linux-kernel - Re: [PATCH] dmaengine: pl330: Fix unbalanced runtime PM

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Date:   Mon, 11 Apr 2022 19:18:59 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Sugar Zhang <sugar.zhang@...k-chips.com>
Cc:     Huibin Hong <huibin.hong@...k-chips.com>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: pl330: Fix unbalanced runtime PM

On 26-03-22, 20:16, Sugar Zhang wrote:
> This driver use runtime PM autosuspend mechanism to manager clk.
> 
>   pm_runtime_use_autosuspend(&adev->dev);
>   pm_runtime_set_autosuspend_delay(&adev->dev, PL330_AUTOSUSPEND_DELAY);
> 
> So, after ref count reached to zero, it will enter suspend
> after the delay time elapsed.
> 
> The unbalanced PM:
> 
> * May cause dmac the next start failed.
> * May cause dmac read unexpected state.
> * May cause dmac stall if power down happen at the middle of the transfer.
>   e.g. may lose ack from AXI bus and stall.
> 
> Considering the following situation:
> 
>       DMA TERMINATE               TASKLET ROUTINE
>             |                            |
>             |                       issue_pending
>             |                            |
>             |                     pch->active = true
>             |                       pm_runtime_get
>   pm_runtime_put(if active)              |
>     pch->active = false                  |
>             |                      work_list empty
>             |                            |
>             |                     pm_runtime_put(force)

maybe unconditional is a better word than force here?

>             |                            |
> 
> At this point, it's unbalanced(1 get / 2 put).
> 
> After this patch:
> 
>       DMA TERMINATE               TASKLET ROUTINE
>             |                            |
>             |                       issue_pending
>             |                            |
>             |                     pch->active = true
>             |                       pm_runtime_get
>   pm_runtime_put(if active)              |
>     pch->active = false                  |
>             |                      work_list empty
>             |                            |
>             |                   pm_runtime_put(if active)
>             |                            |
> 
> Now, it's balanced(1 get / 1 put).
> 
> Fixes:
> commit 5c9e6c2b2ba3 ("dmaengine: pl330: Fix runtime PM support for terminated transfers")
> commit ae43b3289186 ("ARM: 8202/1: dmaengine: pl330: Add runtime Power Management support v12")

That is not the right way for Fixes tag
> 
> Signed-off-by: Huibin Hong <huibin.hong@...k-chips.com>
> Signed-off-by: Sugar Zhang <sugar.zhang@...k-chips.com>
> ---
> 
>  drivers/dma/pl330.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
> index 858400e..ccd430e 100644
> --- a/drivers/dma/pl330.c
> +++ b/drivers/dma/pl330.c
> @@ -2084,7 +2084,7 @@ static void pl330_tasklet(struct tasklet_struct *t)
>  		spin_lock(&pch->thread->dmac->lock);
>  		_stop(pch->thread);
>  		spin_unlock(&pch->thread->dmac->lock);
> -		power_down = true;
> +		power_down = pch->active;
>  		pch->active = false;
>  	} else {
>  		/* Make sure the PL330 Channel thread is active */
> -- 
> 2.7.4

-- 
~Vinod