Re: [PATCH v7 03/12] drm/bridge: cdns-dsi: Fix phy de-init and flag it so

[Tomi Valkeinen]

Hi,

On 14/01/2025 16:44, Aradhya Bhatia wrote:
> Hi Tomi,
>
> On 1/14/25 18:00, Tomi Valkeinen wrote:
> > Hi,
> >
> > On 14/01/2025 07:56, Aradhya Bhatia wrote:
> > > From: Aradhya Bhatia <a-bhatia1@xxxxxx>
> > >
> > > The driver code doesn't have a Phy de-initialization path as yet, and so
> > > it does not clear the phy_initialized flag while suspending. This is a
> > > problem because after resume the driver looks at this flag to determine
> > > if a Phy re-initialization is required or not. It is in fact required
> > > because the hardware is resuming from a suspend, but the driver does not
> > > carry out any re-initialization causing the D-Phy to not work at all.
> > >
> > > Call the counterparts of phy_init() and phy_power_on(), that are
> > > phy_exit() and phy_power_off(), from _bridge_disable(), and clear the
> > > flags so that the Phy can be initialized again when required.
> > >
> > > Fixes: fced5a364dee ("drm/bridge: cdns: Convert to phy framework")
> > > Signed-off-by: Aradhya Bhatia <a-bhatia1@xxxxxx>
> > > Signed-off-by: Aradhya Bhatia <aradhya.bhatia@xxxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c | 6 +++++-
> > >    1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
> > > b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
> > > index 056583e81155..039c5eb7fb66 100644
> > > --- a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
> > > +++ b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
> > > @@ -672,6 +672,11 @@ static void cdns_dsi_bridge_disable(struct
> > > drm_bridge *bridge)
> > >        if (dsi->platform_ops && dsi->platform_ops->disable)
> > >            dsi->platform_ops->disable(dsi);
> > >    +    dsi->phy_initialized = false;
> > > +    dsi->link_initialized = false;
> > > +    phy_power_off(dsi->dphy);
> > > +    phy_exit(dsi->dphy);
> > > +
> >
> > The phy related lines are counterparts to what's done in
> > cdns_dsi_hs_init(), right? Maybe add cdns_dsi_hs_uninit(),
> >
> > But is the phy_initialized even needed? phy_initialized() is called from
> > cdns_dsi_bridge_enable() and cdns_dsi_bridge_pre_enable(). Won't the
> > call in cdns_dsi_bridge_enable() be always skipped, as
> > cdns_dsi_bridge_pre_enable() already set phy_initialized?
>
> Yes, that is how the behavior has been. The initialization calls inside
> the _bridge_enable() end-up getting skipped.
>
> My first thought after reading your comments was to remove the init
> calls entirely from the _bridge_pre_enable(), and drop the
> phy_initialized flag too, and let _bridge_enable() only handle the init.

Isn't that the wrong way around? If currently bridge_pre_enable enables 
the phy, your suggestion above would change that. I would think keeping 
the init calls in bridge_pre_enable, and drop from bridge_enable.

> The _bridge_enable() will anyway get renamed to _bridge_pre_enable(),
> while the existing _bridge_pre_enable() will get dropped, by the last
> patch of this series.

Ok, but you can't do a fix that'll only be right after some future patch 
does more changes =).

> But since this patch is intended as a fix, it will get applied to
> previous versions while that last patch of the series won't... and then

Speaking of which, I think you should cc: stable for the ones that 
should be applied to earlier kernels. And it would be good to have all 
such patches first in the series, to decrease any dependencies.

> we may end up having init calls only from _bridge_enable() for the older
> versions.
> Also, given all the fixes in the series, there is a possibility that an
> older-version of the driver might become functional (except for the
> color shift issue).
>
> My question then is, would it be a cause for concern if all the init
> calls are handled from the _bridge_enable() only?

I'm not sure I follow here. Don't we want the init calls to happen in 
the pre_enable phase, both before and after the sequence change (patch 12)?

But generally speaking, yes, it's good to keep fixes simple, and do 
cleanups later on top. Keeping that in mind, maybe this current patch is 
fine as it is. Although... if the init is done in pre_enable, shouldn't 
the deinit be done in post_disable?

> (one of the potential concerns detailed below)
>
> > Same question for cdns_dsi_init_link(), although that's also called from
> > cdns_dsi_transfer(), so we probably need dsi->link_initialized.
>
> Don't you think we'd need the phy to also be initialized for the DCS
> command to work?

I'm sure we do. But the driver doesn't do that currently, does it? Which 
I did find a bit odd, but I'm not familiar with the HW.

However, my comment was related to calling cdns_dsi_init_link() in both 
cdns_dsi_bridge_enable and cdns_dsi_bridge_pre_enable functions. In this 
case the call in the cdns_dsi_bridge_enable function is a no-op, similar 
to calling cdns_dsi_hs_init().

But, if changed, that's also a cleanup, so maybe better keep away from 
fix patches.

> Usually, since DSI is among the initial bridges to get pre_enabled, the
> Link and Phy are both initialized by the time cdns_dsi_transfer() is
> called. So, even if cdns_dsi_transfer() doesn't call for
> cdns_dsi_hs_init(), it is able to work fine.
>
> If DCS commands do indeed require the cdns_dsi_hs_init(), then shifting
> it to _bridge_enable() (like I suggested above) would be problematic
> without fixing it here.

I don't know what how the HW works, but we definitely need PHY to send 
DCS commands. But we don't necessarily need HS mode, LP works fine 
(usually). It's just not clear to me what exactly cdns_dsi_hs_init() and 
cdns_dsi_init_link() do. What is "link"? Looks like cdns_dsi_init_link 
is doing some PHY stuff, which is kind of strange thing to do, as 
phy_init() and phy_power_on() are only done later.

In any case, yes, the cdns_dsi_transfer() has to make sure we have LP/HS 
working. So indeed it might mean calling both functions. This is, 
however, perhaps a different topic, best left out of this series.

 Tomi