linux-kernel - Re: [PATCH v10 2/2] dmaengine: sun6i: Add driver for the Allwinner A31 DMA controller

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Message-ID: <20140630153305.GA28740@leverpostej>
Date:	Mon, 30 Jun 2014 16:33:05 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	"andriy.shevchenko@...el.com" <andriy.shevchenko@...el.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"zhuzhenhua@...winnertech.com" <zhuzhenhua@...winnertech.com>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"linux-sunxi@...glegroups.com" <linux-sunxi@...glegroups.com>,
	"kevin.z.m.zh@...il.com" <kevin.z.m.zh@...il.com>,
	"sunny@...winnertech.com" <sunny@...winnertech.com>,
	"shuge@...winnertech.com" <shuge@...winnertech.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v10 2/2] dmaengine: sun6i: Add driver for the Allwinner
 A31 DMA controller

On Mon, Jun 30, 2014 at 04:19:06PM +0100, Maxime Ripard wrote:
> On Mon, Jun 30, 2014 at 03:20:54PM +0100, Mark Rutland wrote:
> > Hi Maxime,
> > 
> > On Mon, Jun 30, 2014 at 02:20:54PM +0100, Maxime Ripard wrote:
> > > The Allwinner A31 has a 16 channels DMA controller that it shares with the
> > > newer A23. Although sharing some similarities with the DMA controller of the
> > > older Allwinner SoCs, it's significantly different, I don't expect it to be
> > > possible to share the driver for these two.
> > >
> > > The A31 Controller is able to memory-to-memory or memory-to-device transfers on
> > > the 16 channels in parallel.
> > >
> > > Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> > > Acked-by: Arnd Bergmann <arnd@...db.de>
> > > ---
> > >  drivers/dma/Kconfig     |    8 +
> > >  drivers/dma/Makefile    |    1 +
> > >  drivers/dma/sun6i-dma.c | 1058 +++++++++++++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 1067 insertions(+)
> > >  create mode 100644 drivers/dma/sun6i-dma.c
> > 
> > [...]
> > 
> > > +       sdc->clk = devm_clk_get(&pdev->dev, NULL);
> > > +       if (IS_ERR(sdc->clk)) {
> > > +               dev_err(&pdev->dev, "No clock specified\n");
> > > +               return PTR_ERR(sdc->clk);
> > > +       }
> > > +
> > > +       mux = clk_get(NULL, "ahb1_mux");
> > > +       if (IS_ERR(mux)) {
> > > +               dev_err(&pdev->dev, "Couldn't get AHB1 Mux\n");
> > > +               return PTR_ERR(mux);
> > > +       }
> > > +
> > > +       pll6 = clk_get(NULL, "pll6");
> > > +       if (IS_ERR(pll6)) {
> > > +               dev_err(&pdev->dev, "Couldn't get PLL6\n");
> > > +               clk_put(mux);
> > > +               return PTR_ERR(pll6);
> > > +       }
> > 
> > I'm slightly confused. The binding listed a single unnamed clock (the
> > AHB clock). What is going on here?
> 
> The device itself needs only a single clock to work...
> 
> > 
> > > +       ret = clk_set_parent(mux, pll6);
> > > +       clk_put(pll6);
> > > +       clk_put(mux);
> > > +
> > > +       if (ret) {
> > > +               dev_err(&pdev->dev, "Couldn't reparent AHB1 on PLL6\n");
> > > +               return ret;
> > > +       }
> > 
> > Why do we need to reparent the mux?
> 
> ... but will function only if this clock is derived from PLL6.

Ok, but _why_ is that the case? Could we at least have a comment for
that?

Where does the driver get the named clocks from if they aren't provided
on the device node? Is there a clock-ranges somewhere?

It feels a little fragile to rely on the organisation of the clock tree
and the naming thereof. If the IP block is ever reused on an SoC with a
different clock tree layout then we have to handle things differently.

Cheers,
Mark.
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