Re: [PATCH v6 2/3] media: imx: add a driver for i.MX8MQ mipi csi rx phy and controller

[Martin Kepplinger]

Am Freitag, dem 16.07.2021 um 00:52 +0300 schrieb Laurent Pinchart:
> Hi Martin,
> 
> On Thu, Jul 15, 2021 at 09:37:24AM +0200, Martin Kepplinger wrote:
> > Am Mittwoch, dem 14.07.2021 um 21:24 +0300 schrieb Laurent
> > Pinchart:
> > > On Wed, Jul 14, 2021 at 01:19:30PM +0200, Martin Kepplinger
> > > wrote:
> > > > Add a driver to support the i.MX8MQ MIPI CSI receiver. The
> > > > hardware side
> > > > is based on
> > > > https://source.codeaurora.org/external/imx/linux-imx/tree/drivers/media/platform/imx8/mxc-mipi-csi2_yav.c?h=imx_5.4.70_2.3.0
> > > > 
> > > > It's built as part of VIDEO_IMX7_CSI because that's documented
> > > > to support
> > > > i.MX8M platforms. This driver adds i.MX8MQ support where
> > > > currently only the
> > > > i.MX8MM platform has been supported.
> > > > 
> > > > Signed-off-by: Martin Kepplinger <martin.kepplinger@xxxxxxx>
> > > > ---
> > > >  drivers/staging/media/imx/Makefile           |   1 +
> > > >  drivers/staging/media/imx/imx8mq-mipi-csi2.c | 949
> > > > +++++++++++++++++++
> > > >  2 files changed, 950 insertions(+)
> > > >  create mode 100644 drivers/staging/media/imx/imx8mq-mipi-
> > > > csi2.c
> > > > 
> > > > diff --git a/drivers/staging/media/imx/Makefile
> > > > b/drivers/staging/media/imx/Makefile
> > > > index 6ac33275cc97..19c2fc54d424 100644
> > > > --- a/drivers/staging/media/imx/Makefile
> > > > +++ b/drivers/staging/media/imx/Makefile
> > > > @@ -16,3 +16,4 @@ obj-$(CONFIG_VIDEO_IMX_CSI) += imx6-mipi-
> > > > csi2.o
> 
> [snip]
> 
> > > > +static int imx8mq_mipi_csi_calc_hs_settle(struct csi_state
> > > > *state)
> > > > +{
> > > > +       u32 width = state-
> > > > >format_mbus[MIPI_CSI2_PAD_SINK].width;
> > > > +       u32 height = state-
> > > > >format_mbus[MIPI_CSI2_PAD_SINK].height;
> > > > +       s64 link_freq;
> > > > +       u32 lane_rate;
> > > > +
> > > > +       /* Calculate the line rate from the pixel rate. */
> > > > +       link_freq = v4l2_get_link_freq(state->src_sd-
> > > > >ctrl_handler,
> > > > +                                      state->csi2_fmt->width,
> > > > +                                      state-
> > > > >bus.num_data_lanes * 2);
> > > > +       if (link_freq < 0) {
> > > > +               dev_err(state->dev, "Unable to obtain link
> > > > frequency: %d\n",
> > > > +                       (int)link_freq);
> > > > +               return link_freq;
> > > > +       }
> > > > +
> > > > +       lane_rate = link_freq * 2;
> > > > +       if (lane_rate < 80000000 || lane_rate > 1500000000) {
> > > > +               dev_dbg(state->dev, "Out-of-bound lane rate
> > > > %u\n", lane_rate);
> > > > +               return -EINVAL;
> > > > +       }
> > > > +
> > > > +       /*
> > > > https://community.nxp.com/t5/i-MX-Processors/Explenation-for-HS-SETTLE-parameter-in-MIPI-CSI-D-PHY-registers/m-p/764275/highlight/true#M118744
> > > >  */
> > > > +       if (lane_rate < 250000000)
> > > > +               state->hs_settle = 0xb;
> > > > +       else if (lane_rate < 500000000)
> > > > +               state->hs_settle = 0x8;
> > > > +       else
> > > > +               state->hs_settle = 0x6;
> > > 
> > > We could possibly compute this value based on the formula from
> > > the table
> > > in that page, but maybe that's overkill ? If you want to give it
> > > a try,
> > > it would be along those lines.
> > > 
> > >         /*
> > >          * The D-PHY specification requires Ths-settle to be in
> > > the range
> > >          * 85ns + 6*UI to 140ns + 10*UI, with the unit interval
> > > UI being half
> > >          * the clock period.
> > >          *
> > >          * The Ths-settle value is expressed in the hardware as a
> > > multiple of
> > >          * the Esc clock period:
> > >          *
> > >          * Ths-settle = (PRG_RXHS_SETTLE + 1) * Tperiod of
> > > RxClkInEsc
> > >          *
> > >          * Due to the one cycle inaccuracy introduced by
> > > rounding, the
> > >          * documentation recommends picking a value away from the
> > > boundaries.
> > >          * Let's pick the average.
> > >          */
> > >         esc_clk_rate = clk_get_rate(...);
> > > 
> > >         min_ths_settle = 85 + 6 * 1000000 / (lane_rate / 1000);
> > >         max_ths_settle = 140 + 10 * 1000000 / (lane_rate / 1000);
> > >         ths_settle = (min_ths_settle + max_ths_settle) / 2;
> > > 
> > >         state->hs_settle = ths_settle * esc_clk_rate / 1000000000
> > > - 1;
> > 
> > I experimented a bit but would like to leave this as a task for
> > later
> > if that's ok. it's correct and simple now. also, using clks[i].clk
> > based on the name string would feel better to submit seperately
> > later.
> 
> That's OK with me, but I may then submit a patch on top fairly soon
> :-)
> Have you been able to test if this code works on your device ? The
> main
> reason why I think it's better is that it doesn't hardcode a specific
> escape clock frequency assumption, so it should be able to
> accommodate a
> wider range of use cases. If we change it later, there's always a
> risk
> of regressions, while if we do this from the start, we'll figure out
> quickly if it doesn't work in some cases.
> 

taking your code basically as-is doesn't yet work, but it helps a bit.
tbh I don't even know how to correctly read that table / calculation:
what is the exact relation of the calculated Ths_settle time inverval
to the hs_settle register bits?

if the 2 of us can't quickly figure it out I can ask NXP via that
community forum issue and I created
https://source.puri.sm/Librem5/linux-next/-/issues/340 so I won't
forget about it.

thanks!