Re: [PATCH v4 08/26] drm/sun4i: sun6i_mipi_dsi: Fix VBP size calculation

[Maxime Ripard]

On Thu, Nov 15, 2018 at 11:19:53PM +0530, Jagan Teki wrote:
> On Thu, Nov 15, 2018 at 3:26 PM Maxime Ripard <maxime.ripard@xxxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On Tue, Nov 13, 2018 at 04:46:15PM +0530, Jagan Teki wrote:
> > > The horizontal and vertical back porch calculation in BSP
> > > code is simply following the Linux drm comment diagram, in
> > > include/drm/drm_modes.h which is
> > >
> > > [hv]back porch = [hv]total - [hv]sync_end
> > >
> > > BSP code form BPI-M64-bsp is calculating vertical back porch as
> > > (from linux-sunxi/drivers/video/sunxi/disp2/disp/de/disp_lcd.c)
> > >
> > > timmings->ver_sync_time= panel_info->lcd_vspw;
> > > timmings->ver_back_porch= panel_info->lcd_vbp-panel_info->lcd_vspw;
> > >
> > > vbp = panel->lcd_vbp;
> > > vspw = panel->lcd_vspw;
> > > dsi_dev[sel]->dsi_basic_size0.bits.vbp = vbp-vspw;
> > > dsi_dev[sel]->dsi_basic_size0.bits.vbp = panel->lcd_vbp - panel->lcd_vspw;
> > > =>  timmings->ver_back_porch + panel_info->lcd_vspw - panel_info->lcd_vspw
> > > =>  timmings->ver_back_porch
> > > =>  mode->vtotal - mode->end
> > >
> > > Which evatually same as mode->vtotal - mode->vsync_end so update the
> > > same in SUN6I_DSI_BASIC_SIZE0_VBP
> > >
> > > On the information note, existing SUN6I_DSI_BASIC_SIZE0_VSA is proper
> > > value.
> > >
> > > Signed-off-by: Jagan Teki <jagan@xxxxxxxxxxxxxxxxxxxx>
> >
> > I've tested your changes on my A33 board, and this commit will break
> > it.
> >
> > It creates vblank timeouts, and visual artifacts at the bottom of the
> > display.
> 
> Strange, VBP is earlier gives front porch which is anyway wrong.
> 
> >
> > Later commits seem to fix the issue, but will create some blanking on
> > the upper third of the display.
> >
> > Since the documentation is quite sparse, and a MIPI-DSI analyzer is
> > way too expensive, I'd really like to have at least what each of these
> > commits are actually fixing, and what symptoms each of these were
> > causing, and not just "the BSP does it".
> 
> W/o this 2-lane panel is breaking, same vblank timeout and visual
> artifacts at the bottom of the panel. though the commits may reference
> BSP, I have at-least tested on 3 different panels for us to prove its
> working.
> 
> > Having some datasheet for the panels you had working would help too.
> 
> Unfortunately datasheet doesn't have any required information what we
> actually looking for.

Not even the timings? How did you get that information then?

Maxime

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

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