linux-kernel - Re: [EXT] Re: [PATCH v2 09/15] dt-bindings: display: Add max-res property for mxsfb

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Message-ID: <1565784231.3209.74.camel@nxp.com>
Date:   Wed, 14 Aug 2019 12:03:51 +0000
From:   Robert Chiras <robert.chiras@....com>
To:     "stefan@...er.ch" <stefan@...er.ch>
CC:     dl-linux-imx <linux-imx@....com>, "marex@...x.de" <marex@...x.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "agx@...xcpu.org" <agx@...xcpu.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 v2 09/15] dt-bindings: display: Add max-res
 property for mxsfb

Hi Stefann,
On Mi, 2019-08-14 at 13:25 +0200, Stefan Agner wrote:
> On 2019-08-14 13:14, Robert Chiras wrote:
> > 
> > Hi Stefan,
> > On Mi, 2019-08-14 at 13:03 +0200, Stefan Agner wrote:
> > > 
> > > On 2019-08-14 12:48, Robert Chiras wrote:
> > > > 
> > > > 
> > > > Add new optional property 'max-res', to limit the maximum
> > > > supported
> > > > resolution by the MXSFB_DRM driver.
> > > I would also mention the reason why we need this.
> > > 
> > > I guess this needs a vendor prefix as well (fsl,max-res). I also
> > > would
> > > like to have the ack of the device tree folks here.
> > Rob Herring also aked be about this, and I'll copy here the reply,
> > with
> > explanations:
> > 
> > Indeed, this limitation is actually due to bandwidth limitation,
> > but
> > the problem is that this limitation comes on 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.
> > If you say that more details are needed, I can add them in the
> > description.
> Oh sorry I missed that.
> 
> Rob Herring also wrote:
> > 
> > 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 guess the binding he refers to is max-memory-bandwidth, which is
> used
> in multiple driver already. It makes sense to reuse this property
> instead of inventing a new set of property which is also not taking
> bpp
> into account...
Actually I saw this binding, but I thought it is strictly related to
that driver, where it is documented. Now, that you say this one is used
in many drivers (and recalling that you suggested for my 'max-res' to
add a prefix), I get it now.
> 
> The pl111 driver implements this property, it should be fairly easy
> to
> adopt that code.
Sure, I will do something similar here, too.
> 
> --
> Stefan
> 
> 
> 
> > 
> > > 
> > > 
> > > --
> > > Stefan
> > > 
> > > > 
> > > > 
> > > > 
> > > > 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
> > > > +
> > > >  Example:
> > > > 
> > > >       lcdif1: display-controller@...0000 {
> > Thanks,
> > Robert