Re: [PATCH] drm/rcar-du: dsi: Handle both DRM_MODE_FLAG_N.SYNC and !DRM_MODE_FLAG_P.SYNC

[Marek Vasut <marek.vasut@mailbox.org>]

On 11/8/25 12:23 AM, Laurent Pinchart wrote:
> On Sat, Nov 08, 2025 at 12:04:10AM +0100, Marek Vasut wrote:
>> Since commit 94fe479fae96 ("drm/rcar-du: dsi: Clean up handling of DRM mode flags")
>> the driver does not set TXVMVPRMSET0R_VSPOL_LOW and TXVMVPRMSET0R_HSPOL_LOW
>> for modes which set neither DRM_MODE_FLAG_[PN].SYNC.
> 
> Could you please explain what broke ?

Consider mode->flags, V-ones for simplicity:

Before 94fe479fae96 :

DRM_MODE_FLAG_PVSYNC => vprmset0r |= 0
DRM_MODE_FLAG_NVSYNC => vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW
Neither DRM_MODE_FLAG_[PN]VSYNC => vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW

After 94fe479fae96 :

DRM_MODE_FLAG_PVSYNC => vprmset0r |= 0
DRM_MODE_FLAG_NVSYNC => vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW
Neither DRM_MODE_FLAG_[PN]VSYNC => vprmset0r |= 0 <---------- This broke

The "Neither" case behavior is different. I did not realize that:

DRM_MODE_FLAG_N[HV]SYNC is not equivalent !DRM_MODE_FLAG_P[HV]SYNC

They really are not equivalent .

[...]

>>   	/* Configuration for Video Parameters, input is always RGB888 */
>>   	vprmset0r = TXVMVPRMSET0R_BPP_24;
>> -	if (mode->flags & DRM_MODE_FLAG_NVSYNC)
>> +	if ((mode->flags & DRM_MODE_FLAG_NVSYNC) ||
>> +	    !(mode->flags & DRM_MODE_FLAG_PVSYNC))
>>   		vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW;
> 
> I don't think this restores the previous behaviour. You would need to
> write
> 
> 	if (!(mode->flags & DRM_MODE_FLAG_PVSYNC))
> 		vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW;
This patch covers both the N[HV]SYNC and !P[HV]SYNC , so that should 
restore the behavior to "Before" and explicitly be clear that N[HV]SYNC 
and !P[HV]SYNC are not the same thing.