Re: [PATCH v2 19/46] drm/mediatek: mtk_crtc: Minimize spinlocked time in cmdq callback

["CK Hu (胡俊光)" <ck.hu@mediatek.com>]

On Tue, 2026-07-14 at 13:43 +0200, AngeloGioacchino Del Regno wrote:
> In ddp_cmdq_cb (the CMDQ mailbox callback), the config_lock spin
> lock is being locked to read the config_updating variable in the
> mtk_crtc structure, as, if configuration is updating, the callback
> function shall not run (or shall run at a later time).
> 
> It is therefore unnecessary to keep the spinlock until the end of
> the function, in case pending config, planes, or async planes
> bools need to be updated.
> 
> Though the cmdq callback is not a performance path in this case,
> this may be running for each frame that is pushed to the display,
> so from 30 (or less) up to 144 times (or more) per second.
> 
> Add a temporary "cfg_updating" variable to aid with readability,
> and unlock the spinlock as soon as the config_updating variable
> read operation is done as a micro-optimization.

cfg_updating is used to protect pending_* variables and these variable should be updated in atomic.
In ddp_cmdq_cb(), it just clean all pending_* variables, so you could move mtk_crtc_finish_page_flip() out of this spinlock region.

Regards,
CK

> 
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
> ---
>  drivers/gpu/drm/mediatek/mtk_crtc.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_crtc.c b/drivers/gpu/drm/mediatek/mtk_crtc.c
> index e74f6244606f..2cdf28a0e2ff 100644
> --- a/drivers/gpu/drm/mediatek/mtk_crtc.c
> +++ b/drivers/gpu/drm/mediatek/mtk_crtc.c
> @@ -288,6 +288,7 @@ static void ddp_cmdq_cb(struct mbox_client *cl, void *mssg)
>  	struct mtk_crtc_state *state;
>  	unsigned int i;
>  	unsigned long flags;
> +	bool cfg_updating;
>  
>  	/* release GCE HW usage and start autosuspend */
>  	pm_runtime_mark_last_busy(cmdq_cl->chan->mbox->dev);
> @@ -299,7 +300,10 @@ static void ddp_cmdq_cb(struct mbox_client *cl, void *mssg)
>  	state = to_mtk_crtc_state(mtk_crtc->base.state);
>  
>  	spin_lock_irqsave(&mtk_crtc->config_lock, flags);
> -	if (mtk_crtc->config_updating)
> +	cfg_updating = mtk_crtc->config_updating;
> +	spin_unlock_irqrestore(&mtk_crtc->config_lock, flags);
> +
> +	if (cfg_updating)
>  		goto ddp_cmdq_cb_out;
>  
>  	state->pending_config = false;
> @@ -329,14 +333,11 @@ static void ddp_cmdq_cb(struct mbox_client *cl, void *mssg)
>  	}
>  
>  ddp_cmdq_cb_out:
> -
>  	if (mtk_crtc->pending_needs_vblank) {
>  		mtk_crtc_finish_page_flip(mtk_crtc);
>  		mtk_crtc->pending_needs_vblank = false;
>  	}
>  
> -	spin_unlock_irqrestore(&mtk_crtc->config_lock, flags);
> -
>  	mtk_crtc->cmdq_vblank_cnt = 0;
>  	wake_up(&mtk_crtc->cb_blocking_queue);
>  }