[PATCH 1/1] drm/bridge: ti-sn65dsi83: Fix enable error path

[Alexander Stein]

Hi Luca,

Am Freitag, 1. März 2024, 10:44:49 CET schrieb Luca Ceresoli:
> Hello Alexander,
> 
> On Thu, 29 Feb 2024 12:11:23 +0100
> Alexander Stein <alexander.stein at ew.tq-group.com> wrote:
> 
> > Hi Luca,
> > 
> > Am Donnerstag, 29. Februar 2024, 10:47:23 CET schrieb Luca Ceresoli:
> > > Hello Alexander,
> > > 
> > > On Wed, 28 Feb 2024 09:15:46 +0100
> > > Alexander Stein <alexander.stein at ew.tq-group.com> wrote:
> > > 
> > > 
> > > [...]
> > >   
> > > > Oh I mistook this DSI-LVDS bridge with the DSI-DP bridge on a different
> > > > board, my bad. I hope I can provide some insights. My platform is
> > > > imx8mm-tqma8mqml-mba8mx-lvds-tm070jvhg33.dtb.
> > > > I can easily cause a PLL lock failure by reducing the delay for the
> > > > enable-gpios 'gpio_delays'. This will result in a PLL lock faiure.
> > > > On my platform the vcc-supply counters do look sane:  
> > > > > /sys/kernel/debug/regulator/SN65DSI83_1V8/open_count:1
> > > > > /sys/kernel/debug/regulator/SN65DSI83_1V8/use_count:0    
> > > 
> > > Interesting. Thanks for taking time to report your initial issue!
> > >   
> > > > Once I remove the ti_sn65dsi83 module, the open_count decrements to 0 as
> > > > well. Looks sane to me.
> > > > 
> > > > If I revert commit c81cd8f7c774 ("Revert "drm/bridge: ti-sn65dsi83:
> > > > Fix enable error path""), vcc-supply counters are:  
> > > > > /sys/kernel/debug/regulator/SN65DSI83_1V8/open_count:1
> > > > > /sys/kernel/debug/regulator/SN65DSI83_1V8/use_count:1    
> > > > 
> > > > So in my case the use_count does not decrease! If I remove the module
> > > > ti_sn65dsi83, I get the WARN_ON (enable_count is still non-zero):  
> > > > > WARNING: CPU: 2 PID: 402 at drivers/regulator/core.c:2398 _regulator_put+0x15c/0x164    
> > > > 
> > > > This is on 6.8.0-rc6-next-20240228 with the following diff applied:  
> > > > --->8---    
> > > > diff --git a/arch/arm64/boot/dts/freescale/mba8mx.dtsi b/arch/arm64/boot/dts/freescale/mba8mx.dtsi
> > > > index 427467df42bf..8461e1fd396f 100644
> > > > --- a/arch/arm64/boot/dts/freescale/mba8mx.dtsi
> > > > +++ b/arch/arm64/boot/dts/freescale/mba8mx.dtsi
> > > > @@ -285,7 +285,7 @@ &i2c3 {
> > > >         dsi_lvds_bridge: bridge at 2d {
> > > >                 compatible = "ti,sn65dsi84";
> > > >                 reg = <0x2d>;
> > > > -               enable-gpios = <&gpio_delays 0 130000 0>;
> > > > +               enable-gpios = <&gpio_delays 0 0 0>;
> > > >                 vcc-supply = <&reg_sn65dsi83_1v8>;
> > > >                 status = "disabled";
> > > >  
> > > > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> > > > index 4814b7b6d1fd..57a7ed13f996 100644
> > > > --- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> > > > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> > > > @@ -478,7 +478,6 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
> > > >                 dev_err(ctx->dev, "failed to lock PLL, ret=%i\n", ret);
> > > >                 /* On failure, disable PLL again and exit. */
> > > >                 regmap_write(ctx->regmap, REG_RC_PLL_EN, 0x00);
> > > > -               regulator_disable(ctx->vcc);
> > > >                 return;
> > > >         }  
> > > > --->8---    
> > > > 
> > > > So my patch indeed did fix an actual problem. On the other hand it seems
> > > > sn65dsi83_atomic_disable is not called in my case for some reason.  
> > > 
> > > So you remove the module and atomic_disable is not called, after
> > > having called atomic_pre_enable?  
> > 
> > Yes, that's the case.
> 
> Ah, it's quite obvious looking at the code: removing the module will
> call sn65dsi83_remove()
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/bridge/ti-sn65dsi83.c#L729
> 
> which does just call drm_bridge_remove()
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_bridge.c#L243
> 
> which just removes the bridge from the list.
> 
> So maybe sn65dsi83_remove() should call regulator_disable() as a last
> resort, but I'm not sure this is the correct solution and it would
> involve some housekeeping to not disable the regulator more times than
> it has been enabled.

Actually I think removing the module should be prohibited while the bridge
is enabled in the first place.

> What is the use case you have for removing the driver module?

I was dealing the PLL lock failure myself, caused by some external delays.
For easy testing I was loading/unloading the module.

> I'm not implying removing the modules is wrong, but it definitely looks
> like not supported / not working. I'm just trying to understand the big
> picture.

Unloading should be possible, but not if the bridge is currently enabled.
Thanks for looking into this.

Best regards,
Alexander
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