Re: [PATCH v4 19/33] dmaengine: xilinx-dma: use dmaengine BH instead of tasklets

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] dmaengine: xilinx-dma: HI_SOFTIRQ priority loss in
  xilinx_mcdma_irq_handler()
--

commit 959cca11cf1a0ddb7da4355d351b00c62da10f51
Author: Allen Pais <allen.lkml@gmail.com>

dmaengine: xilinx-dma: use dmaengine BH instead of tasklets

This commit replaces the per-channel tasklet in the Xilinx DMA driver with
the shared dmaengine bottom-half helper.

> diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
> index b89f2c3c1579c..d23658c927c74 100644
> --- a/drivers/dma/xilinx/xilinx_dma.c
> +++ b/drivers/dma/xilinx/xilinx_dma.c

[ ... ]

> @@ -1979,7 +1978,7 @@ static irqreturn_t xilinx_mcdma_irq_handler(int irq, void *data)
>  		spin_unlock(&chan->lock);
>  	}
>  
> -	tasklet_hi_schedule(&chan->tasklet);
> +	dmaengine_schedule_bh(&chan->common);
>  	return IRQ_HANDLED;
>  }

[Severity: Low]
Does replacing tasklet_hi_schedule() with dmaengine_schedule_bh() here
regress MCDMA completion priority?

dma_bus_init() allocates dmaengine_bh_wq without WQ_HIGHPRI:

drivers/dma/dmaengine.c:dma_bus_init() {
	...
	dmaengine_bh_wq = alloc_workqueue("dmaengine_bh",
					  WQ_BH | WQ_PERCPU, 0);
	...
}

Because WQ_HIGHPRI is not set, dmaengine_schedule_bh() runs the bottom
half in TASKLET_SOFTIRQ rather than HI_SOFTIRQ, which silently undoes
c77d4c5081aa ("dmaengine: xilinx_dma: Use tasklet_hi_schedule for timing
critical usecase") and places MCDMA completions behind network softirqs.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1794315350.git.allen.lkml@gmail.com?part=19