linux-kernel - Re: [PATCH v2 31/37] drm: rcar-du: Add support for the nomodeset kernel parameter

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Date:   Fri, 28 Jan 2022 12:34:05 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 31/37] drm: rcar-du: Add support for the nomodeset
 kernel parameter

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@...hat.com>
> >> ---
> >>
> >> (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@...asonboard.com>
> > 
> >>          rcar_du_of_init(rcar_du_of_table);
> >>   
> >>          return platform_driver_register(&rcar_du_platform_driver);

-- 
Regards,

Laurent Pinchart