linux-kernel - Re: [EXT] Re: [PATCH 05/10] dt-bindings: display: Add max-res property for mxsfb

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Date:   Wed, 14 Aug 2019 08:05:09 +0000
From:   Robert Chiras <robert.chiras@....com>
To:     "robh@...nel.org" <robh@...nel.org>
CC:     dl-linux-imx <linux-imx@....com>, "marex@...x.de" <marex@...x.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stefan@...er.ch" <stefan@...er.ch>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "festevam@...il.com" <festevam@...il.com>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>
Subject: Re: [EXT] Re: [PATCH 05/10] dt-bindings: display: Add max-res
 property for mxsfb

On Lu, 2019-07-22 at 11:48 -0600, Rob Herring wrote:
> On Wed, Jun 26, 2019 at 04:32:13PM +0300, Robert Chiras wrote:
> > 
> > Add new optional property 'max-res', to limit the maximum supported
> > resolution by the MXSFB_DRM driver.
> Bindings are for h/w description, not driver config.
> 
> > 
> > 
> > Signed-off-by: Robert Chiras <robert.chiras@....com>
> > ---
> >  Documentation/devicetree/bindings/display/mxsfb.txt | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/display/mxsfb.txt
> > b/Documentation/devicetree/bindings/display/mxsfb.txt
> > index 472e1ea..55e22ed 100644
> > --- a/Documentation/devicetree/bindings/display/mxsfb.txt
> > +++ b/Documentation/devicetree/bindings/display/mxsfb.txt
> > @@ -17,6 +17,12 @@ Required properties:
> >  Required sub-nodes:
> >    - port: The connection to an encoder chip.
> > 
> > +Optional properties:
> > +- max-res:   an array with a maximum of two integers, representing
> > the
> > +             maximum supported resolution, in the form of
> > +             <maxX>, <maxY>; if one of the item is <0>, the
> > default
> > +             driver-defined maximum resolution for that axis is
> > used
> I suppose what you are after is bandwidth limits? IIRC, there's
> already
> some bindings expressing such limits. Also, wouldn't you need to
> account
> for bpp and using the 2nd plane (IIRC that there is one).
I am sorry for this late reply, but I was looking after the existing
bindings expressing such limits. I didn't find such bindings related to
this driver though, I found some limits present in some other drivers.
Indeed, this limitation is actually due to bandwidth limitation, but
the problem is that this limitation comes i.MX8M (known as mScale
850D), where the memory bandwidth cannot support: GPU/VPU workload in
the same time with both DCSS driving 4k@60 and eLCDIF driving 1080p@60.
Since eLCDIF is a secondary display we though to add the posibility to
limit it's bandwidth by limiting the resolution.
Indeed, there is also a second plane, but currently not yet supported
(there is no support for second plane in MXSFB in the upstream driver
or in our internal tree).
If you think this limitation doesn't make sense, I can just drop it.
> 
> Rob

Thanks,
Robert