Avoid hw_desc array overrun in dw-axi-dmac

[ Upstream commit 333e11bf47fa8d477db90e2900b1ed3c9ae9b697 ]

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@synopsys.com>
Link: https://lore.kernel.org/r/1711536564-12919-1-git-send-email-jpinto@synopsys.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c

@@ -213,6 +213,7 @@ static struct axi_dma_desc *axi_desc_alloc(u32 num)
 		kfree(desc);
 		return NULL;
 	}
+	desc->nr_hw_descs = num;
 
 	return desc;
 }
@@ -239,7 +240,7 @@ 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;
 
@@ -1049,9 +1050,6 @@ 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:

drivers/dma/dw-axi-dmac/dw-axi-dmac.h

@@ -101,6 +101,7 @@ struct axi_dma_desc {
 	u32				completed_blocks;
 	u32				length;
 	u32				period_len;
+	u32				nr_hw_descs;
 };
 
 static inline struct device *dchan2dev(struct dma_chan *dchan)