linux-kernel - Re: [RFC v1 0/1] imx-drm: match ipu_di_signal_cfg's clk

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Date:   Thu, 25 Oct 2018 14:15:18 -0400
From:   thesven73@...il.com
To:     svendev@...x.com, p.zabel@...gutronix.de, denis@...rea.com,
        rmk+kernel@....linux.org.uk
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [RFC v1 0/1] imx-drm: match ipu_di_signal_cfg's clk_pol with its observed behaviour.

Hi Russell, thanks for looking at the RFC !

> Given what's in the documentation, I'd opt for it describing the
> edge that the output data changes, not the latch edge.  With that
> interpretation, the existing code is correct.

The clock polarity is reversed in Freescale's non-mainline fbdev code,
which we were using previously, contributing to the confusion.

Yes, I think your interpretation is the correct one. Let's double-check.

Suppose we set this bus flag in the panel/mode definition:

drm/drm_connector.h:
/* drive data on pos. edge */
#define DRM_BUS_FLAG_PIXDATA_POSEDGE	(1<<2)

According to your interpretation, this means we're asking the data to change
on the positive edge, and be stable on the negative edge. Correct?

then clk_pol == 1:
drivers/gpu/drm/imx/ipuv3-crtc.c:
	sig_cfg.clk_pol = !!(imx_crtc_state->bus_flags &
			     DRM_BUS_FLAG_PIXDATA_POSEDGE);

then DI_GENERAL bit 17 is set:
drivers/gpu/ipu-v3/ipu-di.c:
	if (sig->clk_pol)
		di_gen |= DI_GEN_POLARITY_DISP_CLK;

and data is stable on the pixel clock's falling edge, as we observed on
the oscilloscope.

All good. Thank you !