linux-kernel - Re: [PATCH v1] clk: tegra20/30: Add custom EMC clock implementation

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Message-ID: <20190412074658.GF12975@pdeschrijver-desktop.Nvidia.com>
Date:   Fri, 12 Apr 2019 10:46:58 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>
CC:     Prashant Gaikwad <pgaikwad@...dia.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Joseph Lo <josephl@...dia.com>, <linux-clk@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] clk: tegra20/30: Add custom EMC clock implementation

On Fri, Apr 12, 2019 at 01:47:08AM +0300, Dmitry Osipenko wrote:
> A proper External Memory Controller clock rounding and parent selection
> functionality is required by the EMC drivers. It is not available using
> the generic clock implementation, hence add a custom one. The clock rate
> rounding shall be done by the EMC drivers because they have information
> about available memory timings, so the drivers will have to register a
> callback that will round the requested rate. EMC clock users won't be able
> to request EMC clock by getting -EPROBE_DEFER until EMC driver is probed
> and the callback is set up. The functionality is somewhat similar to the
> clk-emc.c which serves Tegra124+ SoC's, the later HW generations support
> more parent clock sources and the HW configuration and integration with
> the EMC drivers differs a tad from the older gens, hence it's not really
> worth to try to squash everything into a single source file.
> 
..

> +	val = readl_relaxed(emc->ioaddr);
> +	val &= ~CLK_SOURCE_EMC_2X_CLK_DIVISOR_MASK;
> +	val |= div;
> +
> +	parent = val >> CLK_SOURCE_EMC_2X_CLK_SRC_SHIFT;
> +
> +	if (parent == EMC_SRC_PLL_M && div == 0 && emc->want_low_jitter)
> +		val |= USE_PLLM_UD;
> +	else
> +		val &= ~USE_PLLM_UD;
> +

Note that low jitter means the divider is bypassed, so you can only use
this when div == 1.

> +	writel_relaxed(val, emc->ioaddr);
> +
> +	return 0;
> +}
> +
> +static int emc_set_rate_and_parent(struct clk_hw *hw,
> +				   unsigned long rate,
> +				   unsigned long parent_rate,
> +				   u8 index)
> +{
> +	struct tegra_clk_emc *emc = to_tegra_clk_emc(hw);
> +	u32 val, div;
> +
> +	div = div_frac_get(rate, parent_rate, 8, 1, 0);
> +
> +	val = readl_relaxed(emc->ioaddr);
> +
> +	val &= ~CLK_SOURCE_EMC_2X_CLK_SRC_MASK;
> +	val |= index << CLK_SOURCE_EMC_2X_CLK_SRC_SHIFT;
> +
> +	val &= ~CLK_SOURCE_EMC_2X_CLK_DIVISOR_MASK;
> +	val |= div;
> +
> +	if (index == EMC_SRC_PLL_M && div == 0 && emc->want_low_jitter)
> +		val |= USE_PLLM_UD;
> +	else
> +		val &= ~USE_PLLM_UD;
> +

Same here ofcourse.

Peter.