linux-kernel - Re: [PATCH v9 2/5] drm/sun4i: sun6i_mipi

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Message-ID: <20190319105648.54i6pd4l74uvp4px@flea>
Date:   Tue, 19 Mar 2019 11:56:48 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Chen-Yu Tsai <wens@...e.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-amarula@...rulasolutions.com,
        Michael Trimarchi <michael@...rulasolutions.com>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH v9 2/5] drm/sun4i: sun6i_mipi_dsi: Fix TCON DRQ set bits

On Mon, Mar 11, 2019 at 08:28:22PM +0530, Jagan Teki wrote:
> On Mon, Mar 11, 2019 at 7:39 PM Maxime Ripard <maxime.ripard@...tlin.com> wrote:
> >
> > On Thu, Mar 07, 2019 at 09:24:02PM +0530, Jagan Teki wrote:
> > > On Thu, Mar 7, 2019 at 9:09 PM Maxime Ripard <maxime.ripard@...tlin.com> wrote:
> > > >
> > > > On Thu, Mar 07, 2019 at 05:49:07PM +0530, Jagan Teki wrote:
> > > > > On Mon, Mar 4, 2019 at 9:13 PM Maxime Ripard <maxime.ripard@...tlin.com> wrote:
> > > > > >
> > > > > > On Sun, Mar 03, 2019 at 11:05:24PM +0530, Jagan Teki wrote:
> > > > > > > TCON DRQ for non-burst DSI mode can computed based on horizontal
> > > > > > > front porch value, but the current driver trying to include sync
> > > > > > > timings along with front porch resulting wrong drq.
> > > > > > >
> > > > > > > This patch is trying to update the drq by subtracting hsync_start
> > > > > > > with hdisplay, which is horizontal front porch.
> > > > > > >
> > > > > > > Current code:
> > > > > > > ------------
> > > > > > > mode->hsync_end - mode->hdisplay => horizontal front porch + sync
> > > > > > >
> > > > > > > With this patch:
> > > > > > > ----------------
> > > > > > > mode->hsync_start - mode->hdisplay => horizontal front porch
> > > > > > >
> > > > > > > BSP code form BPI-M64-bsp is computing TCON DRQ set bits
> > > > > > > for non-burts as (from linux-sunxi/
> > > > > > > drivers/video/sunxi/disp2/disp/de/lowlevel_sun50iw1/de_dsi.c)
> > > > > > >
> > > > > > > => panel->lcd_ht -    panel->lcd_x - panel->lcd_hbp
> > > > > > > => (timmings->hor_front_porch + panel->lcd_hbp + panel->lcd_x)
> > > > > >                                   ^ + sync length +
> > > > > > >    - panel->lcd_x - panel->hbp
> > > > > > > => timmings->hor_front_porch
> > > > > >                                ^ + sync
> > > > > > > => mode->hsync_start - mode->hdisplay
> > > > > >
> > > > > > s/hsync_start/hsync_end/
> > > > >
> > > > > No, it should be front porch so it is hsync_start. This change is
> > > > > trying to update DRQ set to use front porch and above evaluation from
> > > > > BSP, result the same front front porch
> > > > >
> > > > > Current driver has hsync_end - hdisplay which is not front porch
> > > > > timing (it is adding extra sync timing).
> > > >
> > > > It would be if you considered that the back porch actually was the
> > > > back porch plus the sync length. I have found no such evidence, quite
> > > > the opposite actually, everything seems to point at the fact that
> > > > unlike the TCON, the DSI block uses the back porch as only the back
> > > > porch.
> > >
> > > Sorry, I'm not clear about back porch here.
> > >
> > > The current code has mode->hsync_end - mode->hdisplay which is Front
> > > porch + sync time do you think it's not?
> >
> > It is.
> >
> > > DRQ set time is pure front porch value (according BSP) as I didn't
> > > see any information about DRQ set bits in manual or anywhere.
> >
> > This is what I'm telling you. If you consider the back porch as only
> > the back porch, then the result of that BSP calculation you mentionned
> > earlier is the front porch and the sync length.
> >
> > You imply that the back porch should actually be treated as the back
> > porch and the sync length in your calculation. What makes you say so?
>
> I'm consider the computations accordingly to drm_modes.h and which
> matches similar like BSP code.
>
> mode->hsync_end - mode->hdisplay = front porch + sync
>
> but the actual computation is
> mode->hsync_start - mode->hdisplay =>  front porch
>
> Which is similar to what BSP is using.
>
> => panel->lcd_ht -    panel->lcd_x - panel->lcd_hbp
> => (timmings->hor_front_porch + panel->lcd_hbp + panel->lcd_x)
>    - panel->lcd_x - panel->hbp
> => timmings->hor_front_porch
> => mode->hsync_start - mode->hdisplay

You're missing the point. What I'm asking for is why you think that
panel->lcd_ht == (timmings->hor_front_porch + panel->lcd_hbp +
panel->lcd_x) and not timmings->hor_front_porch + panel->lcd_hbp +
sync_width + panel->lcd_x

maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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