linux-kernel - Re: [PATCH 2/3] dma: xilinx_dpdma: Remove unnecessary use of irqsave/restore

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Message-ID: <0652c82f-b0a2-4881-ac51-38399b180ad4@ideasonboard.com>
Date: Wed, 27 Mar 2024 14:27:08 +0200
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Sean Anderson <sean.anderson@...ux.dev>
Cc: Michal Simek <michal.simek@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Vinod Koul <vkoul@...nel.org>, dmaengine@...r.kernel.org
Subject: Re: [PATCH 2/3] dma: xilinx_dpdma: Remove unnecessary use of
 irqsave/restore

Hi,

On 08/03/2024 23:00, Sean Anderson wrote:
> xilinx_dpdma_chan_done_irq and xilinx_dpdma_chan_vsync_irq are always
> called with IRQs disabled from xilinx_dpdma_irq_handler. Therefore we
> don't need to save/restore the IRQ flags.

I think this is fine, but a few thoughts:

- Is spin_lock clearly faster than the irqsave variant, or is this a 
pointless optimization? It's safer to just use irqsave variant, instead 
of making sure the code is always called from the expected contexts.
- Is this style documented/recommended anywhere? Going through docs, I 
only found docs telling to use irqsave when mixing irq and non-irq contexts.
- Does this cause issues on PREEMPT_RT?

  Tomi

> Signed-off-by: Sean Anderson <sean.anderson@...ux.dev>
> ---
> 
>   drivers/dma/xilinx/xilinx_dpdma.c | 10 ++++------
>   1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/dma/xilinx/xilinx_dpdma.c b/drivers/dma/xilinx/xilinx_dpdma.c
> index eb0637d90342..36bd4825d389 100644
> --- a/drivers/dma/xilinx/xilinx_dpdma.c
> +++ b/drivers/dma/xilinx/xilinx_dpdma.c
> @@ -1043,9 +1043,8 @@ static int xilinx_dpdma_chan_stop(struct xilinx_dpdma_chan *chan)
>   static void xilinx_dpdma_chan_done_irq(struct xilinx_dpdma_chan *chan)
>   {
>   	struct xilinx_dpdma_tx_desc *active;
> -	unsigned long flags;
>   
> -	spin_lock_irqsave(&chan->lock, flags);
> +	spin_lock(&chan->lock);
>   
>   	xilinx_dpdma_debugfs_desc_done_irq(chan);
>   
> @@ -1057,7 +1056,7 @@ static void xilinx_dpdma_chan_done_irq(struct xilinx_dpdma_chan *chan)
>   			 "chan%u: DONE IRQ with no active descriptor!\n",
>   			 chan->id);
>   
> -	spin_unlock_irqrestore(&chan->lock, flags);
> +	spin_unlock(&chan->lock);
>   }
>   
>   /**
> @@ -1072,10 +1071,9 @@ static void xilinx_dpdma_chan_vsync_irq(struct  xilinx_dpdma_chan *chan)
>   {
>   	struct xilinx_dpdma_tx_desc *pending;
>   	struct xilinx_dpdma_sw_desc *sw_desc;
> -	unsigned long flags;
>   	u32 desc_id;
>   
> -	spin_lock_irqsave(&chan->lock, flags);
> +	spin_lock(&chan->lock);
>   
>   	pending = chan->desc.pending;
>   	if (!chan->running || !pending)
> @@ -1108,7 +1106,7 @@ static void xilinx_dpdma_chan_vsync_irq(struct  xilinx_dpdma_chan *chan)
>   	spin_unlock(&chan->vchan.lock);
>   
>   out:
> -	spin_unlock_irqrestore(&chan->lock, flags);
> +	spin_unlock(&chan->lock);
>   }
>   
>   /**