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SubjectRe: [PATCH 2/3] dma: xilinx_dpdma: Remove unnecessary use of irqsave/restore
From
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@linux.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);
> }
>
> /**


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