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

Jon Hunter
2020-01-30T18:51:31Z

On 30/01/2020 16:11, Dmitry Osipenko wrote:
> 30.01.2020 17:09, Jon Hunter пишет:
>>
>> 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@gmail.com>
>>> ---
>>> 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().

Yes and that is deliberate. The cyclic transfers the transfers *should*
not stop until terminate_all is called. The tegra_dma_stop in
handle_continuous_head_request() is an error condition and so I am not
sure it is actually necessary to call pm_runtime_put() here.

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

I can see that, but I was thinking that in the case of cyclic transfers,
it should only really be necessary to call the get/put at the beginning
and end. So in my mind there should only be two exit points which are
the ISR handler for SG and terminate_all for SG and cyclic.

Jon

--
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