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Date:   Wed, 12 Sep 2018 10:42:42 +0200
From:   Maxime Ripard <maxime.ripard@bootlin.com>
To:     Kishon Vijay Abraham I <kishon@ti.com>
Cc:     Boris Brezillon <boris.brezillon@bootlin.com>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        linux-media@vger.kernel.org,
        Archit Taneja <architt@codeaurora.org>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Chen-Yu Tsai <wens@csie.org>, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org,
        linux-arm-kernel@lists.infradead.org,
        Krzysztof Witos <kwitos@cadence.com>,
        Rafal Ciepiela <rafalc@cadence.com>
Subject: Re: [PATCH 02/10] phy: Add configuration interface
Message-ID: <20180912084242.skxbwbgluakakyg6@flea>
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Hi!

On Wed, Sep 12, 2018 at 01:12:31PM +0530, Kishon Vijay Abraham I wrote:
> On Thursday 06 September 2018 08:26 PM, Maxime Ripard wrote:
> > Hi Kishon,
> >=20
> > On Thu, Sep 06, 2018 at 02:57:58PM +0530, Kishon Vijay Abraham I wrote:
> >> On Wednesday 05 September 2018 02:46 PM, Maxime Ripard wrote:
> >>> The phy framework is only allowing to configure the power state of th=
e PHY
> >>> using the init and power_on hooks, and their power_off and exit
> >>> counterparts.
> >>>
> >>> While it works for most, simple, PHYs supported so far, some more adv=
anced
> >>> PHYs need some configuration depending on runtime parameters. These P=
HYs
> >>> have been supported by a number of means already, often by using ad-h=
oc
> >>> drivers in their consumer drivers.
> >>>
> >>> That doesn't work too well however, when a consumer device needs to d=
eal
> >>> multiple PHYs, or when multiple consumers need to deal with the same =
PHY (a
> >>> DSI driver and a CSI driver for example).
> >>>
> >>> So we'll add a new interface, through two funtions, phy_validate and
> >>> phy_configure. The first one will allow to check that a current
> >>> configuration, for a given mode, is applicable. It will also allow th=
e PHY
> >>> driver to tune the settings given as parameters as it sees fit.
> >>>
> >>> phy_configure will actually apply that configuration in the phy itsel=
f.
> >>>
> >>> Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
> >>> ---
> >>>  drivers/phy/phy-core.c  | 62 +++++++++++++++++++++++++++++++++++++++=
+++-
> >>>  include/linux/phy/phy.h | 42 ++++++++++++++++++++++++++++-
> >>>  2 files changed, 104 insertions(+)
> >>>
> >>> diff --git a/drivers/phy/phy-core.c b/drivers/phy/phy-core.c
> >>> index 35fd38c5a4a1..6eaf655e370f 100644
> >>> --- a/drivers/phy/phy-core.c
> >>> +++ b/drivers/phy/phy-core.c
> >>> @@ -408,6 +408,68 @@ int phy_calibrate(struct phy *phy)
> >>>  EXPORT_SYMBOL_GPL(phy_calibrate);
> >>> =20
> >>>  /**
> >>> + * phy_configure() - Changes the phy parameters
> >>> + * @phy: the phy returned by phy_get()
> >>> + * @mode: phy_mode the configuration is applicable to.
> >>
> >> mode should be used if the same PHY can be configured in multiple mode=
s. But
> >> with phy_set_mode() and phy_calibrate() we could achieve the same.
> >=20
> > So you would change the prototype to have a configuration applying
> > only to the current mode set previously through set_mode?
>=20
> yeah.
> With phy_configure, if the PHY is not in @mode, it should return an error=
? Or
> will it set the PHY to @mode and apply the configuration in @opts?

I wanted to have it return an error either if it was configured in
another mode or if the mode was unsupported yes.

> > Can we have PHY that operate in multiple modes at the same time?
>=20
> Not at the same time. But the same PHY can operate in multiple modes (For
> example we have PHYs that can be used either with PCIe or USB3)

Ok, that makes sense. I guess we could rely on phy_set_mode then if
you prefer.

> >>> + * @opts: New configuration to apply
> >>
> >> Should these configuration come from the consumer driver?
> >=20
> > Yes
>=20
> How does the consumer driver get these configurations? Is it from user sp=
ace or
> dt associated with consumer device.

It really depends on multiple factors (and I guess on what mode the
PHY is actually supposed to support), but in the case covered by this
serie, the info mostly come from multiple places:
  - The resolutions supported by the panel
  - The resolutions supported by the phy consumer (and its
    integration, for things like the clock rates it can output)
  - The resolutions and timings supported by the phy itself (once
    again, the integration is mostly involved here since it really
    only depends on which clock rates can be achieved)
  - The timings boundaries that the specification has
  - The resolution selected by the user

So we'd have that information coming from multiple places: the
userspace would select the resolution, drivers would be able to filter
out unsupported resolutions, and the DT will provide the integration
details to help them do so.

But I guess from an API standpoint, it really is expected to be
assembled by the phy consumer driver.

> >>> +/**
> >>> + * phy_validate() - Checks the phy parameters
> >>> + * @phy: the phy returned by phy_get()
> >>> + * @mode: phy_mode the configuration is applicable to.
> >>> + * @opts: Configuration to check
> >>> + *
> >>> + * Used to check that the current set of parameters can be handled by
> >>> + * the phy. Implementations are free to tune the parameters passed as
> >>> + * arguments if needed by some implementation detail or
> >>> + * constraints. It will not change any actual configuration of the
> >>> + * PHY, so calling it as many times as deemed fit will have no side
> >>> + * effect.
> >>> + *
> >>> + * Returns: 0 if successful, an negative error code otherwise
> >>> + */
> >>> +int phy_validate(struct phy *phy, enum phy_mode mode,
> >>> +		  union phy_configure_opts *opts)
> >>
> >> IIUC the consumer driver will pass configuration options (or PHY param=
eters)
> >> which will be validated by the PHY driver and in some cases the PHY dr=
iver can
> >> modify the configuration options? And these modified configuration opt=
ions will
> >> again be given to phy_configure?
> >>
> >> Looks like it's a round about way of doing the same thing.
> >=20
> > Not really. The validate callback allows to check whether a particular
> > configuration would work, and try to negotiate a set of configurations
> > that both the consumer and the PHY could work with.
>=20
> Maybe the PHY should provide the list of supported features to the consum=
er
> driver and the consumer should select a supported feature?

It's not really about the features it supports, but the boundaries it
might have on those features. For example, the same phy integrated in
two different SoCs will probably have some limit on the clock rate it
can output because of the phy design itself, but also because of the
clock that is fed into that phy, and that will be different from one
SoC to the other.

This integration will prevent us to use some clock rates on the first
SoC, while the second one would be totally fine with it.

Obviously, the consumer driver shouldn't care about the phy
integration details, especially since some of those consumer drivers
need to interact with multiple phy designs (or the same phy design can
be used by multiple consumers).

So knowing that a feature is supported is really not enough.

With MIPI-DPHY at least, the API is generic enough so that another
mode where the features would make sense could implement a feature
flag if that makes sense.

> > For example, DRM requires this to filter out display modes (ie,
> > resolutions) that wouldn't be achievable by the PHY so that it's never
>=20
> Can't the consumer driver just tell the required resolution to the PHY an=
d PHY
> figuring out all the parameters for the resolution or an error if that
> resolution cannot be supported?

Not really either. With MIPI D-PHY, the phy is fed a clock that is
generated by the phy consumer, which might or might not be an exact
fit for the resolution. There's so many resolutions that in most case,
the clock factors don't allow you to have a perfect match. And
obviously, this imprecision should be taken into account by the PHY as
well.

And then, there's also the matter than due to design constraints, some
consumers would have fixed timings that are not at the spec default
value, but still within the acceptable range. We need to communicate
that to the PHY.

Maxime

--=20
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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