Subject: Re: [PATCH v3] can: c_can: Speed up rx_poll function

From: Marc Kleine-Budde <[email-masked]>
Date: Sun, 03 Nov 2013 22:05:46 +0100

On 11/03/2013 08:20 PM, Wolfgang Grandegger wrote:
> 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 <[email-masked]>
>> ---
>>
>> 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.

Yes, but the code looks IMHO better this way.

Marc

-- 
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