Re: [PATCH v6 11/16] dmaengine: tegra-apb: Keep clock enabled only during of DMA transfer

[Dmitry Osipenko]

30.01.2020 17:09, Jon Hunter ÐÐÑÐÑ:
> 
> On 30/01/2020 04:37, Dmitry Osipenko wrote:
>> It's a bit impractical to enable hardware's clock at the time of DMA
>> channel's allocation because most of DMA client drivers allocate DMA
>> channel at the time of the driver's probing, and thus, DMA clock is kept
>> always-enabled in practice, defeating the whole purpose of runtime PM.
>>
>> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
>> ---
>>  drivers/dma/tegra20-apb-dma.c | 47 ++++++++++++++++++++++++-----------
>>  1 file changed, 32 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
>> index 22b88ccff05d..0ee28d8e3c96 100644
>> --- a/drivers/dma/tegra20-apb-dma.c
>> +++ b/drivers/dma/tegra20-apb-dma.c
>> @@ -436,6 +436,8 @@ static void tegra_dma_stop(struct tegra_dma_channel *tdc)
>>  		tdc_write(tdc, TEGRA_APBDMA_CHAN_STATUS, status);
>>  	}
>>  	tdc->busy = false;
>> +
>> +	pm_runtime_put(tdc->tdma->dev);
>>  }
>>  
>>  static void tegra_dma_start(struct tegra_dma_channel *tdc,
>> @@ -500,18 +502,25 @@ static void tegra_dma_configure_for_next(struct tegra_dma_channel *tdc,
>>  	tegra_dma_resume(tdc);
>>  }
>>  
>> -static void tdc_start_head_req(struct tegra_dma_channel *tdc)
>> +static bool tdc_start_head_req(struct tegra_dma_channel *tdc)
>>  {
>>  	struct tegra_dma_sg_req *sg_req;
>> +	int err;
>>  
>>  	if (list_empty(&tdc->pending_sg_req))
>> -		return;
>> +		return false;
>> +
>> +	err = pm_runtime_get_sync(tdc->tdma->dev);
>> +	if (WARN_ON_ONCE(err < 0))
>> +		return false;
>>  
>>  	sg_req = list_first_entry(&tdc->pending_sg_req, typeof(*sg_req), node);
>>  	tegra_dma_start(tdc, sg_req);
>>  	sg_req->configured = true;
>>  	sg_req->words_xferred = 0;
>>  	tdc->busy = true;
>> +
>> +	return true;
>>  }
>>  
>>  static void tdc_configure_next_head_desc(struct tegra_dma_channel *tdc)
>> @@ -615,6 +624,8 @@ static void handle_once_dma_done(struct tegra_dma_channel *tdc,
>>  	}
>>  	list_add_tail(&sgreq->node, &tdc->free_sg_req);
>>  
>> +	pm_runtime_put(tdc->tdma->dev);
>> +
>>  	/* Do not start DMA if it is going to be terminate */
>>  	if (to_terminate || list_empty(&tdc->pending_sg_req))
>>  		return;
>> @@ -730,9 +741,7 @@ static void tegra_dma_issue_pending(struct dma_chan *dc)
>>  		dev_err(tdc2dev(tdc), "No DMA request\n");
>>  		goto end;
>>  	}
>> -	if (!tdc->busy) {
>> -		tdc_start_head_req(tdc);
>> -
>> +	if (!tdc->busy && tdc_start_head_req(tdc)) {
>>  		/* Continuous single mode: Configure next req */
>>  		if (tdc->cyclic) {
>>  			/*
>> @@ -775,6 +784,13 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
>>  	else
>>  		wcount = status;
>>  
>> +	/*
>> +	 * tegra_dma_stop() will drop the RPM's usage refcount, but
>> +	 * tegra_dma_resume() touches hardware and thus we should keep
>> +	 * the DMA clock active while it's needed.
>> +	 */
>> +	pm_runtime_get(tdc->tdma->dev);
>> +
> 
> Would it work and make it simpler to just enable in the issue_pending
> and disable in the handle_once_dma_done or terminate_all?
> 
> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
> index 3a45079d11ec..86bbb45da93d 100644
> --- a/drivers/dma/tegra20-apb-dma.c
> +++ b/drivers/dma/tegra20-apb-dma.c
> @@ -616,9 +616,14 @@ static void handle_once_dma_done(struct
> tegra_dma_channel *tdc,
>         list_add_tail(&sgreq->node, &tdc->free_sg_req);
> 
>         /* Do not start DMA if it is going to be terminate */
> -       if (to_terminate || list_empty(&tdc->pending_sg_req))
> +       if (to_terminate)
>                 return;
> 
> +       if (list_empty(&tdc->pending_sg_req)) {
> +               pm_runtime_put(tdc->tdma->dev);
> +               return;
> +       }
> +
>         tdc_start_head_req(tdc);
>  }
> 
> @@ -729,6 +734,11 @@ static void tegra_dma_issue_pending(struct dma_chan
> *dc)
>                 goto end;
>         }
>         if (!tdc->busy) {
> +               if (pm_runtime_get_sync(tdc->tdma->dev) < 0) {
> +                       dev_err(tdc2dev(tdc), "Failed to enable DMA!\n");
> +                       goto end;
> +               }
> +
>                 tdc_start_head_req(tdc);
> 
>                 /* Continuous single mode: Configure next req */
> @@ -788,6 +798,7 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
>                                 get_current_xferred_count(tdc, sgreq,
> wcount);
>         }
>         tegra_dma_resume(tdc);
> +       pm_runtime_put(tdc->tdma->dev);
> 
>  skip_dma_stop:
>         tegra_dma_abort_all(tdc);
> 

The tegra_dma_stop() should put RPM anyways, which is missed in yours
sample. Please see handle_continuous_head_request().

I'm also finding the explicit get/put a bit easier to follow in the
code, don't you think so?