linux-kernel - Re: [PATCH] dma: apple-admac: Convert from tasklet to BH workqueue

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Message-ID: <aYZBbfktpZyu5u1k@lizhi-Precision-Tower-5810>
Date: Fri, 6 Feb 2026 14:30:53 -0500
From: Frank Li <Frank.li@....com>
To: Pat Somaru <patso@...ewhatevs.io>
Cc: Tejun Heo <tj@...nel.org>, Sven Peter <sven@...nel.org>,
	Janne Grunau <j@...nau.net>, Vinod Koul <vkoul@...nel.org>,
	Neal Gompa <neal@...pa.dev>, asahi@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma: apple-admac: Convert from tasklet to BH workqueue

On Fri, Feb 06, 2026 at 04:01:37AM -0500, Pat Somaru wrote:
> The only generic interface to execute asynchronously in the BH context
> is tasklet; however, it's marked deprecated and has some design flaws
> such as the execution code accessing the tasklet item after the
> execution is complete which can lead to subtle use-after-free in certain
> usage scenarios and less-developed flush and cancel mechanisms.
>
> To replace tasklets, BH workqueue support was recently added. A BH
> workqueue behaves similarly to regular workqueues except that the queued
> work items are executed in the BH context.
>
> This patch converts drivers/dma/apple-admac.c from tasklet to BH
> workqueue.

Avoid use words "This patch",

Convert apple-admac.c from tasklet to BH workqueue


>
> The Apple ADMAC driver uses a per-channel tasklet to invoke DMA
> completion callbacks for cyclic transactions. This conversion maintains
> the same execution semantics while using the modern BH workqueue
> infrastructure.
>
...

> This patch was tested by:
>     - Building with allmodconfig: no new warnings (compared to v6.18)
>     - Building with allyesconfig: no new warnings (compared to v6.18)
>     - Booting defconfig kernel via vng and running `uname -a`:
>     Linux virtme-ng 6.18.0-virtme #1 SMP PREEMPT_DYNAMIC 0 x86_64 GNU/Linux
>

Should move after ---,

> Semantically, this is an equivalent conversion and there shouldn't be
> any user-visible behavior changes. The BH workqueue implementation uses
> the same softirq infrastructure, and performance-critical networking
> conversions have shown no measurable performance impact.

>
> Maintainers can apply this directly to the DMA subsystem tree or ack it
> for the workqueue tree to carry.

This private talk to maintainer, put after ---

Sorry, the same comments for

https://lore.kernel.org/dmaengine/aYYm2pA6l-ksXvkk@lizhi-Precision-Tower-5810/T/#t

Frank
>
> Signed-off-by: Pat Somaru <patso@...ewhatevs.io>
> ---
>  drivers/dma/apple-admac.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/dma/apple-admac.c b/drivers/dma/apple-admac.c
> index bd49f0374291..8a0e100d5aaf 100644
> --- a/drivers/dma/apple-admac.c
> +++ b/drivers/dma/apple-admac.c
> @@ -16,6 +16,7 @@
>  #include <linux/reset.h>
>  #include <linux/spinlock.h>
>  #include <linux/interrupt.h>
> +#include <linux/workqueue.h>
>
>  #include "dmaengine.h"
>
> @@ -89,7 +90,7 @@ struct admac_chan {
>  	unsigned int no;
>  	struct admac_data *host;
>  	struct dma_chan chan;
> -	struct tasklet_struct tasklet;
> +	struct work_struct work;
>
>  	u32 carveout;
>
> @@ -522,8 +523,8 @@ static int admac_terminate_all(struct dma_chan *chan)
>  		adchan->current_tx = NULL;
>  	}
>  	/*
> -	 * Descriptors can only be freed after the tasklet
> -	 * has been killed (in admac_synchronize).
> +	 * Descriptors can only be freed after the work
> +	 * has been cancelled (in admac_synchronize).
>  	 */
>  	list_splice_tail_init(&adchan->submitted, &adchan->to_free);
>  	list_splice_tail_init(&adchan->issued, &adchan->to_free);
> @@ -543,7 +544,7 @@ static void admac_synchronize(struct dma_chan *chan)
>  	list_splice_tail_init(&adchan->to_free, &head);
>  	spin_unlock_irqrestore(&adchan->lock, flags);
>
> -	tasklet_kill(&adchan->tasklet);
> +	cancel_work_sync(&adchan->work);
>
>  	list_for_each_entry_safe(adtx, _adtx, &head, node) {
>  		list_del(&adtx->node);
> @@ -662,7 +663,7 @@ static void admac_handle_status_desc_done(struct admac_data *ad, int channo)
>  		tx->reclaimed_pos %= 2 * tx->buf_len;
>
>  		admac_cyclic_write_desc(ad, channo, tx);
> -		tasklet_schedule(&adchan->tasklet);
> +		queue_work(system_bh_wq, &adchan->work);
>  	}
>  	spin_unlock_irqrestore(&adchan->lock, flags);
>  }
> @@ -712,9 +713,9 @@ static irqreturn_t admac_interrupt(int irq, void *devid)
>  	return IRQ_HANDLED;
>  }
>
> -static void admac_chan_tasklet(struct tasklet_struct *t)
> +static void admac_chan_work(struct work_struct *work)
>  {
> -	struct admac_chan *adchan = from_tasklet(adchan, t, tasklet);
> +	struct admac_chan *adchan = from_work(adchan, work, work);
>  	struct admac_tx *adtx;
>  	struct dmaengine_desc_callback cb;
>  	struct dmaengine_result tx_result;
> @@ -886,7 +887,7 @@ static int admac_probe(struct platform_device *pdev)
>  		INIT_LIST_HEAD(&adchan->issued);
>  		INIT_LIST_HEAD(&adchan->to_free);
>  		list_add_tail(&adchan->chan.device_node, &dma->channels);
> -		tasklet_setup(&adchan->tasklet, admac_chan_tasklet);
> +		INIT_WORK(&adchan->work, admac_chan_work);
>  	}
>
>  	err = reset_control_reset(ad->rstc);
> --
> 2.52.0
>