Hi Thomas, On Fri, Jan 28, 2022 at 11:46:49AM +0100, Thomas Zimmermann wrote: > Am 28.01.22 um 11:34 schrieb Laurent Pinchart: > > On Fri, Jan 28, 2022 at 10:33:21AM +0100, Thomas Zimmermann wrote: > >> Am 28.01.22 um 10:13 schrieb Kieran Bingham: > >>> Quoting Javier Martinez Canillas (2021-12-17 00:37:46) > >>>> According to disable Documentation/admin-guide/kernel-parameters.txt, this > >>>> parameter can be used to disable kernel modesetting. > >>>> > >>>> DRM drivers will not perform display-mode changes or accelerated rendering > >>>> and only the system framebuffer will be available if it was set-up. > >>> > >>> What is the 'system framebuffer' in this instance? Reading > >>> https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt > >>> it sounds like that means anything already set up by the bootloader. > >> > >> Exactly this. > >> > >>>> But only a few DRM drivers currently check for nomodeset, make this driver > >>>> to also support the command line parameter. > >>>> > >>>> Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx> > >>>> --- > >>>> > >>>> (no changes since v1) > >>>> > >>>> drivers/gpu/drm/rcar-du/rcar_du_drv.c | 3 +++ > >>>> 1 file changed, 3 insertions(+) > >>>> > >>>> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_drv.c b/drivers/gpu/drm/rcar-du/rcar_du_drv.c > >>>> index 5a8131ef81d5..982e450233ed 100644 > >>>> --- a/drivers/gpu/drm/rcar-du/rcar_du_drv.c > >>>> +++ b/drivers/gpu/drm/rcar-du/rcar_du_drv.c > >>>> @@ -701,6 +701,9 @@ static struct platform_driver rcar_du_platform_driver = { > >>>> > >>>> static int __init rcar_du_init(void) > >>>> { > >>>> + if (drm_firmware_drivers_only()) > >>>> + return -ENODEV; > >>>> + > >>> > >>> This will completely disable all control of the display device when > >>> nomodeset is enabled. > >>> > >>> Is there any requirement for us to support outputting to the display if > >>> it was previously set up? presumably without setting or changing any > >>> modes, but simply allowing the existing frame to be updated? > >> > >> There's no requirement for your driver. We just want a parameter where > >> we can conveniently disable most of DRM's drivers and reduce it to a > >> minimum. Helps distributions to provide a simple fallback mode. Most > >> PCI-based drivers already support that. Now we're added it to the other > >> drivers as well. > >> > >>> > >>> I think the implication is that 'firmware drivers' would mean a display > >>> could be updated through some firmware interface, which we won't have > >>> ... so it seems reasonable to accept that this whole driver can be > >>> disabled in that instance. > >> > >> It cannot be 'mode-setted'. We get a pre-configured framebuffer from the > >> firmware or bootloader. Whatever we draw there shows up on the screen. > > > > I doubt that's going to work as you expect, clocks and regulators will > > get disabled at boot if not used by any driver. > > Simpledrm and simplefb attach to these firmware framebuffers. Both > drivers look at the device tree nodes to acquire the relevant clocks and > regulators. How about clocks and regulators for the ancillary devices, such as encoders, or in the R-Car case, the external composer handled by the vsp1 driver (in drivers/media/platform/vsp1) ? This approach may work fine on x86 desktop systems, but for ARM-based devices, the situation is usually more complex. > >>> Reading your mail that brought this thread up in my inbox, I think > >>> you've already hit merge on this, so don't worry about adding a tag in > >>> that instance, but I think this is ok. > >>> > >>> Reviewed-by: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx> > >>> > >>>> rcar_du_of_init(rcar_du_of_table); > >>>> > >>>> return platform_driver_register(&rcar_du_platform_driver); -- Regards, Laurent Pinchart
