linux-kernel - [PATCH] Fix kernel panic and desc unbalance when multi-buffer used

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Date: Thu, 15 Feb 2024 19:05:49 +0000
From: Joao Pinto <Joao.Pinto@...opsys.com>
To: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        Martin McKenny <Martin.McKenny@...opsys.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>
Subject: [PATCH] Fix kernel panic and desc unbalance when multi-buffer used

I have a use case where nr_buffers = 3 and in which each descriptor is composed 
by 3 segments, resulting in the DMA channel descs_allocated to be 9.
Since axi_desc_put() handles the hw_desc considering the descs_allocated, this
scenario would result in a kernel panic (hw_desc array will be overrun).

To fix this, the proposal is to add a new member to the axi_dma_desc structure,
where we keep the number of allocated hw_descs (axi_desc_alloc()) and use it in
axi_desc_put() to handle the hw_desc array correctly.

Additionally I propose to remove the axi_chan_start_first_queued() call after completing
the transfer, since it was identified that unbalance can occur (started descriptors can
be interrupted and transfer ignored due to DMA channel not being enabled). 

Signed-off-by: Joao Pinto <jpinto@...opsys.com>
---
 drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c | 7 ++-----
 drivers/dma/dw-axi-dmac/dw-axi-dmac.h          | 1 +
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c b/drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c
index a86a81f..b39f37a 100644
--- a/drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c
+++ b/drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c
@@ -302,6 +302,7 @@ static struct axi_dma_desc *axi_desc_alloc(u32 num)
 		kfree(desc);
 		return NULL;
 	}
+	desc->nr_hw_descs = num;
 
 	return desc;
 }
@@ -328,7 +329,7 @@ static struct axi_dma_lli *axi_desc_get(struct axi_dma_chan *chan,
 static void axi_desc_put(struct axi_dma_desc *desc)
 {
 	struct axi_dma_chan *chan = desc->chan;
-	int count = atomic_read(&chan->descs_allocated);
+	int count = desc->nr_hw_descs;
 	struct axi_dma_hw_desc *hw_desc;
 	int descs_put;
 
@@ -430,7 +431,6 @@ static void axi_chan_block_xfer_start(struct axi_dma_chan *chan,
 	if (unlikely(axi_chan_is_hw_enable(chan))) {
 		dev_err(chan2dev(chan), "%s is non-idle!\n",
 			axi_chan_name(chan));
-
 		return;
 	}
 
@@ -1139,9 +1139,6 @@ static void axi_chan_block_xfer_complete(struct axi_dma_chan *chan)
 		/* Remove the completed descriptor from issued list before completing */
 		list_del(&vd->node);
 		vchan_cookie_complete(vd);
-
-		/* Submit queued descriptors after processing the completed ones */
-		axi_chan_start_first_queued(chan);
 	}
 
 out:
diff --git a/drivers/dma/dw-axi-dmac/dw-axi-dmac.h b/drivers/dma/dw-axi-dmac/dw-axi-dmac.h
index 454904d..ac571b4 100644
--- a/drivers/dma/dw-axi-dmac/dw-axi-dmac.h
+++ b/drivers/dma/dw-axi-dmac/dw-axi-dmac.h
@@ -104,6 +104,7 @@ struct axi_dma_desc {
 	u32				completed_blocks;
 	u32				length;
 	u32				period_len;
+	u32				nr_hw_descs;
 };
 
 struct axi_dma_chan_config {
-- 
1.8.3.1