[PATCH] drm/panel: boe-tv101wum-nl6: Ensure DSI writes succeed during disable

[Dave Stevenson]

Hi Stephen

On Fri, 13 Jan 2023 at 21:12, Stephen Boyd <swboyd at chromium.org> wrote:
>
> Quoting Dave Stevenson (2023-01-13 08:27:29)
> > Hi Stephen
> >
> > On Fri, 6 Jan 2023 at 03:01, Stephen Boyd <swboyd at chromium.org> wrote:
> > >
> > > The unprepare sequence has started to fail after moving to panel bridge
> > > code in the msm drm driver (commit 007ac0262b0d ("drm/msm/dsi: switch to
> > > DRM_PANEL_BRIDGE")). You'll see messages like this in the kernel logs:
> > >
> > >    panel-boe-tv101wum-nl6 ae94000.dsi.0: failed to set panel off: -22
> > >
> > > This is because boe_panel_enter_sleep_mode() needs an operating DSI link
> > > to set the panel into sleep mode. Performing those writes in the
> > > unprepare phase of bridge ops is too late, because the link has already
> > > been torn down by the DSI controller in post_disable, i.e. the PHY has
> > > been disabled, etc. See dsi_mgr_bridge_post_disable() for more details
> > > on the DSI .
> > >
> > > Split the unprepare function into a disable part and an unprepare part.
> > > For now, just the DSI writes to enter sleep mode are put in the disable
> > > function. This fixes the panel off routine and keeps the panel happy.
> >
> > It is documented that the mipi_dsi_host_ops transfer function should
> > be called with the host in any state [1], so the host driver is
> > failing there.
>
> Thanks for the info! It says "Drivers that need the underlying device to
> be powered to perform these operations will first need to make sure it’s
> been properly enabled." Does that mean the panel driver needs to make
> sure the underlying dsi host device is powered? The sentence is too
> ambiguous for me to understand what 'drivers' and 'underlying device'
> are.

The DSI host driver (ie in your case something under
drivers/gpu/drm/msm/dsi) should ensure that a transfer can be made,
regardless of state.

I must say that this has been documented as the case, but doesn't
necessarily reflect reality in a number of drivers.

> >
> > This sounds like the same issue I was addressing in adding the
> > prepare_prev_first flag to drm_panel, and pre_enable_prev_first to
> > drm_bridge via [2].
> > Defining prepare_prev_first for your panel would result in the host
> > pre_enable being called before the panel prepare, and therefore the
> > transfer calls from boe_panel_init_dcs_cmd boe_panel_prepare won't be
> > relying on the DSI host powering up early. It will also call the panel
> > unprepare before the DSI host bridge post_disable is called, and
> > therefore the DSI host will still be powered up and the transfer won't
> > fail.
> >
> > Actually I've just noted the comment at [3]. [2] is that framework fix
> > that means that the magic workaround isn't required, but it will
> > require this panel to opt in to the ordering change.
>
> Cool. Glad that we can clean that up with your series.
>
> Is it wrong to split unprepare to a disable and unprepare phase? I'm not
> super keen on fixing 6.1 stable kernels by opting this driver into the
> ordering change. Splitting the phase into two is small and simple and
> works. I suspect changing the ordering may uncover more bugs, or be a
> larger task. I'd be glad to test that series[2] from you to get rid of
> [3].

Ah, I hadn't realised it was a regression in a released kernel :(

Splitting into a disable and unprepare is totally fine. Normally
disable would normally disable the panel and backlight (probably by
drm_panel before the panel disable call), with unprepare disabling
power and clocks

Do note that AIUI you will be telling the panel to enter sleep mode
whilst video is still being transmitted. That should be safe as the
panel has to still be partially active to handle an exit sleep mode
command, but actually powering hardware down at that point could cause
DSI bus arbitration errors as clock or data lanes could be pulled down
when the host is trying to adopt HS or LP11.

  Dave

> >
> >
> > [1] https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html#c.mipi_dsi_host_ops
> > [2] https://patchwork.freedesktop.org/series/100252/
> > [3] https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/msm/dsi/dsi_manager.c#L47
> >