linux-kernel - Re: [PATCH 3/6] ARM: dts: da850-lcdk: enable the LCD controller

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Date:   Fri, 30 Sep 2016 11:42:14 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Karl Beldan <karl.beldan@...il.com>
Cc:     Kevin Hilman <khilman@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        Sekhar Nori <nsekhar@...com>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Karl Beldan <kbeldan@...libre.com>,
        LKML <linux-kernel@...r.kernel.org>,
        arm-soc <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/6] ARM: dts: da850-lcdk: enable the LCD controller

2016-09-29 20:40 GMT+02:00 Karl Beldan <karl.beldan@...il.com>:
> Hi,
>
> On Thu, Sep 29, 2016 at 06:31:52PM +0200, Bartosz Golaszewski wrote:
>> From: Karl Beldan <kbeldan@...libre.com>
>>
>> This adds the pins used by the LCD controller, and uses 'tilcdc,panel'
>> with some default timings for 800x600.
>>
>> Tested on an LCDK connected on the VGA port (the LCDC is connected to
>> this port via a THS8135).
>>
>> Signed-off-by: Karl Beldan <kbeldan@...libre.com>
>> [Bartosz:
>>   - fixed whitespace errors
>>   - tweaked the description
>
> The description tweak you mention is the removal of an erratum which is
> in the mentioned commit I put on github @
> (https://github.com/kbeldan/linux/commit/b7720bc983c00a083dece119f68ea9d2f522c6c4)
> it included an erratum wrt FIFO threshold I think is worth keeping:
> {
> There is an erratum (fifo-th) "LCDC: Underflow During Initialization":
> [...]
> "This problem may occur if the LCDC FIFO threshold size (
> LCDDMA_CTRL[TH_FIFO_READY]) is left at its default value after reset.
> Increasing the FIFO threshold size will reduce or eliminate underflows.
> Setting the threshold size to 256 double words or larger is
> recommended."
> }

Isn't this the issue that is fixed by changing the memory priority for lcdc?

>
>>   - fixed the incorrect hback-porch value
>
> It can't be a fix, this value depends on the monitor connected.
>

Thanks, I'm new to drm. From reading the datasheet it seemed to me
that this depends on the resolution. FWIW it seems that most LCDs are
able to adjust to this themselves - I tested with two different
displays and the value I introduced worked on both while the previous
one shifted the image to the right. I'll look into that.

>>   - other minor tweaks]
>
> I didn't see any other change while diffing.
>

Dropped the refresh rate from the timings node name.

Thanks,
Bartosz