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

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Date:   Tue, 7 Jan 2020 20:12:45 +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

Hello Jon,

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?