Subject: [PATCH 2/2 v2] drm: bridge: Add THS8134A/B support to dumb VGA DAC

From: [email-masked] (Laurent Pinchart)
Date: Thu, 19 Oct 2017 00:16:50 +0300

Hi Linus,

Thank you for the patch.

On Tuesday, 17 October 2017 13:19:04 EEST Linus Walleij wrote:
> This extends the dumb VGA DAC bridge to handle the THS8134A
> and THS8134B VGA DACs in addition to those already handled.
> 
> The THS8134A, THS8134B and as it turns out also THS8135 need to
> have data clocked out at the negative edge of the clock pulse,
> since they clock it into the DAC at the positive edge (so by
> then it needs to be stable) so we need some extra logic to flag
> this on the connector to the driver.
> 
> The semantics of the flag DRM_BUS_FLAG_PIXDATA_NEGEDGE in
> <drm/drm_connector.h> clearly indicates that this flag tells
> when to *drive* the data, not when the receiver *reads* it,
> so the TI variants needs to be handled like this.
> 
> Introduce a variant struct and contain the information there,
> and add a bit of helpful comments about how this works so
> people will get it right when adding new DACs or connectiong
> new display drivers to DACs.
> 
> The fact that THS8135 might be working on some systems today
> is probably due to the fact that the display driver cannot
> configure when the data is clocked out and the electronics
> have simply been designed around it so it works anyways.
> 
> The phenomenon is very real on the ARM reference designs using
> PL111 where the hardware can control which edge to push out
> the data.
> 
> Cc: Laurent Pinchart <[email-masked]>
> Cc: Bartosz Golaszewski <[email-masked]>
> Cc: Maxime Ripard <[email-masked]>
> Signed-off-by: Linus Walleij <[email-masked]>
> ---
> ChangeLog v1->v2:
> - Alphabetize includes
> - Use a u32 with the bus polarity flags and just encode the
>   polarity using the DRM define directly.
> - Rename vendor_data to vendor_info.
> - Simplify assignment of the flag as it is just a simple
>   u32 now.
> - Probe all TI variants on the "ti,ths813x" wildcard for now,
>   we only need to know that the device is in this family to
>   set the clock edge flag right.
> ---
>  drivers/gpu/drm/bridge/dumb-vga-dac.c | 51 +++++++++++++++++++++++++++++---
>  1 file changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/dumb-vga-dac.c
> b/drivers/gpu/drm/bridge/dumb-vga-dac.c index 831a606c4706..9cd19e4c33c9
> 100644
> --- a/drivers/gpu/drm/bridge/dumb-vga-dac.c
> +++ b/drivers/gpu/drm/bridge/dumb-vga-dac.c
> @@ -11,6 +11,7 @@
>   */
> 
>  #include <linux/module.h>
> +#include <linux/of_device.h>
>  #include <linux/of_graph.h>
>  #include <linux/regulator/consumer.h>
> 
> @@ -19,9 +20,18 @@
>  #include <drm/drm_crtc.h>
>  #include <drm/drm_crtc_helper.h>
> 
> +/**
> + * struct vga_dac_info - characteristics of the DAC
> + * @clk_edge_latch: this defines the clock edge latch for the variant
> + */
> +struct vga_dac_info {
> +	u32 clk_edge_latch;
> +};
> +
>  struct dumb_vga {
>  	struct drm_bridge	bridge;
>  	struct drm_connector	connector;
> +	struct vga_dac_info const *variant;

Anything wrong with the usual order of keywords (const struct vga_dac_info 
*variant) ?

>  	struct i2c_adapter	*ddc;
>  	struct regulator	*vdd;
> @@ -55,7 +65,9 @@ static int dumb_vga_get_modes(struct drm_connector
> *connector) }
> 
>  	drm_mode_connector_update_edid_property(connector, edid);
> -	return drm_add_edid_modes(connector, edid);
> +	ret = drm_add_edid_modes(connector, edid);
> +	connector->display_info.bus_flags |= vga->variant->clk_edge_latch;

As far as I can tell drm_add_edid_modes() doesn't set bus_flags, so you could 
keep the return live as-is and set the clock edge flag just before it.

> +	return ret;
> 
>  fallback:
>  	/*
> @@ -67,6 +79,8 @@ static int dumb_vga_get_modes(struct drm_connector
> *connector) /* And prefer a mode pretty much anyone can handle */
>  	drm_set_preferred_mode(connector, 1024, 768);
> 
> +	connector->display_info.bus_flags |= vga->variant->clk_edge_latch;
> +
>  	return ret;
>  }
> 
> @@ -183,6 +197,7 @@ static int dumb_vga_probe(struct platform_device *pdev)
>  	if (!vga)
>  		return -ENOMEM;
>  	platform_set_drvdata(pdev, vga);
> +	vga->variant = of_device_get_match_data(&pdev->dev);
> 
>  	vga->vdd = devm_regulator_get_optional(&pdev->dev, "vdd");
>  	if (IS_ERR(vga->vdd)) {
> @@ -226,10 +241,38 @@ static int dumb_vga_remove(struct platform_device
> *pdev) return 0;
>  }
> 
> +static const struct vga_dac_info default_dac_variant = {
> +	/*
> +	 * These DACs read data on the negative edge. For example in the
> +	 * ADV7123 datasheet (revision D, page 8) there is a timing diagram
> +	 * making this clear. So consequently we need to latch the data
> +	 * on the positive edge.
> +	 */
> +	.clk_edge_latch = DRM_BUS_FLAG_PIXDATA_POSEDGE,
> +};
> +
> +static const struct vga_dac_info ti_ths_dac_variant = {
> +	/*
> +	 * The TI DACs read the data on the positive edge of the CLK,
> +	 * so consequently we need to latch the data on the negative
> +	 * edge.
> +	 */
> +	.clk_edge_latch = DRM_BUS_FLAG_PIXDATA_NEGEDGE,
> +};
> +
>  static const struct of_device_id dumb_vga_match[] = {
> -	{ .compatible = "dumb-vga-dac" },
> -	{ .compatible = "adi,adv7123" },
> -	{ .compatible = "ti,ths8135" },

You need to keep support for this compatible string for backward 
compatibility.

> +	{
> +		.compatible = "dumb-vga-dac",
> +		.data = &default_dac_variant,
> +	},
> +	{
> +		.compatible = "adi,adv7123",
> +		.data = &default_dac_variant,
> +	},
> +	{
> +		.compatible = "ti,ths813x",
> +		.data = &ti_ths_dac_variant,
> +	},
>  	{},
>  };
>  MODULE_DEVICE_TABLE(of, dumb_vga_match);

-- 
Regards,

Laurent Pinchart