Re: [PATCH 1/2] drm/msm/dsi: Use correct pm_runtime_put variant during host_init

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Oct 10, 2017 at 08:58:03AM +0530, Archit Taneja wrote:
> 
> 
> On 10/06/2017 07:32 PM, Daniel Vetter wrote:
> > On Fri, Oct 06, 2017 at 04:27:06PM +0530, Archit Taneja wrote:
> > > The DSI runtime PM suspend/resume callbacks check whether
> > > msm_host->cfg_hnd is non-NULL before trying to enable the bus clocks.
> > > This is done to accommodate early calls to these functions that may
> > > happen before the bus clocks are even initialized.
> > > 
> > > Calling pm_runtime_put_autosuspend() in dsi_host_init() can result in
> > > racy behaviour since msm_host->cfg_hnd is set very soon after. If the
> > > suspend callback happens too late, we end up trying to disable clocks
> > > that were never enabled, resulting in a bunch of WARN_ON splats.
> > 
> > Sounds like the correct fix here is to block autosuspend until after
> > everything is set up, including bus clocks. This patch just makes the race
> > harder to hit in practice ...
> 
> Thanks for the review. pm_runtime_put_sync() ensures that the suspend handler
> is called in the same context as that of the caller, right? msm_host->cfg_hnd
> is set to a non-NULL value only when we return from dsi_get_config(). The race
> would never happen in this case.
> 
> This call is a one time thing during DSI probe, we do a pm_runtime_get_sync()
> just so that we can read the block revision number. Once we have the revision
> number, we look at an internal table which maintains IP version specific
> resources, like what bus clocks to get, etc. Having pm_runtime_put_autosuspend()
> here didn't help much anyway.

Maybe this stuff is different on arm than on pci, but on x86 you have to
explicitly enable autosuspend. Before that, the device stays on.

What I mean with properly fixing this, is to delay enabling of autosuspend
until you're fully set up. Which then allows you to drop the check for
clocks and other stuff.

For i915, what we do is hold an artificial runtime pm reference that we
grab first thing in ->probe and drop only once everything is fully set up
(including asynchronous workers that load firmware). That way we make sure
that our runtime pm code never sees a partially initiliazed driver.

Doing something similar sounds best too, i.e. instead of dropping the
runtime pm reference here, only drop it once dsi_get_config has been
called. Pronto, no race, no need for special functions.
-Daniel

> 
> Archit
> 
> 
> > -Daniel
> > 
> > > 
> > > Use pm_runtime_put_sync() so that the suspend callback is called
> > > immediately.
> > > 
> > > Reported-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
> > > Signed-off-by: Archit Taneja <architt@codeaurora.org>
> > > ---
> > >   drivers/gpu/drm/msm/dsi/dsi_host.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> > > index dbb31a014419..deaf869374ea 100644
> > > --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> > > +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> > > @@ -248,7 +248,7 @@ static const struct msm_dsi_cfg_handler *dsi_get_config(
> > >   	clk_disable_unprepare(ahb_clk);
> > >   disable_gdsc:
> > >   	regulator_disable(gdsc_reg);
> > > -	pm_runtime_put_autosuspend(dev);
> > > +	pm_runtime_put_sync(dev);
> > >   put_clk:
> > >   	clk_put(ahb_clk);
> > >   put_gdsc:
> > > -- 
> > > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> > > hosted by The Linux Foundation
> > > 
> > > _______________________________________________
> > > dri-devel mailing list
> > > dri-devel@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > 
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch