Re: [PATCH] drm/stm: Enable RPM during fbdev registration

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Nov 04, 2020 at 01:52:00PM +0100, Marek Vasut wrote:
> Enable runtime PM before registering the fbdev emulation and disable it
> afterward, otherwise register access to the LTDC IP during the fbdev
> emulation registration might hang the system.
> 
> The problem happens because RPM is activated at the end of ltdc_load(),
> but the fbdev emulation registration happens only after that, and ends
> up calling ltdc_crtc_mode_set_nofb(), which checks whether RPM is active
> and only if it is not active, calls pm_runtime_get_sync() to enable the
> clock and so on. If the clock are not enabled, any register access in
> ltdc_crtc_mode_set_nofb() could hang the platform completely.
> 
> This patch makes sure that ltdc_crtc_mode_set_nofb() is called within
> pm_runtime_get_sync(), so with clock enabled.
> 
> Signed-off-by: Marek Vasut <marex@denx.de>
> Cc: Alexandre Torgue <alexandre.torgue@st.com>
> Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
> Cc: Philippe Cornu <philippe.cornu@st.com>
> Cc: Yannick Fertré <yannick.fertre@st.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-stm32@st-md-mailman.stormreply.com

This looks like you're papering over a bug in your modeset code. If
userspace later on does a setpar on the fbdev chardev, the exact same
thing could happen. You need to fix your modeset code to avoid this, not
sprinkle temporary rpm_get/put all over some top level entry points,
because you can't even patch those all.
-Daniel


> ---
>  drivers/gpu/drm/stm/drv.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/stm/drv.c b/drivers/gpu/drm/stm/drv.c
> index 411103f013e2..d8921edc83db 100644
> --- a/drivers/gpu/drm/stm/drv.c
> +++ b/drivers/gpu/drm/stm/drv.c
> @@ -197,7 +197,9 @@ static int stm_drm_platform_probe(struct platform_device *pdev)
>  	if (ret)
>  		goto err_put;
>  
> +	pm_runtime_get_sync(ddev->dev);
>  	drm_fbdev_generic_setup(ddev, 16);
> +	pm_runtime_put_sync(ddev->dev);
>  
>  	return 0;
>  
> -- 
> 2.28.0
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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