Re: [PATCH] drm/tegra: Remove existing framebuffer only if we support display

[Thierry Reding <thierry.reding@gmail.com>]

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On Thu, Aug 31, 2023 at 10:04:29AM +0200, Daniel Vetter wrote:
> On Thu, 31 Aug 2023 at 08:33, Mikko Perttunen <cyndis@kapsi.fi> wrote:
> >
> > On 8/30/23 13:19, Thomas Zimmermann wrote:
> > > Hi
> > >
> > > Am 25.08.23 um 15:22 schrieb Thierry Reding:
> > >> From: Thierry Reding <treding@nvidia.com>
> > >>
> > >> Tegra DRM doesn't support display on Tegra234 and later, so make sure
> > >> not to remove any existing framebuffers in that case.
> > >>
> > >> Signed-off-by: Thierry Reding <treding@nvidia.com>
> > >> ---
> > >>   drivers/gpu/drm/tegra/drm.c | 8 +++++---
> > >>   1 file changed, 5 insertions(+), 3 deletions(-)
> > >>
> > >> diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
> > >> index b1e1a78e30c6..7a38dadbc264 100644
> > >> --- a/drivers/gpu/drm/tegra/drm.c
> > >> +++ b/drivers/gpu/drm/tegra/drm.c
> > >> @@ -1220,9 +1220,11 @@ static int host1x_drm_probe(struct
> > >> host1x_device *dev)
> > >>       drm_mode_config_reset(drm);
> > >> -    err = drm_aperture_remove_framebuffers(&tegra_drm_driver);
> > >> -    if (err < 0)
> > >> -        goto hub;
> > >> +    if (drm->mode_config.num_crtc > 0) {
> > >
> > > If you don't support the hardware, wouldn't it be better to return
> > > -ENODEV if !num_crtc?
> >
> > While display is not supported through TegraDRM on Tegra234+, certain
> > multimedia accelerators are supported, so we need to finish probe for those.
> 
> Ideally you also register the tegra driver without DRIVER_MODESET |
> DRIVER_ATOMIC in that case, to avoid unecessary userspace confusion.
> Most userspace can cope with a display driver without any crtc, but I
> think xorg-modesettting actually falls over. Or at least I've seen
> some hacks that the agx people added to make sure X doesn't
> accidentally open the wrong driver.

That's a good point. However I recall from earlier attempts at doing
something like this in Nouveau (although this is now very long ago) that
it's not very easy. The problem, as I recall, is that the driver is a
singleton, so we would essentially be supporting either modesetting or
not, for any device in the system.

Now, it's unlikely that we'd have a mix of one Tegra DRM driver with
display support and another without, but it's something that I recall
back at the time with Nouveau was problematic because you could have the
Tegra integrated graphics (without display support) and a PCI-connected
discrete GPU (with display support) within the same system.

I need to look into it a bit more to see if I can come up with something
good to account for this.

Thierry

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