Re: [PATCH 4/8] drm/bridge: fsl-ldb: Use clk_round_rate() to validate "ldb" clock rate

[Liu Ying <victor.liu@nxp.com>]

On 09/30/2024, Maxime Ripard wrote:
> On Mon, Sep 30, 2024 at 01:28:59PM GMT, Liu Ying wrote:
>> Multiple display modes could be read from a display device's EDID.
>> Use clk_round_rate() to validate the "ldb" clock rate for each mode
>> in drm_bridge_funcs::mode_valid() to filter unsupported modes out.
>>
>> Also, if the "ldb" clock and the pixel clock are sibling in clock
>> tree, use clk_round_rate() to validate the pixel clock rate against
>> the "ldb" clock.  This is not done in display controller driver
>> because drm_crtc_helper_funcs::mode_valid() may not decide to do
>> the validation or not if multiple encoders are connected to the CRTC,
>> e.g., i.MX93 LCDIF may connect with MIPI DSI controller, LDB and
>> parallel display output simultaneously.
>>
>> Signed-off-by: Liu Ying <victor.liu@nxp.com>
>> ---
>>  drivers/gpu/drm/bridge/fsl-ldb.c | 22 ++++++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/bridge/fsl-ldb.c b/drivers/gpu/drm/bridge/fsl-ldb.c
>> index b559f3e0bef6..ee8471c86617 100644
>> --- a/drivers/gpu/drm/bridge/fsl-ldb.c
>> +++ b/drivers/gpu/drm/bridge/fsl-ldb.c
>> @@ -11,6 +11,7 @@
>>  #include <linux/of_graph.h>
>>  #include <linux/platform_device.h>
>>  #include <linux/regmap.h>
>> +#include <linux/units.h>
>>  
>>  #include <drm/drm_atomic_helper.h>
>>  #include <drm/drm_bridge.h>
>> @@ -64,6 +65,7 @@ struct fsl_ldb_devdata {
>>  	u32 lvds_ctrl;
>>  	bool lvds_en_bit;
>>  	bool single_ctrl_reg;
>> +	bool ldb_clk_pixel_clk_sibling;
>>  };
>>  
>>  static const struct fsl_ldb_devdata fsl_ldb_devdata[] = {
>> @@ -74,11 +76,13 @@ static const struct fsl_ldb_devdata fsl_ldb_devdata[] = {
>>  	[IMX8MP_LDB] = {
>>  		.ldb_ctrl = 0x5c,
>>  		.lvds_ctrl = 0x128,
>> +		.ldb_clk_pixel_clk_sibling = true,
>>  	},
>>  	[IMX93_LDB] = {
>>  		.ldb_ctrl = 0x20,
>>  		.lvds_ctrl = 0x24,
>>  		.lvds_en_bit = true,
>> +		.ldb_clk_pixel_clk_sibling = true,
>>  	},
>>  };
>>  
>> @@ -269,11 +273,29 @@ fsl_ldb_mode_valid(struct drm_bridge *bridge,
>>  		   const struct drm_display_info *info,
>>  		   const struct drm_display_mode *mode)
>>  {
>> +	unsigned long link_freq, pclk_rate, rounded_pclk_rate;
>>  	struct fsl_ldb *fsl_ldb = to_fsl_ldb(bridge);
>>  
>>  	if (mode->clock > (fsl_ldb_is_dual(fsl_ldb) ? 160000 : 80000))
>>  		return MODE_CLOCK_HIGH;
>>  
>> +	/* Validate "ldb" clock rate. */
>> +	link_freq = fsl_ldb_link_frequency(fsl_ldb, mode->clock);
>> +	if (link_freq != clk_round_rate(fsl_ldb->clk, link_freq))
>> +		return MODE_NOCLOCK;
>> +
>> +	/*
>> +	 * Use "ldb" clock to validate pixel clock rate,
>> +	 * if the two clocks are sibling.
>> +	 */
>> +	if (fsl_ldb->devdata->ldb_clk_pixel_clk_sibling) {
>> +		pclk_rate = mode->clock * HZ_PER_KHZ;
>> +
>> +		rounded_pclk_rate = clk_round_rate(fsl_ldb->clk, pclk_rate);
>> +		if (rounded_pclk_rate != pclk_rate)
>> +			return MODE_NOCLOCK;
>> +	}
>> +
> 
> I guess this is to workaround the fact that the parent rate would be
> changed, and thus the sibling rate as well? This should be documented in
> a comment if so.

This is to workaround the fact that the display controller driver
(lcdif_kms.c) cannot do the mode validation against pixel clock, as
the commit message mentions.

The parent clock is IMX8MP_VIDEO_PLL1_OUT and it's clock rate is not
supposed to be changed any more once IMX8MP_VIDEO_PLL1 clock rate is
set by using DT assigned-clock-rates property.  For i.MX8MP EVK, the
clock rate is assigned to 1039500000Hz in imx8mp.dtsi in media_blk_ctrl
node.

> 
> Maxime

-- 
Regards,
Liu Ying