Re: [PATCH 2/4] drm: Add support for ARM's HDLCD controller.

[Liviu Dudau <Liviu.Dudau@arm.com>]

On Sun, Aug 16, 2015 at 09:56:33AM +0100, Emil Velikov wrote:
> Hi Liviu,

Hi Emil,

> 
> On 5 August 2015 at 15:28, Liviu Dudau <Liviu.Dudau@arm.com> wrote:
> > The HDLCD controller is a display controller that supports resolutions
> > up to 4096x4096 pixels. It is present on various development boards
> > produced by ARM Ltd and emulated by the latest Fast Models from the
> > company.
> >
> I believe there is a unofficial requirement(?) for new drm drivers to
> use atomic modesetting. Not 100% sure on this one though. The
> following drivers: tegra, msm, rcar-du, i915, and Daniel's blog [1]
> [2] cat provide some information on the topic.

I am also interested to know if this is a requirement or not.
Thanks for the pointers, I will review them again to see if I did miss
anything. I remember reading them at the beginning of the year but probably
the Christmas haze did not clear enough when I did.

> 
> The driver seems to has has a bit of dead code guarded by
> HDLCD_*_UNDERRUN. Perhaps these macros should become build or runtime
> switch(es) ?

There is a comment in hdlcd_drv.h explaining what HDLCD_SHOW_UNDERRUN can
be used for. As for HDLCD_COUNT_BUFFERUNDERRUNS, you are right, it's dead
code from the earlier debugging code that got removed before submission.
I will correct that.

> 
> Most DRM drivers do not threat dma, bus_error, vsync and/or underrun
> interrupts as debug functionality. They are of limited use in this
> driver, presently, yet the CONFIG_DEBUG_FS guard seems a bit strange
> imho.

The HDLCD device has only 1 interrupt that can fire and there is no other
way to show the reason for the interrupt in the driver. It was useful when
debugging underruns and/or vsync issues so I thought it might be useful
to keep around. Putting it the other way: if you are going to use this device
and the image is not completely rendered I would not be able to give you
support to figure out what went wrong without this debugging information.
With this in place I can tell the difference between a busy system vs one
where the interrupt latency is large.

Best regards,
Liviu

> 
> Cheers,
> Emil
> 
> [1] http://blog.ffwll.ch/2014/11/atomic-modeset-support-for-kms-drivers.html
> [2] http://blog.ffwll.ch/2015/01/update-for-atomic-display-updates.html
> 

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