linux-kernel - Re: [PATCH for-next 1/4] dmaengine: ti: k3-udma: Use ktime/usleep

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Date:   Tue, 28 Jan 2020 17:35:25 +0530
From:   Vignesh Raghavendra <vigneshr@...com>
To:     Vinod Koul <vkoul@...nel.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>
CC:     <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <dan.j.williams@...el.com>, <grygorii.strashko@...com>
Subject: Re: [PATCH for-next 1/4] dmaengine: ti: k3-udma: Use
 ktime/usleep_range based TX completion check

Hi Vinod,

On 1/28/2020 5:18 PM, Vinod Koul wrote:
> On 27-01-20, 15:21, Peter Ujfalusi wrote:
>> From: Vignesh Raghavendra <vigneshr@...com>
>>
>> In some cases (McSPI for example) the jiffie and delayed_work based
>> workaround can cause big throughput drop.
>>
>> Switch to use ktime/usleep_range based implementation to be able
>> to sustain speed for PDMA based peripherals.
>>
>> Signed-off-by: Vignesh Raghavendra <vigneshr@...com>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
>> ---
>>  drivers/dma/ti/k3-udma.c | 80 ++++++++++++++++++++++++++--------------
>>  1 file changed, 53 insertions(+), 27 deletions(-)
>>
>> diff --git a/drivers/dma/ti/k3-udma.c b/drivers/dma/ti/k3-udma.c
>> index ea79c2df28e0..fb59c869a6a7 100644
>> --- a/drivers/dma/ti/k3-udma.c
>> +++ b/drivers/dma/ti/k3-udma.c
>> @@ -5,6 +5,7 @@
>>   */
>>  
>>  #include <linux/kernel.h>
>> +#include <linux/delay.h>
>>  #include <linux/dmaengine.h>
>>  #include <linux/dma-mapping.h>
>>  #include <linux/dmapool.h>
>> @@ -169,7 +170,7 @@ enum udma_chan_state {
>>  
>>  struct udma_tx_drain {
>>  	struct delayed_work work;
>> -	unsigned long jiffie;
>> +	ktime_t tstamp;
>>  	u32 residue;
>>  };
>>  
>> @@ -946,9 +947,10 @@ static bool udma_is_desc_really_done(struct udma_chan *uc, struct udma_desc *d)
>>  	peer_bcnt = udma_tchanrt_read(uc->tchan, UDMA_TCHAN_RT_PEER_BCNT_REG);
>>  	bcnt = udma_tchanrt_read(uc->tchan, UDMA_TCHAN_RT_BCNT_REG);
>>  
>> +	/* Transfer is incomplete, store current residue and time stamp */
>>  	if (peer_bcnt < bcnt) {
>>  		uc->tx_drain.residue = bcnt - peer_bcnt;
>> -		uc->tx_drain.jiffie = jiffies;
>> +		uc->tx_drain.tstamp = ktime_get();
> 
> Any reason why ktime_get() is better than jiffies..?

Resolution of jiffies is 4ms. ktime_t is has better resolution (upto ns
scale). With jiffies, I observed that code was either always polling DMA
progress counters (which affects HW data transfer speed) or sleeping too
long, both causing performance loss. Switching to ktime_t provides
better prediction of how long transfer takes to complete.

Regards
Vignesh