linux-kernel - Re: [PATCH v3 2/7] mmc: atmel-mci: prepare clk before calling enable

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Message-ID: <20130716171324.7d483ada@skate>
Date:	Tue, 16 Jul 2013 17:13:24 +0200
From:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
To:	Boris BREZILLON <b.brezillon@...rkiz.com>
Cc:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Ludovic Desroches <ludovic.desroches@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Chris Ball <cjb@...top.org>, linux-mmc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/7] mmc: atmel-mci: prepare clk before calling
 enable

Dear Boris BREZILLON,

On Tue, 16 Jul 2013 17:06:48 +0200, Boris BREZILLON wrote:

>  	buf = kmalloc(ATMCI_REGS_SIZE, GFP_KERNEL);
>  	if (!buf)
> @@ -389,9 +391,13 @@ static int atmci_regs_show(struct seq_file *s, void *v)
>  	 * consistent.
>  	 */
>  	spin_lock_bh(&host->lock);
> -	clk_enable(host->mck);
> +	ret = clk_prepare_enable(host->mck);

I am not very familiar with the spin_lock_bh() variant, but are you
sure we are allowed to sleep within a spin_lock_bh()-protected critical
section?

Remember that clk_prepare_enable() calls both ->prepare() and
->enable() for the clock, and ->prepare() is allowed to sleep, while
->enable() is guaranteed not to sleep.

Therefore, clk_prepare() is usually called at probe time, while
clk_enable() is called whenever enabling/disabling the clock is really
needed. So not all clk_enable() can transparently be converted into a
clk_prepare_enable().

Best regards,

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com
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