Re: [PATCH v5 0/8] drm: rcar-du: Add Color Management Module (CMM)

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Laurent,

On Fri, Jun 12, 2020 at 5:51 PM Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> On Fri, Jun 12, 2020 at 05:36:07PM +0200, Eugeniu Rosca wrote:
> > On Fri, Jun 12, 2020 at 06:10:05PM +0300, Laurent Pinchart wrote:
> > > On Fri, Jun 12, 2020 at 05:00:32PM +0200, Jacopo Mondi wrote:
> > > > On Tue, Jun 09, 2020 at 04:29:59PM +0200, Eugeniu Rosca wrote:
> > > > > On Sun, Jun 07, 2020 at 05:41:58AM +0300, Laurent Pinchart wrote:
> > > > > > Note that the CMM driver is controlled by the DU driver. As the DU
> > > > > > driver will reenable the display during resume, it will call
> > > > > > rcar_du_cmm_setup() at resume time, which will reprogram the CMM. There
> > > > > > should thus be no need for manual suspend/resume handling in the CMM as
> > > > > > far as I can tell, but we need to ensure that the CMM is suspended
> > > > > > before and resumed after the DU. I believe this could be implemented
> > > > > > using device links.
> > > > >
> > > > > Based on below quote [*] from Jacopo's commit [**], isn't the device
> > > > > link relationship already in place?
> > > >
> > > > Yes, it's in place already.
> > > >
> > > > I added pm_ops to cmm just to be able to printout when suspend/resume
> > > > happens and the sequence is what comment [*] reports
> > > >
> > > > [  222.909002] rcar_du_pm_suspend:505
> > > > [  223.145497] rcar_cmm_pm_suspend:193
> > > >
> > > > [  223.208053] rcar_cmm_pm_resume:200
> > > > [  223.460094] rcar_du_pm_resume:513
> > > >
> > > > However, Laurent mentioned that in his comment here that he expects
> > > > the opposite sequence to happen (CMM to suspend before and resume after
> > > > DU).
> > > >
> > > > I still think what is implemented is correct:
> > > > - CMM is suspended after DU: when CMM is suspended, DU is not feeding
> > > >   it with data
> > > > - CMM is resumed before: once DU restart operations CMM is ready to
> > > >   receive data.
> > > >
> > > > Laurent, what do you think ?
> > >
> > > I think I shouldn't have written the previous e-mail in the middle of
> > > the night :-) Suspending CMM after DU is obviously correct.
> >
> > Thanks to Renesas team (kudos to Gotthard and Michael), we've
> > figured out that below sequence of clock handling (happening during
> > concurrent suspend and HDMI display unplug) leads to SoC lockup:
> >
> > cmm1 OFF      (caused by HDMI unplug)
> > x21-clock OFF         (caused by HDMI unplug)
> > du1 OFF       (caused by HDMI unplug)
> > cmm1 ON (caused by suspend to ram, as preparation for CMM register save)
> > # Freeze happens
> >
> > That seems to be explained by Chapter 35A.4.3 "Restriction of enabling
> > clock signal of the CMM" of HW User's manual (Rev.2.00 Jul 2019):
> >
> >  -----8<-----
> >  When the clock signal of the CMM is enabled (RMSTPCR7.CMMn or
> >  SMSTPCR7.CMMn = 0), the clock signal of the DU should be also enabled
> >  (RMSTPCR7.DUn or SMSTPCR7.DUn = 0).
> >  -----8<-----
> >
> > So, the lesson learned from the above is: do not enable the CMMi clock
> > while the DUi clock is disabled. I expect this to also potentially
> > give some input w.r.t. what to suspend/resume first, CMM or DU.
>
> This may be an ugly one. The DU driver needs to disable the CMM when
> suspending, and enabling the CMM when resuming. To do so, it calls
> functions of the CMM driver, and those functions access CMM registers.
> We can't do so while the CMM is suspended, so the DU has to suspend
> before the CMM, and resume after the CMM.
>
> On the other hand, as you state here, we need to make sure the CMM clock
> is disabled first. The CMM thus needs to suspend before the DU, and
> resume after the DU.
>
> Those are conflicting requirements.
>
> One option could be to use the .suspend_late() and .resume_early() PM
> operations for the DU, to turn the DU clock off late and turn it back on
> early. Integrating it with the DRM suspend/resume helpers will likely be
> complicated though. I wonder if we could find a more elegant solution.
>
> I the above sequence, you list "cmm1 ON" as a preparation for CMM
> register save. We don't need to save any register, the CMM driver has no
> .suspend() handler. The PM core should really skip waking up the device
> at that point (I recall complaining about the spurious wake ups at
> suspend time a while ago, but that neevr got addressed). Geert, any
> opinion on that ?

If there are issues with the PM core, please bring it up with the PM people.

If there's a (too) close integration of the CMM and the DU, perhaps the
CMM should list the DU module clock in its clocks property, too?
We have a similar construction for USB (module clocks 703 and 704).

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds