From mboxrd@z Thu Jan 1 00:00:00 1970 From: daniel.lezcano@linaro.org (Daniel Lezcano) Date: Wed, 16 Dec 2015 11:32:17 +0100 Subject: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value In-Reply-To: <20151216093340.GD8644@n2100.arm.linux.org.uk> References: <1448466169-5230-1-git-send-email-jszhang@marvell.com> <56707F32.3030405@linaro.org> <20151216151125.1e91b4f4@xhacker> <20151216152807.23491eee@xhacker> <20151216153609.0f09f941@xhacker> <56712D33.5080009@linaro.org> <20151216093340.GD8644@n2100.arm.linux.org.uk> Message-ID: <56713DB1.9070301@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 12/16/2015 10:33 AM, Russell King - ARM Linux wrote: > On Wed, Dec 16, 2015 at 10:21:55AM +0100, Daniel Lezcano wrote: >> On 12/16/2015 08:36 AM, Jisheng Zhang wrote: >>> And in fact, clocksource_mmio_readw_down() also has similar issue, but it masks >>> with c->mask before return, the c->mask is less than 32 bit (because the >>> clocksource_mmio_init think number of valid bits > 32 or < 16 is invalid.) >>> the higher 32 bits are masked off, so we never saw such issue. But we'd better >>> to fix that, what's your opinion? >> >> I think we should have a look to this portion closely. > > There is no need to return more bits than are specified. If you have > a N-bit counter, then the high (64-N)-bits can be any value, because: > > static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask) > { > return (now - last) & mask; > } > > where 'now' is the current value returned from the clock source read > function, 'last' is a previously returned value, and 'mask' is the > bit mask. This has the effect of ignoring the high order bits. I think this approach is perfectly sane. When I said we should look at this portion closely, I meant we should double check the bitwise-nor order regarding the explicit cast. The clocksource's mask makes sense and must stay untouched. -- Linaro.org ? Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965805AbbLPKcR (ORCPT ); Wed, 16 Dec 2015 05:32:17 -0500 Received: from mail-wm0-f44.google.com ([74.125.82.44]:33728 "EHLO mail-wm0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933069AbbLPKcP (ORCPT ); Wed, 16 Dec 2015 05:32:15 -0500 Subject: Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value To: Russell King - ARM Linux References: <1448466169-5230-1-git-send-email-jszhang@marvell.com> <56707F32.3030405@linaro.org> <20151216151125.1e91b4f4@xhacker> <20151216152807.23491eee@xhacker> <20151216153609.0f09f941@xhacker> <56712D33.5080009@linaro.org> <20151216093340.GD8644@n2100.arm.linux.org.uk> Cc: Jisheng Zhang , tglx@linutronix.de, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org From: Daniel Lezcano Message-ID: <56713DB1.9070301@linaro.org> Date: Wed, 16 Dec 2015 11:32:17 +0100 User-Agent: Mozilla/5.0 (X11; Linux i686; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <20151216093340.GD8644@n2100.arm.linux.org.uk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/16/2015 10:33 AM, Russell King - ARM Linux wrote: > On Wed, Dec 16, 2015 at 10:21:55AM +0100, Daniel Lezcano wrote: >> On 12/16/2015 08:36 AM, Jisheng Zhang wrote: >>> And in fact, clocksource_mmio_readw_down() also has similar issue, but it masks >>> with c->mask before return, the c->mask is less than 32 bit (because the >>> clocksource_mmio_init think number of valid bits > 32 or < 16 is invalid.) >>> the higher 32 bits are masked off, so we never saw such issue. But we'd better >>> to fix that, what's your opinion? >> >> I think we should have a look to this portion closely. > > There is no need to return more bits than are specified. If you have > a N-bit counter, then the high (64-N)-bits can be any value, because: > > static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask) > { > return (now - last) & mask; > } > > where 'now' is the current value returned from the clock source read > function, 'last' is a previously returned value, and 'mask' is the > bit mask. This has the effect of ignoring the high order bits. I think this approach is perfectly sane. When I said we should look at this portion closely, I meant we should double check the bitwise-nor order regarding the explicit cast. The clocksource's mask makes sense and must stay untouched. -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog