linux-kernel - Re: [PATCH 1/2] dmaengine: sprd: delete enable opreation in probe

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Date: Tue, 19 Dec 2023 13:21:18 +0800
From: liu kaiwei <liukaiwei086@...il.com>
To: Vinod Koul <vkoul@...nel.org>
Cc: Kaiwei Liu <kaiwei.liu@...soc.com>, Orson Zhai <orsonzhai@...il.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Chunyan Zhang <zhang.lyra@...il.com>, 
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Wenming Wu <wenming.wu@...soc.com>
Subject: Re: [PATCH 1/2] dmaengine: sprd: delete enable opreation in probe

On Mon, Dec 11, 2023 at 7:41 PM Vinod Koul <vkoul@...nel.org> wrote:
>
> On 06-12-23, 17:32, liu kaiwei wrote:
> > On Fri, Nov 24, 2023 at 9:11 PM Vinod Koul <vkoul@...nel.org> wrote:
> > >
> > > On 02-11-23, 20:16, Kaiwei Liu wrote:
> > > > From: "kaiwei.liu" <kaiwei.liu@...soc.com>
> > >
> > > Typo is subject line
> > >
> > > >
> > > > In the probe of dma, it will allocate device memory and do some
> > > > initalization settings. All operations are only at the software
> > > > level and don't need the DMA hardware power on. It doesn't need
> > > > to resume the device and set the device active as well. here
> > > > delete unnecessary operation.
> > >
> > > Don't you need to read or write to the device? Without enable that wont
> > > work right?
> > >
> >
> > Yes, it doesn't need to read or write to the device in the probe of DMA.
> > We will enable the DMA when allocating the DMA channel.
>
> So you will probe even if device is not present! I think it makes sense
> to access device registers in probe!

There is another reason why we delete enable/disable and not to access
device in probe. The current driver is applicable to two DMA devices
in different
power domain. For some scenes, one of the domain is power off and when you
probe,  enable the dma with the domain power off may cause crash.

For example, one case is for audio co-processor and DMA serves for it,
DMA's power domain is off during initialization since audio is not used
at that time, so we cannot read/write DMA's register for this kind of cases.

@Baolin Wang
Hi baolin，what's your opinion?

> --
> ~Vinod