Re: drm/msm/mdp5: undefined CONFIG_MSM_BUS_SCALING

From: Greg KH <gregkh@linuxfoundation.org>
To: Rob Clark <robdclark@gmail.com>
Cc: Valentin Rothberg <valentinrothberg@gmail.com>,
	Hai Li <hali@codeaurora.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	David Airlie <airlied@linux.ie>, Paul Bolle <pebolle@tiscali.nl>,
	rupran@einserver.de, stefan.hengelein@fau.de
Subject: Re: drm/msm/mdp5: undefined CONFIG_MSM_BUS_SCALING
Date: Thu, 9 Apr 2015 19:07:07 +0200	[thread overview]
Message-ID: <20150409170707.GA7742@kroah.com> (raw)
In-Reply-To: <CAF6AEGsQ=dkREnU5bA=EJCF6o3h+DwcSUd1V2Hvu-rat5-+ypw@mail.gmail.com>

On Thu, Apr 09, 2015 at 10:50:58AM -0400, Rob Clark wrote:
> On Thu, Apr 9, 2015 at 10:20 AM, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Thu, Apr 09, 2015 at 09:49:58AM -0400, Rob Clark wrote:
> >> On Thu, Apr 9, 2015 at 7:22 AM, Valentin Rothberg
> >> <valentinrothberg@gmail.com> wrote:
> >> > Hi Hai,
> >> >
> >> > your commit d5af49c92a8a ("drm/msm/mdp5: Enable DSI connector in msm drm
> >> > driver") in today's Linux next tree adds an #ifdef with CONFIG_MSM_BUS_SCALING
> >> > as condition.  MSM_BUS_SCALING is not defined in Kconfig, so the code in this
> >> > #ifdef block won't be compiled at its current state.
> >> >
> >> > I saw some references on this Kconfig option in other files; is there a
> >> > reason for the absence of MSM_BUS_SCALING?
> >>
> >> right now, it is something that only exists in downstream kernels (for
> >> example, android device kernels).. but in those kernels it is
> >> mandatory to use, as by default the memory/bus is downclocked and the
> >> display would underflow if we did not request sufficient bandwidth.
> >>
> >> It only exists right now in the upstream kernel to simplify
> >> backporting to various device kernels
> >
> > That's crazy.  You are asking upstream to maintain code in order to just
> > make out of tree crap easier to maintain, which you don't have any plan
> > to ever upstream?  That causes havoc on static analysis tools and
> > prevents anyone from ever being able to even change the code for new api
> > changes and test build it.
> 
> Hey, don't blame me for the downstream kernels.  But at various points
> in time I've had to backport drm/msm to various device kernels in
> order to work on the userspace/mesa end of things.  (And, well, there
> are other crazy folks out there who want to get open source graphics
> drivers working on various phones/tablets.)  It was a choice to make
> my life easier.  You know, because reverse engineering a gpu is a walk
> in the park..

I really don't understand.  Why is this code in the kernel tree if it
can't be built?  How does anyone use this?  By taking it and copying it
where?  If it can't be built, and no one can update it, and of course
not run it, why is it here?  What good is this code doing sitting here?

confused,

greg k-h