[PATCH] drm/ssd130x: Fix an oops when attempting to update a disabled plane

[Javier Martinez Canillas <javierm@redhat.com>]

Geert reports that the following NULL pointer dereference happens for him
after commit 49d7d581ceaf ("drm/ssd130x: Don't allocate buffers on each
plane update"):

    [drm] Initialized ssd130x 1.0.0 20220131 for 0-003c on minor 0
    ssd130x-i2c 0-003c: [drm] surface width(128), height(32), bpp(1)
    and format(R1   little-endian (0x20203152))
    Unable to handle kernel NULL pointer dereference at virtual address 00000000
    Oops [#1]
    CPU: 0 PID: 1 Comm: swapper Not tainted
    6.5.0-rc1-orangecrab-02219-g0a529a1e4bf4 #565
    epc : ssd130x_update_rect.isra.0+0x13c/0x340
     ra : ssd130x_update_rect.isra.0+0x2bc/0x340
    ...
    status: 00000120 badaddr: 00000000 cause: 0000000f
    [<c0303d90>] ssd130x_update_rect.isra.0+0x13c/0x340
    [<c0304200>] ssd130x_primary_plane_helper_atomic_update+0x26c/0x284
    [<c02f8d54>] drm_atomic_helper_commit_planes+0xfc/0x27c
    [<c02f9314>] drm_atomic_helper_commit_tail+0x5c/0xb4
    [<c02f94fc>] commit_tail+0x190/0x1b8
    [<c02f99fc>] drm_atomic_helper_commit+0x194/0x1c0
    [<c02c5d00>] drm_atomic_commit+0xa4/0xe4
    [<c02cce40>] drm_client_modeset_commit_atomic+0x244/0x278
    [<c02ccef0>] drm_client_modeset_commit_locked+0x7c/0x1bc
    [<c02cd064>] drm_client_modeset_commit+0x34/0x64
    [<c0301a78>] __drm_fb_helper_restore_fbdev_mode_unlocked+0xc4/0xe8
    [<c0303424>] drm_fb_helper_set_par+0x38/0x58
    [<c027c410>] fbcon_init+0x294/0x534
    ...

The problem is that fbcon calls fbcon_init() which triggers a DRM modeset
and this leads to drm_atomic_helper_commit_planes() attempting to commit
the atomic state for all planes, even the ones whose CRTC is not enabled.

Since the primary plane buffer is allocated in the encoder .atomic_enable
callback, this happens after that initial modeset commit and leads to the
mentioned NULL pointer dereference.

Fix this by not using the default drm_atomic_helper_commit_tail() helper,
but instead the drm_atomic_helper_commit_tail_rpm() function that doesn't
attempt to commit the atomic state for planes related to inactive CRTCs.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

 drivers/gpu/drm/solomon/ssd130x.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/solomon/ssd130x.c b/drivers/gpu/drm/solomon/ssd130x.c
index afb08a8aa9fc..12820d16b15b 100644
--- a/drivers/gpu/drm/solomon/ssd130x.c
+++ b/drivers/gpu/drm/solomon/ssd130x.c
@@ -795,6 +795,10 @@ static const struct drm_mode_config_funcs ssd130x_mode_config_funcs = {
 	.atomic_commit = drm_atomic_helper_commit,
 };
 
+static const struct drm_mode_config_helper_funcs ssd130x_mode_config_helpers = {
+	.atomic_commit_tail = drm_atomic_helper_commit_tail_rpm,
+};
+
 static const uint32_t ssd130x_formats[] = {
 	DRM_FORMAT_XRGB8888,
 };
@@ -923,6 +927,7 @@ static int ssd130x_init_modeset(struct ssd130x_device *ssd130x)
 	drm->mode_config.max_height = max_height;
 	drm->mode_config.preferred_depth = 24;
 	drm->mode_config.funcs = &ssd130x_mode_config_funcs;
+	drm->mode_config.helper_private = &ssd130x_mode_config_helpers;
 
 	/* Primary plane */
 
-- 
2.41.0