Re: [PATCH] media: stm32-dcmi: Set minimum cpufreq requirement

[Benjamin GAIGNARD <benjamin.gaignard@st.com>]

On 6/2/20 11:31 AM, Valentin Schneider wrote:
> Hi Benjamin,
>
> On 27/05/20 16:16, Benjamin Gaignard wrote:
>> Before start streaming set cpufreq minimum frequency requirement.
>> The cpufreq governor will adapt the frequencies and we will have
>> no latency for handling interrupts.
>>
> Few comments below from someone oblivious to your platform, they may not
> be all that relevant but I figured I'd pitch in anyway.
>
>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
>> ---
>>   drivers/media/platform/stm32/stm32-dcmi.c | 29 ++++++++++++++++++++++++++++-
>>   1 file changed, 28 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/media/platform/stm32/stm32-dcmi.c b/drivers/media/platform/stm32/stm32-dcmi.c
>> index b8931490b83b..97c342351569 100644
>> --- a/drivers/media/platform/stm32/stm32-dcmi.c
>> +++ b/drivers/media/platform/stm32/stm32-dcmi.c
>> @@ -13,6 +13,7 @@
>>
>>   #include <linux/clk.h>
>>   #include <linux/completion.h>
>> +#include <linux/cpufreq.h>
>>   #include <linux/delay.h>
>>   #include <linux/dmaengine.h>
>>   #include <linux/init.h>
>> @@ -99,6 +100,8 @@ enum state {
>>
>>   #define OVERRUN_ERROR_THRESHOLD	3
>>
>> +#define DCMI_MIN_FREQ	650000 /* in KHz */
>> +
> This assumes the handling part is guaranteed to always run on the same CPU
> with the same performance profile (regardless of the platform). If that's
> not guaranteed, it feels like you'd want this to be configurable in some
> way.
Yes I could add a st,stm32-dcmi-min-frequency (in KHz) parameter the 
device tree node.

>
>>   struct dcmi_graph_entity {
>>        struct v4l2_async_subdev asd;
>>
> [...]
>> @@ -2020,6 +2042,8 @@ static int dcmi_probe(struct platform_device *pdev)
>>                goto err_cleanup;
>>        }
>>
>> +	dcmi->policy = cpufreq_cpu_get(0);
>> +
> Ideally you'd want to fetch the policy of the CPU your IRQ (and handling
> thread) is affined to; The only compatible DTS I found describes a single
> A7, which is somewhat limited in the affinity area...
If I move this code just before start streaming and use get_cpu(), would 
it works ?

Benjamin
>
>>        dev_info(&pdev->dev, "Probe done\n");
>>
>>        platform_set_drvdata(pdev, dcmi);
>> @@ -2049,6 +2073,9 @@ static int dcmi_remove(struct platform_device *pdev)
>>
>>        pm_runtime_disable(&pdev->dev);
>>
>> +	if (dcmi->policy)
>> +		cpufreq_cpu_put(dcmi->policy);
>> +
>>        v4l2_async_notifier_unregister(&dcmi->notifier);
>>        v4l2_async_notifier_cleanup(&dcmi->notifier);
>>        media_entity_cleanup(&dcmi->vdev->entity);