Hi Thomas, 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. > > 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
