linux-kernel - Re: [RFC PATCH 1/3] drm: Add support for Amlogic Meson Graphic Controller

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Date:   Tue, 29 Nov 2016 09:50:24 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     airlied@...ux.ie, khilman@...libre.com, carlo@...one.org,
        Xing.Xu@...ogic.com, victor.wan@...ogic.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        jerry.cao@...ogic.com, linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 1/3] drm: Add support for Amlogic Meson Graphic
 Controller

Hi Neil,

On Mon, Nov 28, 2016 at 10:34:58AM +0100, Neil Armstrong wrote:
> On 11/28/2016 09:16 AM, Daniel Vetter wrote:
> > On Fri, Nov 25, 2016 at 05:03:09PM +0100, Neil Armstrong wrote:
> >> +static void meson_cvbs_encoder_disable(struct drm_encoder *encoder)
> >> +{
> >> +	struct meson_cvbs *meson_cvbs = encoder_to_meson_cvbs(encoder);
> >> +
> >> +	meson_venci_cvbs_disable(meson_cvbs->priv);
> >> +}
> >> +
> >> +static void meson_cvbs_encoder_enable(struct drm_encoder *encoder)
> >> +{
> >> +	struct meson_cvbs *meson_cvbs = encoder_to_meson_cvbs(encoder);
> >> +
> >> +	meson_venci_cvbs_enable(meson_cvbs->priv);
> >> +}
> > 
> > Personally I'd remove the indirection above, more direct code is easier to
> > read.
> 
> I understand, I'll maybe change the meson_venci_cvbs_XXable to be
> directly added to the ops.
> 
> I want to keep the registers setup in separate files and keep a clean
> DRM/HW separation.

I figured this is worth clarifying, and I'm somewhat guessing at your
motivation here for a clean drm/hw split. There's of course various levels
of how much you can split the drm side from your hw backend, but in
general that design approach is really unpopular with upstream. It goes by
the name of "midlayer", and the trouble with it is that it makes
subsystem refactoring more complicated.

For the driver itself it's nice, because it isolates you a bit from drm
core. But that exact isolation is the problem when someone wants (or more
often, needs to) refactor something across the entire subsystem. Then all
these driver-private little (or sometimes much bigger) abstractions get in
the way. That's way I suggested to remove it (both here and in the plane
code), because for upstream the overall subsystem matters more than each
individual driver. GPUs change fast, we need to be able to adapt fast,
too.

Anyway you're driver's pretty small, so personally I don't mind much. I'd
still think removing the indirection would be better though.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch