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

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Message-ID: <9f98e3b5194640cf578d2aa290c6d0ff9ab92369.camel@pengutronix.de>
Date:   Thu, 04 May 2023 09:14:57 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Jagan Teki <jagan@...rulasolutions.com>,
        Adam Ford <aford173@...il.com>
Cc:     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

Am Mittwoch, dem 03.05.2023 um 22:05 +0530 schrieb Jagan Teki:
> 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?
> 
The blanking packets are are MIPI long packets, so 4 byte header,
payload, 2 bytes footer. I experimented with different blanking sizes
and haven't found that it would make any difference. I don't think any
real-world horizontal blanking sizes would require multiple packets to
be sent.

Regards,
Lucas