linux-kernel - Re: [PATCH V2 1/6] drm: bridge: samsung-dsim: fix blanking packet size calculation

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Date:   Wed, 3 May 2023 22:05:56 +0530
From:   Jagan Teki <jagan@...rulasolutions.com>
To:     Adam Ford <aford173@...il.com>
Cc:     l.stach@...gutronix.de, dri-devel@...ts.freedesktop.org,
        m.szyprowski@...sung.com, aford@...conembedded.com,
        dario.binacchi@...rulasolutions.com,
        Inki Dae <inki.dae@...sung.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Frieder Schrempf <frieder.schrempf@...tron.de>,
        Marek Vasut <marex@...x.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/6] drm: bridge: samsung-dsim: fix blanking packet
 size calculation

On Mon, Apr 24, 2023 at 3:17 PM Adam Ford <aford173@...il.com> wrote:
>
> On Mon, Apr 24, 2023 at 4:03 AM Jagan Teki <jagan@...rulasolutions.com> wrote:
> >
> > On Sun, Apr 23, 2023 at 5:42 PM Adam Ford <aford173@...il.com> wrote:
> > >
> > > From: Lucas Stach <l.stach@...gutronix.de>
> > >
> > > Scale the blanking packet sizes to match the ratio between HS clock
> > > and DPI interface clock. The controller seems to do internal scaling
> > > to the number of active lanes, so we don't take those into account.
> > >
> > > Signed-off-by: Lucas Stach <l.stach@...gutronix.de>
> > > Signed-off-by: Adam Ford <aford173@...il.com>
> > > ---
> > >  drivers/gpu/drm/bridge/samsung-dsim.c | 18 +++++++++++++++---
> > >  1 file changed, 15 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> > > index e0a402a85787..2be3b58624c3 100644
> > > --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> > > +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> > > @@ -874,17 +874,29 @@ static void samsung_dsim_set_display_mode(struct samsung_dsim *dsi)
> > >         u32 reg;
> > >
> > >         if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO) {
> > > +               int byte_clk_khz = dsi->burst_clk_rate / 1000 / 8;
> > > +               int hfp = (m->hsync_start - m->hdisplay) * byte_clk_khz / m->clock;
> >
> > I do not quite understand why it depends on burst_clk_rate, would you
> > please explain? does it depends on bpp something like this
> >
> > mipi_dsi_pixel_format_to_bpp(format) / 8
>
> The pixel clock is currently set to the burst clock rate.  Dividing
> the clock by 1000 gets the pixel clock in KHz, and dividing by 8
> converts bits to bytes.
> Later in the series, I change the clock from the burst clock to the
> cached value returned from samsung_dsim_set_pll.

Okay.

>
> >
> > > +               int hbp = (m->htotal - m->hsync_end) * byte_clk_khz / m->clock;
> > > +               int hsa = (m->hsync_end - m->hsync_start) * byte_clk_khz / m->clock;
> > > +
> > > +               /* remove packet overhead when possible */
> > > +               hfp = max(hfp - 6, 0);
> > > +               hbp = max(hbp - 6, 0);
> > > +               hsa = max(hsa - 6, 0);
> >
> > 6 blanking packet overhead here means, 4 bytes + payload + 2 bytes
> > format? does this packet overhead depends on the respective porch's
> > like hpf, hbp and hsa has different packet overheads?
>
> Lucas might be able to explain this better.  However, it does match
> the values of the downstream NXP kernel, and I tried playing with
> these values manually, and 6 appeared to be the only number that
> seemed to work for me too.  I abandoned my approach for Lucas'
> implementation, because it seemed more clear than mine.
> Maybe Lucas can chime in, since this is really his patch.

Lucan, any inputs?

Jagan.