Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts

From: Adam Ford <aford173@gmail.com>
To: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Linux-OMAP <linux-omap@vger.kernel.org>,
	"Adam Ford" <adam.ford@logicpd.com>,
	"David Airlie" <airlied@linux.ie>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Benoît Cousson" <bcousson@baylibre.com>,
	"Tony Lindgren" <tony@atomide.com>,
	dri-devel@lists.freedesktop.org,
	devicetree <devicetree@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts
Date: Tue, 28 May 2019 08:09:57 -0700	[thread overview]
Message-ID: <CAHCN7xLzoCNW6q5yDCsqMHeNvdNegkGhd0N+q9+Gd8JUGbG=_g@mail.gmail.com> (raw)
In-Reply-To: <af325707-3e42-493d-e858-77878ef06138@ti.com>

On Tue, May 28, 2019 at 4:11 AM Tomi Valkeinen <tomi.valkeinen@ti.com> wrote:
>
> Hi,
>
> On 10/05/2019 22:42, Adam Ford wrote:
> > Currently the source code is compiled using hard-coded values
> > from CONFIG_OMAP2_DSS_MIN_FCK_PER_PCK.  This patch allows this
> > clock divider value to be moved to the device tree and be changed
> > without having to recompile the kernel.
> >
> > Signed-off-by: Adam Ford <aford173@gmail.com>
>
> I understand why you want to do this, but I'm not sure it's a good idea.
> It's really something the driver should figure out, and if we add it to
> the DT, it effectively becomes an ABI.
>
> That said... I'm not sure how good of a job the driver could ever do, as
> it can't know the future scaling needs of the userspace at the time it
> is configuring the clock. And so, I'm not nacking this patch, but I
> don't feel very good about this patch...
>
> The setting also affects all outputs (exluding venc), which may not be
> what the user wants. Then again, I think this setting is really only
> needed on OMAP2 & 3, which have only a single output. But that's the
> same with the current kconfig option, of course.
>
> So, the current CONFIG_OMAP2_DSS_MIN_FCK_PER_PCK is an ugly hack, in my
> opinion, and moving it to DT makes it a worse hack =). But I don't have
> any good suggestions either.

As it stands the Logic PD OMAP35 and AM37/DM37 boards (SOM-LV and
Torpedo) require this to be hard coded to 4 or it hangs during start.
This is the case for all versions 4.2+.  I haven't tested it with
older stuff.  Tony has a DM3730 Torpedo kit and reported the hanging
issue to me. I told him to set that value to 4 to make it not hang.
He asked that I move it to the DT to avoid custom kernels.  I agree
it's a hack, but if it's create a customized defconfig file for 4
boards or modify the device tree, it seems like the device tree
approach is less intrusive.

adam
>
>   Tomi
>
> --
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