Re: [RFC PATCH 03/10] drm/mipi-dsi: add API for manual control over the DSI link power state

Alexander Stein
2023-10-23T06:52:17Z

Hi Dmitry,

Am Sonntag, 22. Oktober 2023, 12:49:41 CEST schrieb Dmitry Baryshkov:
> On Thu, 19 Oct 2023 at 14:42, Alexander Stein
>
> <alexander.stein@ew.tq-group.com> wrote:
> > Hi,
> >
> > Am Donnerstag, 19. Oktober 2023, 13:19:51 CEST schrieb Dmitry Baryshkov:
> > > On Thu, 19 Oct 2023 at 12:26, Maxime Ripard <mripard@kernel.org> wrote:
> > > > On Mon, Oct 16, 2023 at 07:53:48PM +0300, Dmitry Baryshkov wrote:
> > > > > The MIPI DSI links do not fully fall into the DRM callbacks model.
> > > >
> > > > Explaining why would help
> > >
> > > A kind of explanation comes afterwards, but probably I should change
> > > the order of the phrases and expand it:
> > >
> > > The atomic_pre_enable / atomic_enable and correspondingly
> > > atomic_disable / atomic_post_disable expect that the bridge links
> > > follow a simple paradigm: either it is off, or it is on and streaming
> > > video. Thus, it is fine to just enable the link at the enable time,
> > > doing some preparations during the pre_enable.
> > >
> > > But then it causes several issues with DSI. First, some of the DSI
> > > bridges and most of the DSI panels would like to send commands over
> > > the DSI link to setup the device. Next, some of the DSI hosts have
> > > limitations on sending the commands. The proverbial sunxi DSI host can
> > > not send DSI commands after the video stream has started. Thus most of
> > > the panels have opted to send all DSI commands from pre_enable (or
> > > prepare) callback (before the video stream has started).
> > >
> > > However this leaves no good place for the DSI host to power up the DSI
> > > link. By default the host's pre_enable callback is called after the
> > > DSI bridge's pre_enable. For quite some time we were powering up the
> > > DSI link from mode_set. This doesn't look fully correct. And also we
> > > got into the issue with ps8640 bridge, which requires for the DSI link
> > > to be quiet / unpowered at the bridge's reset time.
> >
> > There are also bridges (e.g. tc358767) which require DSI-LP11 upon bridge
> > reset. And additionally this DSI-(e)DP bridge requires LP11 while
> > accessing
> > DP-AUX channel, e.g. reading EDID. So bridges need at least some control
> > over DSI line state.
>
> For sending commands in LP11 it is typical to toggle the
> MIPI_DSI_MODE_LPM flag, for example see panel-=jdi-lt070me05000.c or
> some other drives. It might be a good idea to make that more explicit.
> All suggestions here would be appreciated.

The biggest difference between that display and the tc358767 bridge is that
the display uses DSI commands, while the bridge is using i2c transfer to issue
DP-AUX commands. There is no host_transfer [1] which would enable LP-11.
It seems this DSI-DP bridge requires LP-11/HS on DSI lanes all times. This
contradicts current Linux behaviour.

Best regards,
Alexander

[1] https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html#mipi-dsi-bridge-operation

>
> > > Dave has come with the idea of pre_enable_prev_first /
> > > prepare_prev_first flags, which attempt to solve the issue by
> > > reversing the order of pre_enable callbacks. This mostly solves the
> > > issue. However during this cycle it became obvious that this approach
> > > is not ideal too. There is no way for the DSI host to know whether the
> > > DSI panel / bridge has been updated to use this flag or not, see the
> > > discussion at [1].
> > >
> > > Thus comes this proposal. It allows for the panels to explicitly bring
> > > the link up and down at the correct time, it supports automatic use
> > > case, where no special handling is required. And last, but not least,
> > > it allows the DSI host to note that the bridge / panel were not
> > > updated to follow new protocol and thus the link should be powered on
> > > at the mode_set time. This leaves us with the possibility of dropping
> > > support for this workaround once all in-kernel drivers are updated.
> >
> > I want to use this series to support tc358767 properly, because
> > pre_enable_prev_first is not enough, see AUX channel above. I hope I'll
> > find any time soon.
> >
> > Best regards,
> > Alexander
> >
> > > > > The drm_bridge_funcs abstraction.
> > > >
> > > > Is there a typo or missing words?
> > >
> > > missing comma in front of The
> > >
> > > > > Instead of having just two states (off and on) the DSI hosts have
> > > > > separate LP-11 state. In this state the host is on, but the video
> > > > > stream is not yet enabled.
> > > > >
> > > > > Introduce API that allows DSI bridges / panels to control the DSI
> > > > > host
> > > > > power up state.
> > >
> > > [1]
> > > https://lore.kernel.org/dri-devel/6c0dd9fd-5d8e-537c-804f-7a03d5899a07@l
> > > ina
> > > ro.org/
> > >
> > > > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> > > > > ---
> > > > >
> > > > > drivers/gpu/drm/drm_mipi_dsi.c | 31 +++++++++++++++++++++++++++++++
> > > > > include/drm/drm_mipi_dsi.h | 29 +++++++++++++++++++++++++----
> > > > > 2 files changed, 56 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/drm_mipi_dsi.c
> > > > > b/drivers/gpu/drm/drm_mipi_dsi.c index 14201f73aab1..c467162cb7d8
> > > > > 100644
> > > > > --- a/drivers/gpu/drm/drm_mipi_dsi.c
> > > > > +++ b/drivers/gpu/drm/drm_mipi_dsi.c
> > > > > @@ -428,6 +428,37 @@ int devm_mipi_dsi_attach(struct device *dev,
> > > > >
> > > > > }
> > > > > EXPORT_SYMBOL_GPL(devm_mipi_dsi_attach);
> > > > >
> > > > > +bool mipi_dsi_host_power_control_available(struct mipi_dsi_host
> > > > > *host)
> > > > > +{
> > > > > + const struct mipi_dsi_host_ops *ops = host->ops;
> > > > > +
> > > > > + return ops && ops->power_up;
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(mipi_dsi_host_power_control_available);
> > > > > +
> > > > > +int mipi_dsi_host_power_up(struct mipi_dsi_host *host)
> > > > > +{
> > > > > + const struct mipi_dsi_host_ops *ops = host->ops;
> > > > > +
> > > > > + if (!mipi_dsi_host_power_control_available(host))
> > > > > + return -EOPNOTSUPP;
> > > > > +
> > > > > + return ops->power_up ? ops->power_up(host) : 0;
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(mipi_dsi_host_power_up);
> > > > > +
> > > > > +void mipi_dsi_host_power_down(struct mipi_dsi_host *host)
> > > > > +{
> > > > > + const struct mipi_dsi_host_ops *ops = host->ops;
> > > > > +
> > > > > + if (!mipi_dsi_host_power_control_available(host))
> > > > > + return;
> > > > > +
> > > > > + if (ops->power_down)
> > > > > + ops->power_down(host);
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(mipi_dsi_host_power_down);
> > > > > +
> > > >
> > > > If this API is supposed to be used by DSI devices, it should probably
> > > > take a mipi_dsi_device pointer as a parameter?
> > >
> > > Ack.
> > >
> > > > We should probably make sure it hasn't been powered on already too?
> > >
> > > Ack, I can add an atomic count there and a WARN_ON. However I don't
> > > think that it is a real problem.
> > >
> > > > Maxime
> > >
> > > --
> > > With best wishes
> > >
> > > Dmitry
> >
> > --
> > TQ-Systems GmbH | Mühlstraße 2, Gut Delling | 82229 Seefeld, Germany
> > Amtsgericht München, HRB 105018
> > Geschäftsführer: Detlef Schneider, Rüdiger Stahl, Stefan Schneider
> > http://www.tq-group.com/

--
TQ-Systems GmbH | Mühlstraße 2, Gut Delling | 82229 Seefeld, Germany
Amtsgericht München, HRB 105018
Geschäftsführer: Detlef Schneider, Rüdiger Stahl, Stefan Schneider