hangs in
the common case.

Regards,
Lucas