linux-kernel - Re: [PATCH 1/2] ARM: dts: bcm2711: force CMA into first GB of memory

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Date:   Mon, 04 Nov 2019 19:53:39 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Stefan Wahren <wahrenst@....net>, catalin.marinas@....com,
        devicetree@...r.kernel.org, bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Eric Anholt <eric@...olt.net>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] ARM: dts: bcm2711: force CMA into first GB of memory

On Mon, 2019-11-04 at 09:51 -0800, Florian Fainelli wrote:
> On 11/4/19 9:09 AM, Stefan Wahren wrote:
> 
> [snip]
> 
> > > +	reserved-memory {
> > > +		#address-cells = <2>;
> > > +		#size-cells = <1>;
> > > +		ranges;
> > > +
> > > +		/*
> > > +		 * arm64 reserves the CMA by default somewhere in ZONE_DMA32,
> > > +		 * that's not good enough for the Raspberry Pi 4 as some
> > > +		 * devices can only address the lower 1G of memory (ZONE_DMA).
> > > +		 */
> > > +		linux,cma {
> > > +			compatible = "shared-dma-pool";
> > > +			size = <0x2000000>; /* 32MB */
> > > +			alloc-ranges = <0x0 0x00000000 0x40000000>;
> > > +			reusable;
> > > +			linux,cma-default;
> > > +		};
> > > +	};
> > > +
> > 
> > i think this is a SoC-specific issue not a board specifc one. Please
> > move this to bcm2711.dtsi
> 
> This sounds like a possibly fragile solution if someone changes
> CONFIG_CMA_SIZE_MBYTES to a value greater than 32MB no?

I agree it's not the most flexible solution in the world. It also bypasses the
command line interface. But I can't see any alternatives as of today.

Overall, I suggest that we set CMA's size to whatever is needed for a sensible
desktop use. And let odd users with custom HW modify it trough overlays (which
they will most likely be forced to do anyway).

That said I'm open to suggestions.

Regards,
Nicolas


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