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

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Date:   Thu, 13 Jul 2023 16:53:57 +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 Thu, Jul 13, 2023 at 4:13 PM Javier Martinez Canillas
<javierm@...hat.com> wrote:
> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
> > On Thu, Jul 13, 2023 at 3:21 PM Javier Martinez Canillas
> > <javierm@...hat.com> wrote:
> >> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
> >> > 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:
> >> >
> >>
> >> Thanks for reporting this issue.
> >>
> >> > [   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
> >> >
> >>
> >> I wonder how this could be too late. I thought that the encoder
> >> .atomic_enable callback would be called before any plane .atomic_update.

[...]

> Thanks for the log, so I think the problem is that the default struct
> drm_mode_config_helper_funcs .atomic_commit_tail is
> drm_atomic_helper_commit_tail():
>
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_atomic_helper.c#L1710
>
> That helper calls drm_atomic_helper_commit_planes() and attempts to commit
> the state for all planes even for CRTC that are not enabled. I see that
> there is a drm_atomic_helper_commit_tail_rpm() helper that instead calls:
>
> drm_atomic_helper_commit_planes(dev, old_state, DRM_PLANE_COMMIT_ACTIVE_ONLY),
> which I thought that was the default behaviour.
>
> Can you please try the following change [0] ? If that works then I can
> propose as a proper patch.
>
> [0]:
> diff --git a/drivers/gpu/drm/solomon/ssd130x.c b/drivers/gpu/drm/solomon/ssd130x.c
> index afb08a8aa9fc..c543caa3ceee 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 */
>

Thanks, that works!

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