DRM Driver implementation question

DRM Driver implementation question

[Gareth Williams]

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?

Kind Regards,

Gareth	

RE: DRM Driver implementation question

[Yoshihiro Shimoda]

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.

Best regards,
Yoshihiro Shimoda

RE: DRM Driver implementation question

[Gareth Williams]

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.
> 
> Best regards,
> Yoshihiro Shimoda

Best Regards,

Gareth Williams

Re: DRM Driver implementation question

[Laurent Pinchart]

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.

-- 
Regards,

Laurent Pinchart

RE: DRM Driver implementation question

[Gareth Williams]

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