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

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Message-ID: <c077c687-57dc-c33f-f434-57918492c789@gmail.com>
Date:   Wed, 8 Jan 2020 18:10:30 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Jon Hunter <jonathanh@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Vinod Koul <vkoul@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     dmaengine@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 09/13] dmaengine: tegra-apb: Remove runtime PM usage

07.01.2020 21:38, Jon Hunter пишет:
> 
> On 07/01/2020 17:12, Dmitry Osipenko wrote:
>> 07.01.2020 18:13, Jon Hunter пишет:
>>>
>>> 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.