Re: [PATCH] drm/bridge: ti-sn65dsi86: Implement lane reordering + polarity

From: Doug Anderson
Date: Tue May 05 2020 - 17:12:38 EST

Hi,

On Tue, May 5, 2020 at 2:06 PM Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> Hi Doug,
>
> On Tue, May 05, 2020 at 10:59:30AM -0700, Doug Anderson wrote:
> > On Tue, May 5, 2020 at 1:24 AM Laurent Pinchart wrote:
> > > 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@xxxxxxxxxxxx>
> > > > ---
> > > > 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@xxxxxxxxxxxx
> > > >
> > > > 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.
>
> As long as it doesn't crash and we eventually catch the error I'm fine.
> I usually try to catch them early as otherwise it gets harder to make
> sure all code paths are sanitized. Up to you.

OK, I'll leave it as-is for now unless someone else chimes in and puts
my opinion in the minority or unless you change your mind and decide
you really hate the way I'm doing it.


> > > > + 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?
>
> If we don't want to test that, I would at least document it in the DT
> bindings. It will be a good occasion to switch the bindings to YAML ;-)

Interesting that you bring this up. Conversion to yaml is sitting
waiting to land (or additional review comments):

https://lore.kernel.org/r/20200430124442.v4.4.Ifcdc4ecb12742a27862744ee1e8753cb95a38a7f@changeid/

I'll add this documentation into the comments of the yaml, but I'm not
going to try to implement enforcement at the yaml level.

-Doug