[PATCH v3 0/2] dmaengine: sun6i: Fix two bugs in the DMA status path

[PATCH v3 0/2] dmaengine: sun6i: Fix two bugs in the DMA status path

[Christian Lugnberg]

Thank you for the review, Frank.

Changes in v3:
  - Patch 1: trim commit message to the root-cause explanation and fix
    description, dropping the debug/reproduction part per feedback
  - Patch 2: unchanged since v2

Christian Lugnberg (2):
  dmaengine: sun6i: fix non-atomic read of DMA position registers
  dmaengine: sun6i: fix undefined behaviour in sun6i_dma_tx_status

 drivers/dma/sun6i-dma.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

-- 
2.54.0 (Apple Git-156)

[PATCH v3 1/2] dmaengine: sun6i: fix non-atomic read of DMA position registers

[Christian Lugnberg]

sun6i_get_chan_size() reads DMA_CHAN_LLI_ADDR and DMA_CHAN_CUR_CNT in two
separate readl() calls with no synchronisation between them:

    pos   = readl(pchan->base + DMA_CHAN_LLI_ADDR);
    bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);

DMA_CHAN_LLI_ADDR holds the physical address of the *next* descriptor the
engine will load once the current one completes. DMA_CHAN_CUR_CNT holds the
remaining byte count for the *current* descriptor. If the DMA engine
advances to the next LLI entry between the two reads, pos becomes stale: it
still points to what was the next descriptor at the time of the first read,
but that descriptor is now the current one and CUR_CNT reflects its initial
(full) byte count. The subsequent virtual-chain walk starts one entry too
early and accumulates an extra full period's worth of bytes into the
residue estimate.

Fix this by re-reading DMA_CHAN_LLI_ADDR after DMA_CHAN_CUR_CNT and
retrying if the value changed. This double-read pattern guarantees that
both registers were sampled during the same descriptor interval. The cost
is at most one extra readl() pair per call in the racy case, which occurs
only at descriptor boundaries (~every 2 ms) and is negligible.

Fixes: a90e173f3faf ("dmaengine: sun6i: Add cyclic capability")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Christian Lugnberg <christian.lugnberg@soundtrack.io>
---
 drivers/dma/sun6i-dma.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
index f47a326dd7ff..04fe1f5042e9 100644
--- a/drivers/dma/sun6i-dma.c
+++ b/drivers/dma/sun6i-dma.c
@@ -354,8 +354,10 @@ static size_t sun6i_get_chan_size(struct sun6i_pchan *pchan)
 	size_t bytes;
 	dma_addr_t pos;
 
-	pos = readl(pchan->base + DMA_CHAN_LLI_ADDR);
-	bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
+	do {
+		pos = readl(pchan->base + DMA_CHAN_LLI_ADDR);
+		bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
+	} while (pos != readl(pchan->base + DMA_CHAN_LLI_ADDR));
 
 	if (pos == LLI_LAST_ITEM)
 		return bytes;
-- 
2.54.0 (Apple Git-156)

Re: [PATCH v3 1/2] dmaengine: sun6i: fix non-atomic read of DMA position registers

[Frank Li]

On Mon, Aug 17, 2026 at 03:51:22PM +0200, Christian Lugnberg wrote:
> sun6i_get_chan_size() reads DMA_CHAN_LLI_ADDR and DMA_CHAN_CUR_CNT in two
> separate readl() calls with no synchronisation between them:
>
>     pos   = readl(pchan->base + DMA_CHAN_LLI_ADDR);
>     bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
>
> DMA_CHAN_LLI_ADDR holds the physical address of the *next* descriptor the
> engine will load once the current one completes. DMA_CHAN_CUR_CNT holds the
> remaining byte count for the *current* descriptor. If the DMA engine
> advances to the next LLI entry between the two reads, pos becomes stale: it
> still points to what was the next descriptor at the time of the first read,
> but that descriptor is now the current one and CUR_CNT reflects its initial
> (full) byte count. The subsequent virtual-chain walk starts one entry too
> early and accumulates an extra full period's worth of bytes into the
> residue estimate.
>
> Fix this by re-reading DMA_CHAN_LLI_ADDR after DMA_CHAN_CUR_CNT and
> retrying if the value changed. This double-read pattern guarantees that
> both registers were sampled during the same descriptor interval. The cost
> is at most one extra readl() pair per call in the racy case, which occurs
> only at descriptor boundaries (~every 2 ms) and is negligible.
>
> Fixes: a90e173f3faf ("dmaengine: sun6i: Add cyclic capability")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-sonnet-4-6
> Signed-off-by: Christian Lugnberg <christian.lugnberg@soundtrack.io>
> ---

Reviewed-by: Frank Li <Frank.Li@nxp.com>

>  drivers/dma/sun6i-dma.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
> index f47a326dd7ff..04fe1f5042e9 100644
> --- a/drivers/dma/sun6i-dma.c
> +++ b/drivers/dma/sun6i-dma.c
> @@ -354,8 +354,10 @@ static size_t sun6i_get_chan_size(struct sun6i_pchan *pchan)
>  	size_t bytes;
>  	dma_addr_t pos;
>
> -	pos = readl(pchan->base + DMA_CHAN_LLI_ADDR);
> -	bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
> +	do {
> +		pos = readl(pchan->base + DMA_CHAN_LLI_ADDR);
> +		bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
> +	} while (pos != readl(pchan->base + DMA_CHAN_LLI_ADDR));
>
>  	if (pos == LLI_LAST_ITEM)
>  		return bytes;
> --
> 2.54.0 (Apple Git-156)
>

[PATCH v3 2/2] dmaengine: sun6i: fix undefined behaviour in sun6i_dma_tx_status

[Christian Lugnberg]

sun6i_dma_tx_status() calls vchan_find_desc() to look up the virtual
descriptor for a given cookie, before checking whether the pointer
vd is NULL:

    vd = vchan_find_desc(&vchan->vc, cookie);
    txd = to_sun6i_desc(&vd->tx);   /* vd may be NULL here */

    if (vd) {
        for (lli = txd->v_lli; ...)

vchan_find_desc() returns NULL when the descriptor has already been
completed or is in-flight on a physical channel and no longer present
in the virtual channel's descriptor list. When vd is NULL,
to_sun6i_desc() is called unconditionally on &vd->tx before the NULL
check, which is undefined behaviour. Move the call inside the if (vd)
guard to ensure it is only reached with a valid pointer.

    vd = vchan_find_desc(&vchan->vc, cookie);
    if (vd) {
        struct sun6i_desc *txd = to_sun6i_desc(&vd->tx);
        for (lli = txd->v_lli; ...)

Fixes: 555859308723 ("dmaengine: sun6i: Add driver for the Allwinner A31 DMA controller")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Christian Lugnberg <christian.lugnberg@soundtrack.io>
---
 drivers/dma/sun6i-dma.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
index 04fe1f5042e9..7704b016aed8 100644
--- a/drivers/dma/sun6i-dma.c
+++ b/drivers/dma/sun6i-dma.c
@@ -981,7 +981,6 @@ static enum dma_status sun6i_dma_tx_status(struct dma_chan *chan,
 	struct sun6i_pchan *pchan = vchan->phy;
 	struct sun6i_dma_lli *lli;
 	struct virt_dma_desc *vd;
-	struct sun6i_desc *txd;
 	enum dma_status ret;
 	unsigned long flags;
 	size_t bytes = 0;
@@ -993,9 +992,9 @@ static enum dma_status sun6i_dma_tx_status(struct dma_chan *chan,
 	spin_lock_irqsave(&vchan->vc.lock, flags);
 
 	vd = vchan_find_desc(&vchan->vc, cookie);
-	txd = to_sun6i_desc(&vd->tx);
 
 	if (vd) {
+		struct sun6i_desc *txd = to_sun6i_desc(&vd->tx);
 		for (lli = txd->v_lli; lli != NULL; lli = lli->v_lli_next)
 			bytes += lli->len;
 	} else if (!pchan || !pchan->desc) {
-- 
2.54.0 (Apple Git-156)

Re: [PATCH v3 2/2] dmaengine: sun6i: fix undefined behaviour in sun6i_dma_tx_status

[Frank Li]

On Mon, Aug 17, 2026 at 03:51:23PM +0200, Christian Lugnberg wrote:
> sun6i_dma_tx_status() calls vchan_find_desc() to look up the virtual
> descriptor for a given cookie, before checking whether the pointer
> vd is NULL:
>
>     vd = vchan_find_desc(&vchan->vc, cookie);
>     txd = to_sun6i_desc(&vd->tx);   /* vd may be NULL here */
>
>     if (vd) {
>         for (lli = txd->v_lli; ...)
>
> vchan_find_desc() returns NULL when the descriptor has already been
> completed or is in-flight on a physical channel and no longer present
> in the virtual channel's descriptor list. When vd is NULL,
> to_sun6i_desc() is called unconditionally on &vd->tx before the NULL
> check, which is undefined behaviour. Move the call inside the if (vd)
> guard to ensure it is only reached with a valid pointer.
>
>     vd = vchan_find_desc(&vchan->vc, cookie);
>     if (vd) {
>         struct sun6i_desc *txd = to_sun6i_desc(&vd->tx);
>         for (lli = txd->v_lli; ...)
>
> Fixes: 555859308723 ("dmaengine: sun6i: Add driver for the Allwinner A31 DMA controller")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-sonnet-4-6
> Signed-off-by: Christian Lugnberg <christian.lugnberg@soundtrack.io>
> ---

Reviewed-by: Frank Li <Frank.Li@nxp.com>

>  drivers/dma/sun6i-dma.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
> index 04fe1f5042e9..7704b016aed8 100644
> --- a/drivers/dma/sun6i-dma.c
> +++ b/drivers/dma/sun6i-dma.c
> @@ -981,7 +981,6 @@ static enum dma_status sun6i_dma_tx_status(struct dma_chan *chan,
>  	struct sun6i_pchan *pchan = vchan->phy;
>  	struct sun6i_dma_lli *lli;
>  	struct virt_dma_desc *vd;
> -	struct sun6i_desc *txd;
>  	enum dma_status ret;
>  	unsigned long flags;
>  	size_t bytes = 0;
> @@ -993,9 +992,9 @@ static enum dma_status sun6i_dma_tx_status(struct dma_chan *chan,
>  	spin_lock_irqsave(&vchan->vc.lock, flags);
>
>  	vd = vchan_find_desc(&vchan->vc, cookie);
> -	txd = to_sun6i_desc(&vd->tx);
>
>  	if (vd) {
> +		struct sun6i_desc *txd = to_sun6i_desc(&vd->tx);
>  		for (lli = txd->v_lli; lli != NULL; lli = lli->v_lli_next)
>  			bytes += lli->len;
>  	} else if (!pchan || !pchan->desc) {
> --
> 2.54.0 (Apple Git-156)
>