linux-kernel - Re: [PATCH v4 09/20] memory: tegra: Adapt to Tegra20 device-tree binding changes

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Date:   Mon, 24 Sep 2018 12:02:05 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Jonathan Hunter <jonathanh@...dia.com>,
        Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 09/20] memory: tegra: Adapt to Tegra20 device-tree
 binding changes

On Mon, Sep 24, 2018 at 03:41:42AM +0300, Dmitry Osipenko wrote:
> The tegra20-mc device-tree binding has been changed, GART has been
> squashed into Memory Controller and now the clock property is mandatory
> for Tegra20, the DT compatible has been changed as well. Adapt driver to
> the DT changes.
> 
> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> ---
>  drivers/memory/tegra/mc.c | 21 ++++++++-------------
>  drivers/memory/tegra/mc.h |  6 ------
>  include/soc/tegra/mc.h    |  2 +-
>  3 files changed, 9 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/memory/tegra/mc.c b/drivers/memory/tegra/mc.c
> index e56862495f36..1b4ceefd82f9 100644
> --- a/drivers/memory/tegra/mc.c
> +++ b/drivers/memory/tegra/mc.c
> @@ -51,7 +51,7 @@
>  
>  static const struct of_device_id tegra_mc_of_match[] = {
>  #ifdef CONFIG_ARCH_TEGRA_2x_SOC
> -	{ .compatible = "nvidia,tegra20-mc", .data = &tegra20_mc_soc },
> +	{ .compatible = "nvidia,tegra20-mc-gart", .data = &tegra20_mc_soc },

Technically we now regress because we no longer support the older device
tree bindings. I know that it doesn't really matter because this driver
doesn't really do much interesting yet other than reporting memory
access violations, but if that's enough to warrant a change of the
compatible string, then I think we also need to preserve compatibility
in the code.

That said, I think compatibility would be easier to preserve if we stuck
with the old compatible string and used a "reg-names" property to
specify which version of the binding we're referring to.

For example, we could have:

	memory-controller@...0f000 {
		compatible = "nvidia,tegra20-mc";
		reg = <0x7000f000 0x024
		       0x7000f03c 0x3c4>;
		...
	};

for the old binding and:

	memory-controller@...0f000 {
		compatible = "nvidia,tegra20-mc";
		reg = <0x7000f000 0x00000400>,
		      <0x58000000 0x02000000>;
		reg-names = "mc", "gart";
		...
	};

for the new binding. The driver can then easily check for the existence
of the reg-names property and take the legacy or new code paths.

Thierry

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