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

G Thomas, Rohan
2025-11-26T06:07:31Z

Hi Maxime,

On 11/25/2025 11:33 PM, Maxime Chevallier wrote:
>
>
> On 25/11/2025 18:15, Maxime Chevallier wrote:
>> Hi Rohan,
>>
>> 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>
>>
>> Should this be a fix ?
>>
>> Can you elaborate on how to reproduce this ? I've given this a try on
>> CycloneV and I can't see any difference in the ping results and iperf3
>> results.
>>
>> From the DUT, I've tried :
>> - iperf3 -c 192.168.X.X -u -b 0 -l 64
>> - iperf3 -c 192.168.X.X -u -b 0 -l 64 -R
>
> Ah ! my iperf3 peer wasn't sending packets hard enough. I switched to a
> more powerful LP and I can now see the huge latencies by doing :
>
> 1 - ping the CycloneV from test machine :
>
> PING 192.168.2.41 (192.168.2.41) 56(84) bytes of data.
> 64 bytes from 192.168.2.41: icmp_seq=1 ttl=64 time=0.387 ms
> 64 bytes from 192.168.2.41: icmp_seq=2 ttl=64 time=0.196 ms
> 64 bytes from 192.168.2.41: icmp_seq=3 ttl=64 time=0.193 ms
> 64 bytes from 192.168.2.41: icmp_seq=4 ttl=64 time=0.207 ms
>
> 2 - on cycloneV, Run iperf3 -c 192.168.X.X -u -b 0 -l 64 -R
>
> 3 - Re-ping :
>
> PING 192.168.2.41 (192.168.2.41) 56(84) bytes of data.
> 64 bytes from 192.168.2.41: icmp_seq=1 ttl=64 time=1022 ms
> 64 bytes from 192.168.2.41: icmp_seq=2 ttl=64 time=1024 ms
> 64 bytes from 192.168.2.41: icmp_seq=3 ttl=64 time=1024 ms
>
>
> This behaviour disapears after your patch :)
>
> Maxime
>

Thanks for testing the patch.

Yes, this is the scenario adressed with the patch. By default driver
configures for SF_DMA_MODE and issue is reproducible when we stress the
mac with Rx Buffer Unavailable scenarios repeatedly.

>
>> - iperf3 -c 192.168.X.X
>> - iperf3 -c 192.168.X.X -R
>>
>> I'm reading the same results with and without the patch
>>
>> I've done ping tests as well, the latency seems to be the same with and
>> without this patch, at around 0.193ms RTT.
>>
>> I'm not familiar with the SF_DMA_MODE though, any thing special to do to
>> enter that mode ?
>>
>> Thanks,
>>
>> 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,
>>
>>
>

Best Regards,
Rohan