RE: [PATCH net v2] net: xilinx: axienet: Fix IRQ coalescing packet count overflow

[Pandey, Radhey Shyam]

> -----Original Message-----
> From: Sean Anderson <sean.anderson@xxxxxxxxx>
> Sent: Thursday, September 12, 2024 8:01 PM
> To: Pandey, Radhey Shyam <radhey.shyam.pandey@xxxxxxx>; David S . Miller
> <davem@xxxxxxxxxxxxx>; Eric Dumazet <edumazet@xxxxxxxxxx>; Jakub Kicinski
> <kuba@xxxxxxxxxx>; Paolo Abeni <pabeni@xxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx;
> Gupta, Suraj <Suraj.Gupta2@xxxxxxx>; Katakam, Harini
> <harini.katakam@xxxxxxx>
> Cc: Andy Chiu <andy.chiu@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Simon
> Horman <horms@xxxxxxxxxx>; Ariane Keller <ariane.keller@xxxxxxxxxxxxxx>; Daniel
> Borkmann <daniel@xxxxxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; Simek,
> Michal <michal.simek@xxxxxxx>
> Subject: Re: [PATCH net v2] net: xilinx: axienet: Fix IRQ coalescing packet count
> overflow
> 
> On 9/11/24 03:01, Pandey, Radhey Shyam wrote:
> >> -----Original Message-----
> >> From: Sean Anderson <sean.anderson@xxxxxxxxx>
> >> Sent: Tuesday, September 10, 2024 4:39 AM
> >> To: Pandey, Radhey Shyam <radhey.shyam.pandey@xxxxxxx>; David S .
> >> Miller <davem@xxxxxxxxxxxxx>; Eric Dumazet <edumazet@xxxxxxxxxx>;
> >> Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo Abeni <pabeni@xxxxxxxxxx>;
> >> netdev@xxxxxxxxxxxxxxx
> >> Cc: Andy Chiu <andy.chiu@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Simon
> >> Horman <horms@xxxxxxxxxx>; Ariane Keller <ariane.keller@xxxxxxxxxxxxxx>;
> >> Daniel Borkmann <daniel@xxxxxxxxxxxxx>; linux-arm-
> >> kernel@xxxxxxxxxxxxxxxxxxx; Simek, Michal <michal.simek@xxxxxxx>; Sean
> >> Anderson <sean.anderson@xxxxxxxxx>
> >> Subject: [PATCH net v2] net: xilinx: axienet: Fix IRQ coalescing packet count
> >> overflow
> >>
> >> If coalece_count is greater than 255 it will not fit in the register and
> >> will overflow. This can be reproduced by running
> >>
> >>     # ethtool -C ethX rx-frames 256
> >>
> >> which will result in a timeout of 0us instead. Fix this by clamping the
> >> counts to the maximum value.
> > After this fix - what is o/p we get on rx-frames read? I think silent clamping is not a
> great
> > idea and user won't know about it.  One alternative is to add check in set_coalesc
> > count for valid range? (Similar to axienet_ethtools_set_ringparam so that user is
> notified
> > for incorrect range)
> 
> The value reported will be unclamped. In [1] I improve the driver to
> return the actual (clamped) value.
> 
> Remember that without this commit, we have silent wraparound instead. I
> think clamping is much friendlier, since you at least get something
> close to the rx-frames value, instead of zero!
> 
> This commit is just a fix for the overflow issue. To ensure it is
> appropriate for backporting I have omitted any other
> changes/improvements.

But the point is the fix also can be to avoid setting coalesce count 
to invalid (or not supported range) value - like done in existing 
axienet_ethtools_set_ringparam() implementation.

And we don't clamp on every dma_start().

> 
> --Sean
> 
> [1] https://lore.kernel.org/netdev/20240909235208.1331065-6-
> sean.anderson@xxxxxxxxx/
> 
> >>
> >> Signed-off-by: Sean Anderson <sean.anderson@xxxxxxxxx>
> >> Fixes: 8a3b7a252dca ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet
> >> driver")
> >> ---
> >>
> >> Changes in v2:
> >> - Use FIELD_MAX to extract the max value from the mask
> >> - Expand the commit message with an example on how to reproduce this
> >>   issue
> >>
> >>  drivers/net/ethernet/xilinx/xilinx_axienet.h      | 5 ++---
> >>  drivers/net/ethernet/xilinx/xilinx_axienet_main.c | 8 ++++++--
> >>  2 files changed, 8 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet.h
> >> b/drivers/net/ethernet/xilinx/xilinx_axienet.h
> >> index 1223fcc1a8da..54db69893565 100644
> >> --- a/drivers/net/ethernet/xilinx/xilinx_axienet.h
> >> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet.h
> >> @@ -109,11 +109,10 @@
> >>  #define XAXIDMA_BD_CTRL_TXEOF_MASK	0x04000000 /* Last tx packet
> >> */
> >>  #define XAXIDMA_BD_CTRL_ALL_MASK	0x0C000000 /* All control bits
> >> */
> >>
> >> -#define XAXIDMA_DELAY_MASK		0xFF000000 /* Delay timeout
> >> counter */
> >> -#define XAXIDMA_COALESCE_MASK		0x00FF0000 /* Coalesce
> >> counter */
> >> +#define XAXIDMA_DELAY_MASK		((u32)0xFF000000) /* Delay
> >> timeout counter */
> >
> > Adding typecast here looks odd. Any reason for it?
> > If needed we do it in specific case where it is required.
> >
> >> +#define XAXIDMA_COALESCE_MASK		((u32)0x00FF0000) /*
> >> Coalesce counter */
> >>
> >>  #define XAXIDMA_DELAY_SHIFT		24
> >> -#define XAXIDMA_COALESCE_SHIFT		16
> >>
> >>  #define XAXIDMA_IRQ_IOC_MASK		0x00001000 /* Completion
> >> intr */
> >>  #define XAXIDMA_IRQ_DELAY_MASK		0x00002000 /* Delay
> >> interrupt */
> >> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> >> b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> >> index 9eb300fc3590..89b63695293d 100644
> >> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> >> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> >> @@ -252,7 +252,9 @@ 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 = FIELD_PREP(XAXIDMA_COALESCE_MASK,
> >> +				   min(lp->coalesce_count_rx,
> >> +
> >> FIELD_MAX(XAXIDMA_COALESCE_MASK))) |
> >>  			XAXIDMA_IRQ_IOC_MASK |
> >> XAXIDMA_IRQ_ERROR_MASK;
> >>  	/* Only set interrupt delay timer if not generating an interrupt on
> >>  	 * the first RX packet. Otherwise leave at 0 to disable delay interrupt.
> >> @@ -264,7 +266,9 @@ static void axienet_dma_start(struct axienet_local
> >> *lp)
> >>  	axienet_dma_out32(lp, XAXIDMA_RX_CR_OFFSET, lp->rx_dma_cr);
> >>
> >>  	/* Start updating the Tx channel control register */
> >> -	lp->tx_dma_cr = (lp->coalesce_count_tx <<
> >> XAXIDMA_COALESCE_SHIFT) |
> >> +	lp->tx_dma_cr = FIELD_PREP(XAXIDMA_COALESCE_MASK,
> >> +				   min(lp->coalesce_count_tx,
> >> +
> >> FIELD_MAX(XAXIDMA_COALESCE_MASK))) |
> >>  			XAXIDMA_IRQ_IOC_MASK |
> >> XAXIDMA_IRQ_ERROR_MASK;
> >>  	/* Only set interrupt delay timer if not generating an interrupt on
> >>  	 * the first TX packet. Otherwise leave at 0 to disable delay interrupt.
> >> --
> >> 2.35.1.1320.gc452695387.dirty
> >