linux-kernel - Re: [PATCH 1/4] irqchip, gicv3-its: Read typer register outside the loop

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Date:	Mon, 06 Jul 2015 13:10:56 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Robert Richter <rric@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>
CC:	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Robert Richter <rrichter@...ium.com>
Subject: Re: [PATCH 1/4] irqchip, gicv3-its: Read typer register outside the
 loop

On 30/06/15 15:14, Robert Richter wrote:
> From: Robert Richter <rrichter@...ium.com>
> 
> No need to read the typer register in the loop. Values do not change.
> 
> Signed-off-by: Robert Richter <rrichter@...ium.com>
> ---
>  drivers/irqchip/irq-gic-v3-its.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index 1b7e155869f6..c5ce21277920 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -803,6 +803,8 @@ static int its_alloc_tables(struct its_node *its)
>  	int psz = SZ_64K;
>  	u64 shr = GITS_BASER_InnerShareable;
>  	u64 cache = GITS_BASER_WaWb;
> +	u64 typer = readq_relaxed(its->base + GITS_TYPER);
> +	u32 ids = GITS_TYPER_DEVBITS(typer);
>  
>  	for (i = 0; i < GITS_BASER_NR_REGS; i++) {
>  		u64 val = readq_relaxed(its->base + GITS_BASER + i * 8);
> @@ -825,9 +827,6 @@ static int its_alloc_tables(struct its_node *its)
>  		 * For other tables, only allocate a single page.
>  		 */
>  		if (type == GITS_BASER_TYPE_DEVICE) {
> -			u64 typer = readq_relaxed(its->base + GITS_TYPER);
> -			u32 ids = GITS_TYPER_DEVBITS(typer);
> -
>  			/*
>  			 * 'order' was initialized earlier to the default page
>  			 * granule of the the ITS.  We can't have an allocation
> 

The TYPER value certainly don't change, but there is also only one
device table per ITS instance, so we only read it once. What do we gain
here?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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