linux-kernel - Re: [PATCH] ARM: dts: i.MX25: define SSI FIFO depth

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Message-ID: <20180309164008.52d41dd5@karo-electronics.de>
Date:   Fri, 9 Mar 2018 16:40:08 +0100
From:   Lothar Waßmann <LW@...O-electronics.de>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     Martin Kaiser <martin@...ser.cx>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Sascha Hauer <kernel@...gutronix.de>,
        Fabio Estevam <fabio.estevam@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: dts: i.MX25: define SSI FIFO depth

Hi,

On Fri, 09 Mar 2018 10:27:12 +0100 Lucas Stach wrote:
> Hi Lothar,
> 
> Am Freitag, den 09.03.2018, 09:37 +0100 schrieb Lothar Waßmann:
> > Hi,
> > 
> > On Thu, 8 Mar 2018 16:38:32 +0100 Martin Kaiser wrote:
> > > Hi Lothar,
> > > 
> > > > > Thus wrote Lothar Waßmann (LW@...O-electronics.de):
> > > 
> > > > > diff --git a/arch/arm/boot/dts/imx25.dtsi b/arch/arm/boot/dts/imx25.dtsi
> > > > > index 9725705..cf70df2 100644
> > > > > --- a/arch/arm/boot/dts/imx25.dtsi
> > > > > +++ b/arch/arm/boot/dts/imx25.dtsi
> > > > > @@ -269,6 +269,7 @@
> > > > > > > > >  				dmas = <&sdma 24 1 0>,
> > > > > > > > >  				       <&sdma 25 1 0>;
> > > > > > > > >  				dma-names = "rx", "tx";
> > > > > > > > > +				fsl,fifo-depth = <15>;
> > > > > > > > >  				status = "disabled";
> > > > > > > > >  			};
> > > > > @@ -329,6 +330,7 @@
> > > > > > > > >  				dmas = <&sdma 28 1 0>,
> > > > > > > > >  				       <&sdma 29 1 0>;
> > > > > > > > >  				dma-names = "rx", "tx";
> > > > > > > > > +				fsl,fifo-depth = <15>;
> > > > > > > > >  				status = "disabled";
> > > > >  			};
> > > > You are changing the global .dtsi file. Did you test this change with
> > > > all devices that are affected by it?
> > > 
> > > I changed the hardware description of the imx25 SSI to match the
> > > reference manual.
> > > 
> > > I did test this change on an imx25 board with audio playback. This uses
> > > the SSI description I modified. I verified that the driver is actually
> > > taking the modified setting into account and that this causes no
> > > problems.
> > > 
> > > As of today, this setting is used by the fsl_ssi driver to set the fifo
> > > water level for dma requests.
> > > 
> > > Of course, I don't have access to the enitre range of supported imx25
> > > boards and I don't think this is required for submitting patches.
> > > 
> > > Do you have any indication why this patch should not be merged?
> > > 
> > 
> > Usually patches should not willy-nilly change the behaviour of existing
> > configurations unless they fix any real life bugs.
> 
> With this logic we wouldn't be able to get most driver changes applied.
> If it is matching the reference manual and has been tested on the
> affected hardware it should be good to go.
> 
> If you know about any specific corner cases that might break with this
> change, please speak up now. Don't reject patches based on the
> overarching "it might break something" argument.
> 
I didn't reject anything, I just wanted to make sure, that the
implications of this patch were sufficiently considered.


Lothar Waßmann