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

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Am 07.09.23 um 10:03 schrieb Thierry Reding:
On Thu, Aug 31, 2023 at 10:04:29AM +0200, Daniel Vetter wrote:
On Thu, 31 Aug 2023 at 08:33, Mikko Perttunen <cyndis@xxxxxxxx> wrote:

On 8/30/23 13:19, Thomas Zimmermann wrote:

Am 25.08.23 um 15:22 schrieb Thierry Reding:
From: Thierry Reding <treding@xxxxxxxxxx>

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@xxxxxxxxxx>
   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)
-    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.

Take a look at struct drm_device.driver_features. It let's you clear the flags that your device doesn't support.

Best regards

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.


Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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