Message-ID: <1443604022.3185.17.camel@pengutronix.de>
Date:	Wed, 30 Sep 2015 11:07:02 +0200
From:	Lucas Stach <l.stach@...gutronix.de>
To:	Daniel Kurtz <djkurtz@...omium.org>
Cc:	Matthias Brugger <matthias.bgg@...il.com>,
	Sascha Hauer <kernel@...gutronix.de>,
	James Liao <jamesjj.liao@...iatek.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>,
	srv_heupstream <srv_heupstream@...iatek.com>,
	Kevin Hilman <khilman@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] soc: mediatek: Fix random hang up issue while kernel
 init

Hi Daniel,

Am Dienstag, den 29.09.2015, 18:25 +0800 schrieb Daniel Kurtz:
> Hi Matthias & Lucas,
> 
> On Sun, Sep 27, 2015 at 7:25 PM, Matthias Brugger
> <matthias.bgg@...il.com> wrote:
> >
> >
> >
> > On 25/09/15 10:26, Lucas Stach wrote:
> >>
> >> Am Freitag, den 25.09.2015, 14:31 +0800 schrieb James Liao:
> >>>
> >>> In kernel late init, it turns off all unused clocks, which
> >>> needs to access subsystem registers such as VENC and VENC_LT.
> >>>
> >>> Accessing MT8173 VENC registers needs two top clocks, mm_sel and
> >>> venc_sel. Accessing VENC_LT registers needs mm_sel and venclt_sel.
> >>> So we need to keep these clocks on before accessing their registers.
> >>>
> >>> This patch keeps venc_sel / venclt_sel clock on when
> >>> VENC / VENC_LT's power is on, to prevent system hang up while
> >>> accessing its registeres.
> >>>
> >> This changes the binding of an existing driver, so it needs some more
> >> consideration. Are VENC_SEL and VENC_LT_SEL really direct clock inputs
> >> to the VENC module, or are they some kind of parent clock for one of the
> >> existing clocks used by the old VENC binding?
> >>
> >> If they are direct clock inputs to the VENC module, changing the binding
> >> might be still ok at that point, as there shouldn't be many users of
> >> that yet.
> 
> The MT8173 VENC subsystem clock registers are modeled by the vencsys
> clock-controller node:
> 
> vencsys: clock-controller@...00000 {
>   compatible = "mediatek,mt8173-vencsys", "syscon";
>   reg = <0 0x18000000 0 0x1000>;
>   #clock-cells = <1>;
> };
> 
> These registers, however, are only accessible if:
>   (a) the VENC power domain is enabled, and
>   (b) the "venc_sel" clock is enabled.
> 
> According to James, venc_sel is a direct clock input the VENC module,
> and it is an ancestor to some, but not all, of the subsystem clocks, in
> particular it is not an ancestor to "venc_cke0":
> 
>  static const struct mtk_gate venc_clks[] __initconst = {
>          GATE_VENC(CLK_VENC_CKE0, "venc_cke0", "mm_sel", 0),
>          GATE_VENC(CLK_VENC_CKE1, "venc_cke1", "venc_sel", 4),
>          GATE_VENC(CLK_VENC_CKE2, "venc_cke2", "venc_sel", 8),
>          GATE_VENC(CLK_VENC_CKE3, "venc_cke3", "venc_sel", 12),
>  };
> 
> If the VENC power domain is disabled, then accesses to the vencsys
> registers just silently fail (i.e., reads probably
> return all 0s). 

If this ever happens it is really unfortunate and needs fixing, too. If
you need the power domain to be powered ON to properly read or even
change the clock registers, the clock driver needs to be a consumer of
the power domain, so any clock operations powers the domain up.

> However, if the power domain is ON but venc_sel is
> disabled, then accessing the clock-controller registers results in a
> system hang (until the power domain is disabled).
> 
> At boot the VENC power domain is forced on by scpsys (like all other
> power domains).  Later both unused clocks and power domains get
> disabled. If clocks are disabled first, and venc_sel is disabled
> before venc_cke0, then when unused_clock tries to disable venc_cke0,
> the system will hang when it reads the vencsys "is clock enabled" bit
> since:
>   (a) VENC power domain is still enabled, and
>   (b) venc_sel is already disabled
> 
> In theory, we could add "clocks=<&topckgen CLK_TOP_VENC_SEL>;" to the
> vencsys clock-controller node.  On initialization, mtk_vencsys_init()
> could then pass venc_sel to the generic MT8173 gate clock driver, and
> it would then clk_enable(venc_sel)/_disable(venc_sel) around any
> access to the clock-controller registers.  James, however, thinks
> this is a lot of extra overhead, and instead has proposed the fix in this patch,
> where venc_sel is forced on whenever the VENC power domain is enabled.
> 
I would still say this is the correct solution. If the vencsys clock
registers are itself clocked by VENC_SEL this driver needs to make sure
this clock is running at the appropriate times. I understand that this
may be a bit of an overhead, but clock enable/disable paths are not
really performance critical.

> So, this patch is a bit of a hack, but the mtk scpsys driver already
> does something similar for the MM & MFG clocks - these clocks are
> always enabled whenever certain power domains are enabled, and they
> are only disabled when all such power domains are disabled.  I'm not
> 100% why these clocks must always be kept on whenever these power
> domains are enabled, but probably to solve a similar problem where
> these clocks are needed to access some registers at a time when these
> clocks would not otherwise be explicitly enabled.
> 
I can't really argue with this being the wrong solution if we already
have precedent of the same solution being used for other domains. But I
would still ask you to re-evaluate with the above in mind.

> Is the above clear?
> 
> >> But then we at least need a corresponding change to the binding documentation.
> 
> Agreed.
> 
> > Yes, we will need a really good reason to change the bindings.
> 
> There is a race where the system can sometimes hang at boot with the
> existing implementation & bindings.
> This seems like a really good reason to change the bindings, if the
> proposed solution is acceptable (it may be possible to fix the hang
> without changing the bindings).
> 
> > Apart from that we would need to find a solution which works with the old (and wrong bindings) as well.
> 
> Why do we need to support the old bindings?
> This patch fixes a problem in MT8173, for which the mainline kernel is
> still under active bringup.
> If the existing bindings are not used anywhere yet (*), it seems like
> unnecessary overhead to enforce backwards compatibility at this stage.
> 
I tend to agree. DT backwards compatibility isn't easily done (and is a
constant burden, as it gets harder to test over time) and if the SoC is
in early bringup one might just admit that the first iteration of the
bindings were wrong and need an incompatible change.

But that's not up to me to decide, but something Matthias needs to make
a decision on.

Regards,
Lucas
-- 
Pengutronix e.K.             | Lucas Stach                 |
Industrial Linux Solutions   | http://www.pengutronix.de/  |

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