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

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

Geert Uytterhoeven <geert@linux-m68k.org> writes:

Hello Geert,

> Hi Javier,
>
> On Thu, Jul 13, 2023 at 6:32 PM Javier Martinez Canillas
> <javierm@redhat.com> 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@linux-m68k.org>
>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
>
> Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
>

Thanks for reporting the issue in the first place and for the testing!

>> --- 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.
>

Indeed, I thought the same. But I believe we can do that as a follow-up patch.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat