linux-kernel - Re: [PATCH v2 4/5] drm/ssd130x: Don't allocate buffers on each plane update

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Message-ID: <CAMuHMdVXhi52KfpCmnum+9t74UWP+AOLE95xQU6VV6Nz=VHk1Q@mail.gmail.com>
Date:   Thu, 13 Jul 2023 14:44:15 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Maxime Ripard <mripard@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 4/5] drm/ssd130x: Don't allocate buffers on each plane update

Hi Javier,

On Fri, Jun 9, 2023 at 7:09 PM Javier Martinez Canillas
<javierm@...hat.com> wrote:
> The resolutions for these panels are fixed and defined in the Device Tree,
> so there's no point to allocate the buffers on each plane update and that
> can just be done once.
>
> Let's do the allocation and free on the encoder enable and disable helpers
> since that's where others initialization and teardown operations are done.
>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
> ---
>
> (no changes since v1)

Thanks for your patch, which is now commit 49d7d581ceaf4cf8
("drm/ssd130x: Don't allocate buffers on each plane update") in
drm-misc/for-linux-next.

> --- a/drivers/gpu/drm/solomon/ssd130x.c
> +++ b/drivers/gpu/drm/solomon/ssd130x.c
> @@ -701,14 +709,22 @@ static void ssd130x_encoder_helper_atomic_enable(struct drm_encoder *encoder,
>                 return;
>
>         ret = ssd130x_init(ssd130x);
> -       if (ret) {
> -               ssd130x_power_off(ssd130x);
> -               return;
> -       }
> +       if (ret)
> +               goto power_off;
> +
> +       ret = ssd130x_buf_alloc(ssd130x);

This appears to be too late, causing a NULL pointer dereference:

[   59.302761] [<c0303d90>] ssd130x_update_rect.isra.0+0x13c/0x340
[   59.304231] [<c0304200>]
ssd130x_primary_plane_helper_atomic_update+0x26c/0x284
[   59.305716] [<c02f8d54>] drm_atomic_helper_commit_planes+0xfc/0x27c

Bailing out from ssd130x_update_rect() when data_array is still NULL
fixes that.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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