Hi Thomas, On Fri, Jan 28, 2022 at 12:26:03PM +0100, Thomas Zimmermann wrote: > Am 28.01.22 um 12:04 schrieb Laurent Pinchart: > > 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. > > Well, in that case the problem has always been there. We don't make it > worse. What's the point of adding nomodeset support in those drivers then, if it's known not to work ? > >>>>> 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
