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.

Convert apple-admac.c from tasklet to BH workqueue

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.

Signed-off-by: Pat Somaru <patso@likewhatevs.io>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
---
 v2: Updated commit message

 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

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

 Thanks for letting me know about talk to maintainer section, Frank!

 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