Re: [PATCH v3 09/13] dmaengine: tegra-apb: Remove runtime PM usage

[Dmitry Osipenko]

07.01.2020 21:38, Jon Hunter ÐÐÑÐÑ:
> 
> On 07/01/2020 17:12, Dmitry Osipenko wrote:
>> 07.01.2020 18:13, Jon Hunter ÐÐÑÐÑ:
>>>
>>> On 06/01/2020 01:17, Dmitry Osipenko wrote:
>>>> There is no benefit from runtime PM usage for the APB DMA driver because
>>>> it enables clock at the time of channel's allocation and thus clock stays
>>>> enabled all the time in practice, secondly there is benefit from manually
>>>> disabled clock because hardware auto-gates it during idle by itself.
>>>
>>> This assumes that the channel is allocated during a driver
>>> initialisation. That may not always be the case. I believe audio is one
>>> case where channels are requested at the start of audio playback.
>>
>> At least serial, I2C, SPI and T20 FUSE are permanently keeping channels
>> allocated, thus audio is an exception here. I don't think that it's
>> practical to assume that there is a real-world use-case where audio
>> driver is the only active DMA client.
>>
>> The benefits of gating the DMA clock are also dim, do you have any
>> power-consumption numbers that show that it's really worth to care about
>> the clock-gating?
> 
> No, but at the same time, I really don't see the point in this. In fact,
> I think it is a step backwards. If we wanted to only enable clocks while
> DMA channels are active we could. So I request you drop this.

I'll take a look at making RPM active only during the time of DMA
activity, otherwise it's pretty much a dead code as it is now.