Re: [PATCH net] net: xilinx: axienet: Fix IRQ coalescing packet count overflow

[Simon Horman <horms@kernel.org>]

On Thu, Sep 05, 2024 at 10:34:15AM -0400, Sean Anderson wrote:
> On 9/4/24 12:00, Simon Horman wrote:
> > On Tue, Sep 03, 2024 at 02:00:59PM -0400, Sean Anderson wrote:
> >> If coalesce_count is greater than 255 it will not fit in the register and
> >> will overflow. Clamp it to 255 for more-predictable results.
> > 
> > Hi Sean,
> > 
> > Can this occur in practice?
> 
> Yes. Simply do `ethtool -C ethX rx-frames 300` or something similar and
> you will end up with a limit of 44 instead. I ran into this with DIM and
> was wondering why the highest-throughput setting (256) was behaving so
> poorly...

I think it would be nice to include an explanation along those
lines in the patch description.

> 
> >> 
> >> Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
> >> Fixes: 8a3b7a252dca ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
> > 
> > nit: I think it is usual for the order of these tags to be reversed.
> 
> OK
> 
> >> ---
> >> 
> >>  drivers/net/ethernet/xilinx/xilinx_axienet_main.c | 6 ++++--
> >>  1 file changed, 4 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> >> index 9aeb7b9f3ae4..5f27fc1c4375 100644
> >> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> >> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> >> @@ -252,7 +252,8 @@ static u32 axienet_usec_to_timer(struct axienet_local *lp, u32 coalesce_usec)
> >>  static void axienet_dma_start(struct axienet_local *lp)
> >>  {
> >>  	/* Start updating the Rx channel control register */
> >> -	lp->rx_dma_cr = (lp->coalesce_count_rx << XAXIDMA_COALESCE_SHIFT) |
> >> +	lp->rx_dma_cr = (min(lp->coalesce_count_rx, 255) <<
> >> +			 XAXIDMA_COALESCE_SHIFT) |
> >>  			XAXIDMA_IRQ_IOC_MASK | XAXIDMA_IRQ_ERROR_MASK;
> > 
> > nit: it would be nice to avoid using a naked 255 here.
> >      Perhaps: #define XAXIDMA_COALESCE_MAX 0xff
> 
> OK, but this is the same as the limit used in axienet_usec_to_timer.

Ok, that does muddy the waters a bit.