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

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Message-ID: <01660250-0489-870a-6f0e-d74c5041e8e3@nvidia.com>
Date:   Tue, 7 Jan 2020 18:38:36 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Dmitry Osipenko <digetx@...il.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


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.

Jon

-- 
nvpublic