linux-kernel - RE: DRM Driver implementation question

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Message-ID: <TY2PR01MB2924FAE6E54718ED44E0E4ABDF8E0@TY2PR01MB2924.jpnprd01.prod.outlook.com>
Date:   Wed, 18 Sep 2019 14:34:04 +0000
From:   Gareth Williams <gareth.williams.jx@...esas.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Phil Edworthy <phil.edworthy@...esas.com>,
        Fabrizio Castro <fabrizio.castro@...renesas.com>,
        "kieran.bingham+renesas@...asonboard.com" 
        <kieran.bingham+renesas@...asonboard.com>
Subject: RE: DRM Driver implementation question

Hi Laurent,

On Wed, Sep 18, 2019 at 04:04:34PM +0000, Laurent Pinchart wrote:
> 
> Hi Gareth,
> 
> Sorry for the delayed response, I was travelling for the Linux 
> Plumbers Conference followed by a code camp.
> 
> On Tue, Sep 17, 2019 at 03:27:34PM +0000, Gareth Williams wrote:
> > Hi Yoshihiro,
> >
> > This looks like an elegant solution that I can implement.
> > Many thanks for pointing me in a good direction.
> >
> > > From: Yoshihiro Shimoda, Sent: Tuesday, September 17, 2019 05:39 
> > > PM
> > >
> > > Hi Gareth,
> > >
> > > > From: Gareth Williams, Sent: Monday, September 16, 2019 10:56 PM
> > > >
> > > > Hi Laurent/Kieran,
> > > >
> > > > I need to upstream a driver for a display controller that within 
> > > > its registers memory region contains registers related to a PWM 
> > > > device. The PWM device is for controlling the backlight of the 
> > > > display.
> > > >
> > > > Ideally, I would like to create a separated driver for the PWM, 
> > > > so that I can re-use "pwm-backlight", but since the registers 
> > > > for the PWM are right in the middle of the registers for the 
> > > > display controller I would need to ioremap the memory region for 
> > > > the PWM registers region twice, once from the display controller 
> > > > driver, and once from the PWM driver.
> > > >
> > > > Do you think that the double ioremap would be acceptable upstream?
> > >
> > > I think that an MFD driver can support such hardware. I checked 
> > > Documentation/devicetree/bindings/mfd roughly, and then 
> > > atmel-hlcdc.txt seems to have a display controller and a PWM device.
> 
> While MFD should technically work, I think it's quite overkill. You 
> could instead bundle both the display controller and the PWM 
> controller in the same driver. The driver would create both a DRM/KMS 
> device and a PWM device. The DT node for your device would contain a 
> #pwm-cells property and could thus be referenced as a PWM controller 
> by the backlight using the pwms property.
This makes sense, many thanks for pointing me in the direction.

> 
> --
> Regards,
> 
> Laurent Pinchart

Kind Regards,

Gareth Williams