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SubjectRe: [PATCH v3] can: c_can: Speed up rx_poll function
On 11/01/2013 10:36 AM, Markus Pargmann wrote:
> This patch speeds up the rx_poll function by reducing the number of
> register reads.
>
> Replace the 32bit register read by a 16bit register read. Currently
> the 32bit register read is implemented by using 2 16bit reads. This is
> inefficient as we only use the lower 16bit in rx_poll.
>
> The for loop reads the pending interrupts in every iteration. This
> leads up to 16 reads of pending interrupts. The patch introduces a new
> outer loop to read the pending interrupts as long as 'quota' is above 0.
> This reduces the total number of reads.
>
> The third change is to replace the for-loop by a ffs loop.
>
> Tested on AM335x. I removed all 'static' and 'inline' from c_can.c to
> see the timings for all functions. I used the function tracer with
> trace_stats.
>
> 125kbit:
> Function Hit Time Avg s^2
> -------- --- ---- --- ---
> c_can_do_rx_poll 63960 10168178 us 158.977 us 1493056 us
> With patch:
> c_can_do_rx_poll 63941 3764057 us 58.867 us 776162.2 us
>
> 1Mbit:
> Function Hit Time Avg s^2
> -------- --- ---- --- ---
> c_can_do_rx_poll 69489 30049498 us 432.435 us 9271851 us
> With patch:
> c_can_do_rx_poll 207109 24322185 us 117.436 us 171469047 us
>
> Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
> ---
>
> Notes:
> Changes in v3:
> - Update commit message (measurements and ffs)
>
> Changes in v2:
> - Small changes, find_next_bit -> ffs and other
>
> drivers/net/can/c_can/c_can.c | 22 ++++++++++++----------
> 1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/net/can/c_can/c_can.c b/drivers/net/can/c_can/c_can.c
> index a668cd4..428681e 100644
> --- a/drivers/net/can/c_can/c_can.c
> +++ b/drivers/net/can/c_can/c_can.c
> @@ -798,17 +798,19 @@ static int c_can_do_rx_poll(struct net_device *dev, int quota)
> u32 num_rx_pkts = 0;
> unsigned int msg_obj, msg_ctrl_save;
> struct c_can_priv *priv = netdev_priv(dev);
> - u32 val = c_can_read_reg32(priv, C_CAN_INTPND1_REG);
> + u16 val;
> +
> + /*
> + * It is faster to read only one 16bit register. This is only possible
> + * for a maximum number of 16 objects.
> + */
> + BUILD_BUG_ON_MSG(C_CAN_MSG_OBJ_RX_LAST > 16,
> + "Implementation does not support more message objects than 16");
> +
> + while (quota > 0 && (val = priv->read_reg(priv, C_CAN_INTPND1_REG))) {
> + while ((msg_obj = ffs(val)) && quota > 0) {
> + val &= ~BIT(msg_obj - 1);

IIRC, we should avoid assignment in if/while statements.

Wolfgang.



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