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Date:   Tue, 5 May 2020 10:59:30 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/bridge: ti-sn65dsi86: Implement lane reordering + polarity

Hi,

On Tue, May 5, 2020 at 1:24 AM Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Douglas,
>
> Thank you for the patch.
>
> On Mon, May 04, 2020 at 09:36:31PM -0700, Douglas Anderson wrote:
> > The ti-sn65dsi86 MIPI DSI to eDP bridge chip supports arbitrary
> > remapping of eDP lanes and also polarity inversion.  Both of these
> > features have been described in the device tree bindings for the
> > device since the beginning but were never implemented in the driver.
> > Implement both of them.
> >
> > Part of this change also allows you to (via the same device tree
> > bindings) specify to use fewer than the max number of DP lanes that
> > the panel reports.  This could be useful if your display supports more
> > lanes but only a few are hooked up on your board.
> >
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > ---
> > This patch is based upon my my outstanding series[1] not because there
> > is any real requirement but simply to avoid merge conflicts.  I
> > believe that my previous series is ready to land.  If, however, you'd
> > prefer that I rebase this patch somewhere atop something else then
> > please shout.
> >
> > [1] https://lore.kernel.org/r/20200430194617.197510-1-dianders@chromium.org
> >
> >  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 75 ++++++++++++++++++++++-----
> >  1 file changed, 62 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > index 1a125423eb07..52cca54b525f 100644
> > --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > @@ -50,8 +50,12 @@
> >  #define SN_CHA_VERTICAL_BACK_PORCH_REG               0x36
> >  #define SN_CHA_HORIZONTAL_FRONT_PORCH_REG    0x38
> >  #define SN_CHA_VERTICAL_FRONT_PORCH_REG              0x3A
> > +#define SN_LN_ASSIGN_REG                     0x59
> > +#define  LN_ASSIGN_WIDTH                     2
> >  #define SN_ENH_FRAME_REG                     0x5A
> >  #define  VSTREAM_ENABLE                              BIT(3)
> > +#define  LN_POLRS_OFFSET                     4
> > +#define  LN_POLRS_MASK                               0xf0
> >  #define SN_DATA_FORMAT_REG                   0x5B
> >  #define  BPP_18_RGB                          BIT(0)
> >  #define SN_HPD_DISABLE_REG                   0x5C
> > @@ -98,6 +102,7 @@
> >
> >  #define SN_REGULATOR_SUPPLY_NUM              4
> >
> > +#define SN_MAX_DP_LANES                      4
> >  #define SN_NUM_GPIOS                 4
> >
> >  /**
> > @@ -115,6 +120,8 @@
> >   * @enable_gpio:  The GPIO we toggle to enable the bridge.
> >   * @supplies:     Data for bulk enabling/disabling our regulators.
> >   * @dp_lanes:     Count of dp_lanes we're using.
> > + * @ln_assign:    Value to program to the LN_ASSIGN register.
> > + * @ln_polr:      Value for the 4-bit LN_POLRS field of SN_ENH_FRAME_REG.
> >   *
> >   * @gchip:        If we expose our GPIOs, this is used.
> >   * @gchip_output: A cache of whether we've set GPIOs to output.  This
> > @@ -140,6 +147,8 @@ struct ti_sn_bridge {
> >       struct gpio_desc                *enable_gpio;
> >       struct regulator_bulk_data      supplies[SN_REGULATOR_SUPPLY_NUM];
> >       int                             dp_lanes;
> > +     u8                              ln_assign;
> > +     u8                              ln_polrs;
> >
> >       struct gpio_chip                gchip;
> >       DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);
> > @@ -707,26 +716,20 @@ static void ti_sn_bridge_enable(struct drm_bridge *bridge)
> >       int dp_rate_idx;
> >       unsigned int val;
> >       int ret = -EINVAL;
> > +     int max_dp_lanes;
> >
> > -     /*
> > -      * Run with the maximum number of lanes that the DP sink supports.
> > -      *
> > -      * Depending use cases, we might want to revisit this later because:
> > -      * - It's plausible that someone may have run fewer lines to the
> > -      *   sink than the sink actually supports, assuming that the lines
> > -      *   will just be driven at a higher rate.
> > -      * - The DP spec seems to indicate that it's more important to minimize
> > -      *   the number of lanes than the link rate.
> > -      *
> > -      * If we do revisit, it would be important to measure the power impact.
> > -      */
> > -     pdata->dp_lanes = ti_sn_get_max_lanes(pdata);
> > +     max_dp_lanes = ti_sn_get_max_lanes(pdata);
> > +     pdata->dp_lanes = min(pdata->dp_lanes, max_dp_lanes);
> >
> >       /* DSI_A lane config */
> >       val = CHA_DSI_LANES(4 - pdata->dsi->lanes);
> >       regmap_update_bits(pdata->regmap, SN_DSI_LANES_REG,
> >                          CHA_DSI_LANES_MASK, val);
> >
> > +     regmap_write(pdata->regmap, SN_LN_ASSIGN_REG, pdata->ln_assign);
> > +     regmap_update_bits(pdata->regmap, SN_ENH_FRAME_REG, LN_POLRS_MASK,
> > +                        pdata->ln_polrs << LN_POLRS_OFFSET);
> > +
> >       /* set dsi clk frequency value */
> >       ti_sn_bridge_set_dsi_rate(pdata);
> >
> > @@ -1063,6 +1066,50 @@ static int ti_sn_setup_gpio_controller(struct ti_sn_bridge *pdata)
> >       return ret;
> >  }
> >
> > +static void ti_sn_bridge_parse_lanes(struct ti_sn_bridge *pdata,
> > +                                  struct device_node *np)
> > +{
> > +     u32 lane_assignments[SN_MAX_DP_LANES] = { 0, 1, 2, 3 };
> > +     u32 lane_polarities[SN_MAX_DP_LANES] = { };
> > +     struct device_node *endpoint;
> > +     u8 ln_assign = 0;
> > +     u8 ln_polrs = 0;
> > +     int dp_lanes;
> > +     int i;
> > +
> > +     /*
> > +      * Read config from the device tree about lane remapping and lane
> > +      * polarities.  These are optional and we assume identity map and
> > +      * normal polarity if nothing is specified.  It's OK to specify just
> > +      * data-lanes but not lane-polarities but not vice versa.
> > +      */
> > +     endpoint = of_graph_get_endpoint_by_regs(np, 1, -1);
>
> Shouldn't you check for endpoint == NULL and fail probe if it is ?

I will if you feel strongly, but I don't think it's necessary.  Specifically:

1. By design of_property_count_u32_elems() will return an error if
passed a NULL node pointer.

2. When we see an error this function will just init things to defaults.

3. Later code which really needs the endpoint to hook things up
properly will catch the error and yell.

...so while I could add a yell here it doesn't seem like it gains much.


> > +     dp_lanes = of_property_count_u32_elems(endpoint, "data-lanes");
> > +     if (dp_lanes > 0) {
> > +             of_property_read_u32_array(endpoint, "data-lanes",
> > +                                        lane_assignments, dp_lanes);
> > +             of_property_read_u32_array(endpoint, "lane-polarities",
> > +                                        lane_polarities, dp_lanes);
>
> Similarly, with a buggy DT, you may have a buffer overrun here. I would
> first check that dp_lanes <= SN_MAX_DP_LANES and error out otherwise.

I will definitely add that.  Buffer overrun is no bueno.


> > +     } else {
> > +             dp_lanes = SN_MAX_DP_LANES;
> > +     }
> > +
> > +     /*
> > +      * Convert into register format.  Loop over all lanes even if
> > +      * data-lanes had fewer elements so that we nicely initialize
> > +      * the LN_ASSIGN register.
> > +      */
> > +     for (i = SN_MAX_DP_LANES - 1; i >= 0; i--) {
> > +             ln_assign = ln_assign << LN_ASSIGN_WIDTH | lane_assignments[i];
> > +             ln_polrs = ln_polrs << 1 | lane_polarities[i];
> > +     }
>
> The datasheet documents the lane remapping register as allowing pretty
> much any combination, but "Table 12. Logical to Physical Supported
> Combinations" only documents a subset (for instance data-lanes = <2 3>
> isn't allowed in that table). Should we guard against invalid
> configurations ?

As I understand it, in general standard kernel policy is to not sanity
check the DT _too_ much.  This feels a bit on the border.  It's up to
the person designing the board and writing the dts to not get things
like this wrong just like it's up to them to make sure they've setup
the i2c pins for our bus w/ the right pullups, configured our
interrupt properly, not overvolted things, put in the correct address
for MMIO, etc.

I wrote this code (untested) and it feels a bit much:

  if (dp_lanes == 1) {
    if (lane_assignments[0] == 1) {
      pr_warn("Lane 0 to physical pin 1 not suggested\n");
    } else if (lane_assignments[0] != 0) {
      pr_err("Unsupported logical to physical pin mapping\n");
      return -EINVAL;
    }
  } else if (dp_lanes == 2 || dp_lanes == 4) {
    u8 good_mask = dp_lanes == 2 ? 0x3 : 0xf;
    u8 mask = 0;

    for (i = 0; i < dp_lanes; i++)
      mask |= BIT(lane_assignments[i])

    if (mask != good_mask) {
      pr_err("Unsupported logical to physical pin mapping\n");
      return -EINVAL;
    }
  } else {
    pr_err("Invalid number of DP lanes: %d\n", dp_lanes);
  }

If you feel strongly I'll add it to the next version.  Does anyone
else have any opinions of whether they'd like all that checking or
whether we should just trust the person designing the hardware and
writing the device tree to put the right values in?


-Doug

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