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

[Liu Ying]

On 10/11/2024, Marek Vasut wrote:
> 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 ?

Yes.

static unsigned long fsl_ldb_link_frequency(struct fsl_ldb *fsl_ldb, int clock)  
{                                                                                
        if (fsl_ldb_is_dual(fsl_ldb))                                            
                return clock * 3500;                                             
        else                                                                     
                return clock * 7000;                                             
}    

> I don't have such a panel right now to test.

You can add a panel DT node for test to see the relationship
between the clocks, without a real dual-link LVDS panel.

> 
> [...]
> 
>>> 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.

A wild idea is to change the order between the CRTC atomic_enable
callback and the bridge one by implementing your own
atomic_commit_tail...  I don't think there is any place to do this
other than atomic_enable callback.

Anyway, I don't think it is necessary to manage the clk_set_rate()
function calls between this driver and mxsfb_kms or lcdif_kms
because "video_pll1" clock rate is supposed to be assigned in DT...

> 
>> 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.

... just like I said in patch 1/2, "video_pll1" clock rate needs
to be x2 "media_ldb" clock rate for dual LVDS link mode. Without
an assigned "video_pll1" clock rate in DT, this driver cannot
achieve that.  And, the i.MX8MP LDB + Samsung MIPI DSI case is
not simple considering using one single PLL and display modes
read from EDID.  

-- 
Regards,
Liu Ying