Hi Javier,
On Thu, Jul 13, 2023 at 6:32 PM Javier Martinez Canillas
<javierm@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
Tested-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> --- 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,
The docs say this is intended for drivers that support runtime_pm or
need the CRTC to be enabled to perform a commit. Might be worthwhile
to add basic Runtime PM, so the I2C controller can go to sleep when
the display is not used.
> +};
> +
> static const uint32_t ssd130x_formats[] = {
> DRM_FORMAT_XRGB8888,
> };
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
