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Date:   Fri, 16 Jul 2021 00:52:20 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Martin Kepplinger <martin.kepplinger@puri.sm>
Cc:     festevam@gmail.com, krzk@kernel.org, devicetree@vger.kernel.org,
        kernel@pengutronix.de, kernel@puri.sm,
        linux-arm-kernel@lists.infradead.org, linux-imx@nxp.com,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        linux-staging@lists.linux.dev, m.felsch@pengutronix.de,
        mchehab@kernel.org, phone-devel@vger.kernel.org, robh@kernel.org,
        shawnguo@kernel.org, slongerbeam@gmail.com
Subject: Re: [PATCH v6 2/3] media: imx: add a driver for i.MX8MQ mipi csi rx
 phy and controller
Message-ID: <YPCuFA+utjudv11H@pendragon.ideasonboard.com>
References: <20210714111931.324485-1-martin.kepplinger@puri.sm>
 <20210714111931.324485-3-martin.kepplinger@puri.sm>
 <YO8r6pZAduu1ZMK4@pendragon.ideasonboard.com>
 <ce71a71a358247eca3b72ddcddd703206c90f284.camel@puri.sm>
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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@puri.sm>
> > > ---
> > >  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.

-- 
Regards,

Laurent Pinchart