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

[Pat Somaru]

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.

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

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.

Signed-off-by: Pat Somaru <patso at likewhatevs.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