linux-kernel - Re: [PATCH 1/6] platform-drivers: msm: add single-wire serial bus interface (SSBI) driver

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Message-ID: <20130312132719.GA3421@kroah.com>
Date:	Tue, 12 Mar 2013 06:27:19 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	David Brown <davidb@...eaurora.org>
Cc:	Kenneth Heitke <kheitke@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/6] platform-drivers: msm: add single-wire serial bus
 interface (SSBI) driver

On Mon, Mar 11, 2013 at 11:51:08PM -0700, David Brown wrote:
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
> 
> >> +static int ssbi_wait_mask(struct msm_ssbi *ssbi, u32 set_mask, u32 clr_mask)
> >> +{
> >> +	u32 timeout = SSBI_TIMEOUT_US;
> >> +	u32 val;
> >> +
> >> +	while (timeout--) {
> >> +		val = ssbi_readl(ssbi, SSBI2_STATUS);
> >> +		if (((val & set_mask) == set_mask) && ((val & clr_mask) == 0))
> >> +			return 0;
> >> +		udelay(1);
> >
> > Busy loop?  Really?
> 
> Finally was able to dig up some of the reason for this.  The
> transactions typically take about 5us.  In the case of contention with
> another CPU, it could take as much as 20us.
> 
> Would it be sufficient to just explain this in a comment?

That would be good to do, especially if it turns out to be a longer
delay and people start to wonder why their system load is increasing for
no noticeable reason.

thanks,

greg k-h
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