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

[Alexander Stein <alexander.stein@ew.tq-group.com>]

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.

> 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@lina
> 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


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