Re: [PATCH v5 2/5] phy: Add LVDS configuration options

[Liu Ying <victor.liu@nxp.com>]

Hi Kishon,

First of all, thanks for your review.

On Wed, 2021-03-31 at 19:02 +0530, Kishon Vijay Abraham I wrote:
> Hi,
> 
> On 25/03/21 2:30 pm, Liu Ying wrote:
> > This patch allows LVDS PHYs to be configured through
> > the generic functions and through a custom structure
> > added to the generic union.
> > 
> > The parameters added here are based on common LVDS PHY
> > implementation practices.  The set of parameters
> > should cover all potential users.
> > 
> > Cc: Kishon Vijay Abraham I <kishon@ti.com>
> > Cc: Vinod Koul <vkoul@kernel.org>
> > Cc: NXP Linux Team <linux-imx@nxp.com>
> > Signed-off-by: Liu Ying <victor.liu@nxp.com>
> > ---
> > v4->v5:
> > * Align kernel-doc style to include/linux/phy/phy.h. (Vinod)
> > * Trivial tweaks.
> > * Drop Robert's R-b tag.
> > 
> > v3->v4:
> > * Add Robert's R-b tag.
> > 
> > v2->v3:
> > * No change.
> > 
> > v1->v2:
> > * No change.
> > 
> >  include/linux/phy/phy-lvds.h | 32 ++++++++++++++++++++++++++++++++
> >  include/linux/phy/phy.h      |  4 ++++
> >  2 files changed, 36 insertions(+)
> >  create mode 100644 include/linux/phy/phy-lvds.h
> > 
> > diff --git a/include/linux/phy/phy-lvds.h b/include/linux/phy/phy-lvds.h
> > new file mode 100644
> > index 00000000..7a2f474
> > --- /dev/null
> > +++ b/include/linux/phy/phy-lvds.h
> > @@ -0,0 +1,32 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Copyright 2020 NXP
> > + */
> > +
> > +#ifndef __PHY_LVDS_H_
> > +#define __PHY_LVDS_H_
> > +
> > +/**
> > + * struct phy_configure_opts_lvds - LVDS configuration set
> > + * @bits_per_lane_and_dclk_cycle:	Number of bits per data lane and
> > + *					differential clock cycle.
> 
> phy_set_bus_width() instead?

This member aims to configure the number of bits transmitted during a
period of time(a clock cycle). It doesn't sound like a concept of 'bus
width'?

> > + * @differential_clk_rate:		Clock rate, in Hertz, of the LVDS
> > + *					differential clock.
> 
> Please use clk API's to get rate.

I like your idea. But, this rate is likely runtime-configurable, e.g.,
a LVDS to HDMI chip is connected. It seems that there is no appropriate
driver to set the rate by calling clk_set_rate() then?

> > + * @lanes:				Number of active, consecutive,
> > + *					data lanes, starting from lane 0,
> > + *					used for the transmissions.
> > + * @is_slave:				Boolean, true if the phy is a slave
> > + *					which works together with a master
> > + *					phy to support dual link transmission,
> > + *					otherwise a regular phy or a master phy.
> 
> For parameters that are known at design time, it doesn't have to be
> passed from consumer driver. So all these parameters do they really have
> to be passed at runtime?

Yes for all, perhaps. Details below:

1) bits_per_lane_and_dclk_cycle
i.MX8qxp LVDS phy can only do 7, while i.MX8qm LVDS phy(a different IP)
can do either 7 or 10(configurable by setting a phy register).

2) differential_clk_rate
It's likely runtime-configurable, as I mentioned above.

3) lanes
The higher color depth is, the more lanes are used:
RGB666 - 3 lanes
RGB888 - 4 lanes
RGB101010 - 5 lanes

That means a phy with 5 lanes(like i.MX8qm LVDS phy) support up to the
3 types of RGB pixels.

Though the i.MX LVDS phys don't have any register to configure the
lanes to be used, it would be good to define it for phy_validate() or
other potential phys?

4) is_slave
Any i.MX8qxp LVDS phy instance can act as a regular phy or a master phy
or a slave phy. Changing mode at runtime is probably unneeded. But,
it's difficult for the phy driver to get the mode from device tree(see
drm_of_lvds_get_port_pixels_type()), I think. Export an i.MX8qxp LVDS
phy specific function to set this instead?

Regards,
Liu Ying

> 
> Thanks
> Kishon
> > + *
> > + * This structure is used to represent the configuration state of a LVDS phy.
> > + */
> > +struct phy_configure_opts_lvds {
> > +	unsigned int	bits_per_lane_and_dclk_cycle;
> > +	unsigned long	differential_clk_rate;
> > +	unsigned int	lanes;
> > +	bool		is_slave;
> > +};
> > +
> > +#endif /* __PHY_LVDS_H_ */
> > diff --git a/include/linux/phy/phy.h b/include/linux/phy/phy.h
> > index e435bdb..d450b44 100644
> > --- a/include/linux/phy/phy.h
> > +++ b/include/linux/phy/phy.h
> > @@ -17,6 +17,7 @@
> >  #include <linux/regulator/consumer.h>
> >  
> >  #include <linux/phy/phy-dp.h>
> > +#include <linux/phy/phy-lvds.h>
> >  #include <linux/phy/phy-mipi-dphy.h>
> >  
> >  struct phy;
> > @@ -51,10 +52,13 @@ enum phy_mode {
> >   *		the MIPI_DPHY phy mode.
> >   * @dp:		Configuration set applicable for phys supporting
> >   *		the DisplayPort protocol.
> > + * @lvds:	Configuration set applicable for phys supporting
> > + *		the LVDS phy mode.
> >   */
> >  union phy_configure_opts {
> >  	struct phy_configure_opts_mipi_dphy	mipi_dphy;
> >  	struct phy_configure_opts_dp		dp;
> > +	struct phy_configure_opts_lvds		lvds;
> >  };
> >  
> >  /**
> >