Re: [PATCH net-next] net: stmmac: dwmac: Disable flushing frames on Rx Buffer Unavailable

Maxime Chevallier
2025-11-25T18:05:54Z

Hi,

On 25/11/2025 17:37, Rohan G Thomas via B4 Relay wrote:
> From: Rohan G Thomas <rohan.g.thomas@altera.com>
>
> In Store and Forward mode, flushing frames when the receive buffer is
> unavailable, can cause the MTL Rx FIFO to go out of sync. This results
> in buffering of a few frames in the FIFO without triggering Rx DMA
> from transferring the data to the system memory until another packet
> is received. Once the issue happens, for a ping request, the packet is
> forwarded to the system memory only after we receive another packet
> and hece we observe a latency equivalent to the ping interval.
>
> 64 bytes from 192.168.2.100: seq=1 ttl=64 time=1000.344 ms
>
> Also, we can observe constant gmacgrp_debug register value of
> 0x00000120, which indicates "Reading frame data".
>
> The issue is not reproducible after disabling frame flushing when Rx
> buffer is unavailable. But in that case, the Rx DMA enters a suspend
> state due to buffer unavailability. To resume operation, software
> must write to the receive_poll_demand register after adding new
> descriptors, which reactivates the Rx DMA.
>
> This issue is observed in the socfpga platforms which has dwmac1000 IP
> like Arria 10, Cyclone V and Agilex 7. Issue is reproducible after
> running iperf3 server at the DUT for UDP lower packet sizes.
>
> Signed-off-by: Rohan G Thomas <rohan.g.thomas@altera.com>
> Reviewed-by: Matthew Gerlach <matthew.gerlach@altera.com>

I'll let others comment on the correctness of the dwmac1000 part, but
still :

Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Maxime

> ---
> drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c | 5 +++--
> drivers/net/ethernet/stmicro/stmmac/dwmac_dma.h | 1 +
> drivers/net/ethernet/stmicro/stmmac/dwmac_lib.c | 5 +++++
> drivers/net/ethernet/stmicro/stmmac/hwif.h | 3 +++
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 ++
> 5 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c b/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c
> index 6d9b8fac3c6d0fd76733ab4a1a8cce2420fa40b4..5877fec9f6c30ed18cdcf5398816e444e0bd0091 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c
> @@ -135,10 +135,10 @@ static void dwmac1000_dma_operation_mode_rx(struct stmmac_priv *priv,
>
> if (mode == SF_DMA_MODE) {
> pr_debug("GMAC: enable RX store and forward mode\n");
> - csr6 |= DMA_CONTROL_RSF;
> + csr6 |= DMA_CONTROL_RSF | DMA_CONTROL_DFF;
> } else {
> pr_debug("GMAC: disable RX SF mode (threshold %d)\n", mode);
> - csr6 &= ~DMA_CONTROL_RSF;
> + csr6 &= ~(DMA_CONTROL_RSF | DMA_CONTROL_DFF);
> csr6 &= DMA_CONTROL_TC_RX_MASK;
> if (mode <= 32)
> csr6 |= DMA_CONTROL_RTC_32;
> @@ -262,6 +262,7 @@ const struct stmmac_dma_ops dwmac1000_dma_ops = {
> .dma_rx_mode = dwmac1000_dma_operation_mode_rx,
> .dma_tx_mode = dwmac1000_dma_operation_mode_tx,
> .enable_dma_transmission = dwmac_enable_dma_transmission,
> + .enable_dma_reception = dwmac_enable_dma_reception,
> .enable_dma_irq = dwmac_enable_dma_irq,
> .disable_dma_irq = dwmac_disable_dma_irq,
> .start_tx = dwmac_dma_start_tx,
> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac_dma.h b/drivers/net/ethernet/stmicro/stmmac/dwmac_dma.h
> index d1c149f7a3dd9e472b237101666e11878707f0f2..054ecb20ce3f68bce5da3efaf36acf33e430d3f0 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac_dma.h
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac_dma.h
> @@ -169,6 +169,7 @@ static inline u32 dma_chan_base_addr(u32 base, u32 chan)
> #define NUM_DWMAC4_DMA_REGS 27
>
> void dwmac_enable_dma_transmission(void __iomem *ioaddr, u32 chan);
> +void dwmac_enable_dma_reception(void __iomem *ioaddr, u32 chan);
> void dwmac_enable_dma_irq(struct stmmac_priv *priv, void __iomem *ioaddr,
> u32 chan, bool rx, bool tx);
> void dwmac_disable_dma_irq(struct stmmac_priv *priv, void __iomem *ioaddr,
> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac_lib.c b/drivers/net/ethernet/stmicro/stmmac/dwmac_lib.c
> index 467f1a05747ecf0be5b9f3392cd3d2049d676c21..97a803d68e3a2f120beaa7c3254748cf404236df 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac_lib.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac_lib.c
> @@ -33,6 +33,11 @@ void dwmac_enable_dma_transmission(void __iomem *ioaddr, u32 chan)
> writel(1, ioaddr + DMA_CHAN_XMT_POLL_DEMAND(chan));
> }
>
> +void dwmac_enable_dma_reception(void __iomem *ioaddr, u32 chan)
> +{
> + writel(1, ioaddr + DMA_CHAN_RCV_POLL_DEMAND(chan));
> +}
> +
> void dwmac_enable_dma_irq(struct stmmac_priv *priv, void __iomem *ioaddr,
> u32 chan, bool rx, bool tx)
> {
> diff --git a/drivers/net/ethernet/stmicro/stmmac/hwif.h b/drivers/net/ethernet/stmicro/stmmac/hwif.h
> index f257ce4b6c66e0bbd3180d54ac7f5be934153a6b..df6e8a567b1f646f83effbb38d8e53441a6f6150 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/hwif.h
> +++ b/drivers/net/ethernet/stmicro/stmmac/hwif.h
> @@ -201,6 +201,7 @@ struct stmmac_dma_ops {
> void (*dma_diagnostic_fr)(struct stmmac_extra_stats *x,
> void __iomem *ioaddr);
> void (*enable_dma_transmission)(void __iomem *ioaddr, u32 chan);
> + void (*enable_dma_reception)(void __iomem *ioaddr, u32 chan);
> void (*enable_dma_irq)(struct stmmac_priv *priv, void __iomem *ioaddr,
> u32 chan, bool rx, bool tx);
> void (*disable_dma_irq)(struct stmmac_priv *priv, void __iomem *ioaddr,
> @@ -261,6 +262,8 @@ struct stmmac_dma_ops {
> stmmac_do_void_callback(__priv, dma, dma_diagnostic_fr, __args)
> #define stmmac_enable_dma_transmission(__priv, __args...) \
> stmmac_do_void_callback(__priv, dma, enable_dma_transmission, __args)
> +#define stmmac_enable_dma_reception(__priv, __args...) \
> + stmmac_do_void_callback(__priv, dma, enable_dma_reception, __args)
> #define stmmac_enable_dma_irq(__priv, __args...) \
> stmmac_do_void_callback(__priv, dma, enable_dma_irq, __priv, __args)
> #define stmmac_disable_dma_irq(__priv, __args...) \
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index 6cacedb2c9b3fefdd4c9ec8ba98d389443d21ebd..1ecca60baf74286da7f156b4c3c835b3cbabf1ba 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -4973,6 +4973,8 @@ static inline void stmmac_rx_refill(struct stmmac_priv *priv, u32 queue)
> rx_q->rx_tail_addr = rx_q->dma_rx_phy +
> (rx_q->dirty_rx * sizeof(struct dma_desc));
> stmmac_set_rx_tail_ptr(priv, priv->ioaddr, rx_q->rx_tail_addr, queue);
> + /* Wake up Rx DMA from the suspend state if required */
> + stmmac_enable_dma_reception(priv, priv->ioaddr, queue);
> }
>
> static unsigned int stmmac_rx_buf1_len(struct stmmac_priv *priv,
>
> ---
> base-commit: e3daf0e7fe9758613bec324fd606ed9caa187f74
> change-id: 20251125-a10_ext_fix-5951805b9906
>
> Best regards,