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

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Message-ID: <CACvgo53KWsC1ZeYYF2bW=4=BeuexaEgBHT=+h2U60iS9nYvYpw@mail.gmail.com>
Date:	Tue, 18 Aug 2015 17:41:59 +0100
From:	Emil Velikov <emil.l.velikov@...il.com>
To:	Liviu Dudau <Liviu.Dudau@....com>
Cc:	David Airlie <airlied@...ux.ie>,
	ML dri-devel <dri-devel@...ts.freedesktop.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <Mark.Rutland@....com>,
	Jon Medhurst <tixy@...aro.org>, Arnd Bergmann <arnd@...db.de>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Will Deacon <Will.Deacon@....com>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	Robin Murphy <Robin.Murphy@....com>
Subject: Re: [PATCH 2/4] drm: Add support for ARM's HDLCD controller.

On 17 August 2015 at 16:15, Liviu Dudau <Liviu.Dudau@....com> wrote:
> 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@....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.
>
There was a similar question from the freescale dcu people [1]. I
believe that the answer still applies.

[1] http://lists.freedesktop.org/archives/dri-devel/2015-March/078689.html

>>
>> 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.
>
I haven't seen other drm drivers follow the "set a macro that is
documented in a header and rebuild" approach. Devs either add a CONFIG
option for it, or expose it as a module parameter. IIRC the nouveau
driver does even more - it enables the underrun machinery
unconditionally.

>>
>> 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.
>
Out of curiosity - are there any implications/side-effects from having
these enabled ?

Thanks,
Emil
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