Re: [PATCH 2/2] drm: bridge: ldb: Configure LDB clock in .mode_set

[Marek Vasut <marex@denx.de>]

On 10/10/24 9:15 AM, Liu Ying wrote:
> On 10/09/2024, Marek Vasut wrote:
>> The LDB serializer clock operate at either x7 or x14 rate of the input
> 
> Isn't it either x7 or 3.5x?

Is it 3.5 for the dual-link LVDS ?
I don't have such a panel right now to test.

[...]

>> diff --git a/drivers/gpu/drm/bridge/fsl-ldb.c b/drivers/gpu/drm/bridge/fsl-ldb.c
>> index 0e4bac7dd04ff..a3a31467fcc85 100644
>> --- a/drivers/gpu/drm/bridge/fsl-ldb.c
>> +++ b/drivers/gpu/drm/bridge/fsl-ldb.c
>> @@ -278,6 +278,16 @@ fsl_ldb_mode_valid(struct drm_bridge *bridge,
>>   	return MODE_OK;
>>   }
>>   
>> +static void fsl_ldb_mode_set(struct drm_bridge *bridge,
>> +			       const struct drm_display_mode *mode,
>> +			       const struct drm_display_mode *adj)
>> +{
>> +	struct fsl_ldb *fsl_ldb = to_fsl_ldb(bridge);
>> +	unsigned long requested_link_freq = fsl_ldb_link_frequency(fsl_ldb, mode->clock);
>> +
>> +	clk_set_rate(fsl_ldb->clk, requested_link_freq);
> 
> The mode_set callback won't be called when only crtc_state->active
> is changed from false to true in an atomic commit, e.g., blanking
> the emulated fbdev first and then unblanking it.  So, in this case,
> the clk_set_rate() in fsl_ldb_atomic_enable() is still called after
> those from mxsfb_kms or lcdif_kms.
> 
> Also, it doesn't look neat to call clk_set_rate() from both mode_set
> callback and atomic_enable callback.

I agree the mode_set callback is not the best place for this.
Do you know of a better callback where to do this ? I couldn't find one.

> The idea is to assign a reasonable PLL clock rate in DT to make
> display drivers' life easier, especially for i.MX8MP where LDB,
> Samsung MIPI DSI may use a single PLL at the same time.
I would really like to avoid setting arbitrary clock in DT, esp. if it 
can be avoided. And it surely can be avoided for this simple use case.