Re: [PATCH] drm/mcde: Fix stability issue

[Stephan Gerhold <stephan@gerhold.net>]

On Sun, Jul 19, 2020 at 01:33:22AM +0200, Linus Walleij wrote:
> Whenener a display update was sent, apart from updating
> the memory base address we called mcde_display_send_one_frame()
> which also sent a command to the display requesting the TE IRQ
> and enabling the FIFO.
> 
> When continous updates are running this is wrong: we need
> to only send this to start the flow to the display on
> the very first update. This lead to the display pipeline
> locking up and crashing.
> 
> Check if the flow is already running and in that case
> do not call mcde_display_send_one_frame().
> 
> This fixes crashes on the Samsung GT-S7710 (Skomer).
> 
> Cc: Stephan Gerhold <stephan@gerhold.net>
> Cc: stable@vger.kernel.org
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>  drivers/gpu/drm/mcde/mcde_display.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mcde/mcde_display.c b/drivers/gpu/drm/mcde/mcde_display.c
> index 212aee60cf61..1d8ea8830a17 100644
> --- a/drivers/gpu/drm/mcde/mcde_display.c
> +++ b/drivers/gpu/drm/mcde/mcde_display.c
> @@ -1086,9 +1086,14 @@ static void mcde_display_update(struct drm_simple_display_pipe *pipe,
>  	 */
>  	if (fb) {
>  		mcde_set_extsrc(mcde, drm_fb_cma_get_gem_addr(fb, pstate, 0));
> -		if (!mcde->video_mode)
> -			/* Send a single frame using software sync */
> -			mcde_display_send_one_frame(mcde);
> +		if (!mcde->video_mode) {
> +			/*
> +			 * Send a single frame using software sync if the flow
> +			 * is not active yet.
> +			 */
> +			if (mcde->flow_active == 0)
> +				mcde_display_send_one_frame(mcde);
> +		}

I think this makes sense as a fix for the issue you described, so FWIW:
Acked-by: Stephan Gerhold <stephan@gerhold.net>

While looking at this I had a few thoughts for potential future patches:

 - Clearly mcde_display_send_one_frame() does not only send a single
   frame only in some cases (when te_sync = true), so maybe it should
   be named differently?

 - I was a bit confused because with this change we also call
   mcde_dsi_te_request() only once. Looking at the vendor driver the
   nova_dsilink_te_request() function that is very similar is only
   called within mcde_add_bta_te_oneshot_listener(), which is only
   called for MCDE_SYNCSRC_BTA.

   However, the rest of the MCDE code looks more similar to
   MCDE_SYNCSRC_TE0, which does not call that function in the vendor
   driver. I wonder if mcde_dsi_te_request() is needed at all?

Thanks,
Stephan

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