From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ray Jui Subject: Re: [PATCH v6 2/3] i2c: iproc: Add Broadcom iProc I2C Driver Date: Mon, 19 Jan 2015 13:31:25 -0800 Message-ID: <54BD77AD.3080209@broadcom.com> References: <1421695428-19102-1-git-send-email-rjui@broadcom.com> <1421695428-19102-3-git-send-email-rjui@broadcom.com> <20150119194420.GG26493@n2100.arm.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150119194420.GG26493-l+eeeJia6m9vn6HldHNs0ANdhmdF6hFW@public.gmane.org> Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Russell King - ARM Linux Cc: Wolfram Sang , =?windows-1252?Q?Uwe_Kleine-K=F6n?= =?windows-1252?Q?ig?= , Arend van Spriel , Rob Herring , Pawel Moll , Mark Rutland , Ian Campbell , Kumar Gala , Grant Likely , Christian Daudt , Matt Porter , Florian Fainelli , Scott Branden , linux-i2c-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, bcm-kernel-feedback-list-dY08KVG/lbpWk0Htik3J/w@public.gmane.org, devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: devicetree@vger.kernel.org On 1/19/2015 11:44 AM, Russell King - ARM Linux wrote: > To see why atomic_t is pure obfuscation: > > typedef struct { > int counter; > } atomic_t; > > So, counter is a plain int. > > On Mon, Jan 19, 2015 at 11:23:47AM -0800, Ray Jui wrote: >> +static irqreturn_t bcm_iproc_i2c_isr(int irq, void *data) >> +{ >> + struct bcm_iproc_i2c_dev *iproc_i2c = data; >> + u32 status = readl(iproc_i2c->base + IS_OFFSET); >> + >> + status &= ISR_MASK; >> + >> + if (!status) >> + return IRQ_NONE; >> + >> + writel(status, iproc_i2c->base + IS_OFFSET); >> + atomic_set(&iproc_i2c->xfer_is_done, 1); > > #define atomic_set(v,i) (((v)->counter) = (i)) > > So, this is the same as doing: > > iproc_i2c->xfer_is_done.counter = 1; > > which is merely setting the 'int' to 1. > >> + time_left = wait_for_completion_timeout(&iproc_i2c->done, time_left); >> + >> + /* disable all interrupts */ >> + writel(0, iproc_i2c->base + IE_OFFSET); >> + >> + if (!time_left && !atomic_read(&iproc_i2c->xfer_is_done)) { > > #define atomic_read(v) ACCESS_ONCE((v)->counter) > > This is practically the same as: > > if (!time_left && !iproc_i2c->xfer_is_done.counter) { > > except that this access will be guaranteed to happen just once at this > location (see ACCESS_ONCE() in include/linux/compiler.h). > > However, complete()..wait_for_completion() ensures that there are > barriers in the way: complete takes a spinlock on the waiter, so the > write to iproc_i2c->xfer_is_done.counter will be visible by the time > wait_for_completion() returns, and wait_for_completion() also does. > The same spinlock is also manipulated by wait_for_completion(), which > means there's barriers there as well, so it can't cache the value of > "counter" across that call. > > So, the "volatile" access guaranteed by ACCESS_ONCE() isn't even > needed here. > > (It would be needed if you were spinning in a loop, calling no other > functions - but then you're supposed to use cpu_relax() in that > circumstance, which has a compiler barrier in it, which ensures that > it will re-read such a variable each time.) > I really learned a good lesson here. Thanks for the thorough explanation! -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html