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

[G Thomas, Rohan]

Hi Russell,

On 11/26/2025 5:16 AM, Russell King (Oracle) wrote:
> On Wed, Nov 26, 2025 at 12:37:12AM +0800, Rohan G Thomas via B4 Relay wrote:
> > From: Rohan G Thomas <rohan.g.thomas@xxxxxxxxxx>
> >
> > 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 seems like a sensible writeup, which all seems to make sense,
> even though the databook I have seems vague on the effect of the
> DFF bit.
>
> > 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@xxxxxxxxxx>
> > Reviewed-by: Matthew Gerlach <matthew.gerlach@xxxxxxxxxx>
>
> Given the above, that Maxime has also tested it which shows a net
> benefit, and I've looked through this, even though I can't positively
> say it's correct due to the databook vagueness:
>
> Reviewed-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
>
> Thanks!

Thanks for reviewing the patch.

Best Regards,
Rohan