linux-kernel - Re: [PATCH] drm/stm: dsi: relax mode

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Date: Tue, 30 Apr 2024 09:48:05 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Sean Nyekjaer <sean@...nix.com>
Cc: Yannick Fertre <yannick.fertre@...s.st.com>, 
	Raphael Gallais-Pou <raphael.gallais-pou@...s.st.com>, Philippe Cornu <philippe.cornu@...s.st.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Alexandre Torgue <alexandre.torgue@...s.st.com>, 
	Robert Foss <rfoss@...nel.org>, Antonio Borneo <antonio.borneo@...s.st.com>, 
	dri-devel@...ts.freedesktop.org, linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/stm: dsi: relax mode_valid clock tolerance

Hi,

On Mon, Apr 29, 2024 at 10:17:45AM +0200, Sean Nyekjaer wrote:
> On Wed, Apr 24, 2024 at 09:21:17AM UTC, Maxime Ripard wrote:
> > Hi,
> > 
> > Sorry, my previous review didn't go through.
> > 
> > On Fri, Mar 22, 2024 at 11:47:31AM +0100, Sean Nyekjaer wrote:
> > > When using the DSI interface via DSI2LVDS bridge, it seems a bit harsh
> > > to reguire the requested and the actual px clock to be within
> > > 50Hz. A typical LVDS display requires the px clock to be within +-10%.
> > > 
> > > In case for HDMI .5% tolerance is required.
> > > 
> > > Fixes: e01356d18273 ("drm/stm: dsi: provide the implementation of mode_valid()")
> > > Signed-off-by: Sean Nyekjaer <sean@...nix.com>
> > > ---
> > >  drivers/gpu/drm/stm/dw_mipi_dsi-stm.c | 7 +++----
> > >  1 file changed, 3 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/stm/dw_mipi_dsi-stm.c b/drivers/gpu/drm/stm/dw_mipi_dsi-stm.c
> > > index d5f8c923d7bc..97936b0ef702 100644
> > > --- a/drivers/gpu/drm/stm/dw_mipi_dsi-stm.c
> > > +++ b/drivers/gpu/drm/stm/dw_mipi_dsi-stm.c
> > > @@ -322,8 +322,6 @@ dw_mipi_dsi_phy_get_timing(void *priv_data, unsigned int lane_mbps,
> > >  	return 0;
> > >  }
> > >  
> > > -#define CLK_TOLERANCE_HZ 50
> > > -
> > >  static enum drm_mode_status
> > >  dw_mipi_dsi_stm_mode_valid(void *priv_data,
> > >  			   const struct drm_display_mode *mode,
> > > @@ -375,9 +373,10 @@ dw_mipi_dsi_stm_mode_valid(void *priv_data,
> > >  		/*
> > >  		 * Filter modes according to the clock value, particularly useful for
> > >  		 * hdmi modes that require precise pixel clocks.
> > > +		 * Check that px_clock is within .5% tolerance.
> > >  		 */
> > > -		if (px_clock_hz < target_px_clock_hz - CLK_TOLERANCE_HZ ||
> > > -		    px_clock_hz > target_px_clock_hz + CLK_TOLERANCE_HZ)
> > > +		if (px_clock_hz < mult_frac(target_px_clock_hz, 995, 1000) ||
> > > +		    px_clock_hz > mult_frac(target_px_clock_hz, 1005, 1000))
> > >  			return MODE_CLOCK_RANGE;
> > 
> > I wonder if it's not something that should be made into a helper. We
> > have a couple of drivers doing it already, so it might be worth creating
> > a function that checks for a given struct clk pointer and pixel clock if
> > it's within parameters.
> > 
> > Maxime
> 
> Yes agree, if the same calculation is happening other places.
> I can't identify where it happens though.

sun4i has one:
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/sun4i/sun4i_rgb.c#L123

> Would that helper function exist in drivers/gpu/drm/drm_hdmi_helper.c ?

There's nothing related to HDMI per se, so in drm_modes is probably
better.

Maxime


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