Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932376AbaFSQbO (ORCPT ); Thu, 19 Jun 2014 12:31:14 -0400 Received: from mail-ve0-f181.google.com ([209.85.128.181]:64164 "EHLO mail-ve0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757533AbaFSQbM (ORCPT ); Thu, 19 Jun 2014 12:31:12 -0400 MIME-Version: 1.0 In-Reply-To: <53A30D08.6070107@gmail.com> References: <1403167145-5267-1-git-send-email-amit.daniel@samsung.com> <53A2B9C3.1050906@gmail.com> <53A30D08.6070107@gmail.com> Date: Thu, 19 Jun 2014 09:31:11 -0700 X-Google-Sender-Auth: 6xkTN-fgQvkZ6H9geTJ8VIHXVPU Message-ID: Subject: Re: [PATCH v2] clocksource: exynos-mct: Register the timer for stable udelay From: Doug Anderson To: Tomasz Figa Cc: Amit Daniel Kachhap , linux-samsung-soc , Kukjin Kim , Daniel Lezcano , Thomas Gleixner , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , David Riley Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Tomasz, On Thu, Jun 19, 2014 at 9:17 AM, Tomasz Figa wrote: > On 19.06.2014 18:01, Doug Anderson wrote: >> Hi, >> >> On Thu, Jun 19, 2014 at 3:21 AM, Tomasz Figa wrote: >>>> +static struct delay_timer exynos4_delay_timer; >>>> + >>>> +static unsigned long exynos4_read_current_timer(void) >> >> Note: I think this should return a cycles_t, not an unsigned long. >> They're the same (right now), but probably shouldn't be (see below). >> >> >>>> +{ >>>> +#ifdef ARM >>>> + return __raw_readl(reg_base + EXYNOS4_MCT_G_CNT_L); >>>> +#else /* ARM64, etc */ >>>> + return exynos4_frc_read(&mct_frc); >>>> +#endif >>>> +} >>>> + >>> >>> No need for anything like this. Even if running on ARM64, the delay >>> timer code should be able to cope with different timer widths. For >>> delays, 32 bits are enough, so just always read the lower part. >> >> I agree that the timer code should cope but it doesn't appear to. I see: >> >> cycles_t start = get_cycles(); >> while ((get_cycles() - start) < cycles) >> cpu_relax(); >> >> Right now cycles_t is defined as "unsigned long". If that's 64-bits >> on ARM64 then this function will have problems with wraparound. > > Well, first of all, I'm not sure why we're discussing ARM64 here, if it > doesn't have support for register_current_timer_delay() at all. > Moreover, the only user if it is ARM32 and, if I remember correctly, the > assumption was that you need a >= 32 bit timer to use it for delays and > so with 32 bit unsigned long everything should work. Now if ARM64 also > wants to use this infrastructure, instead of hacking drivers now and > encouraging ARM64 people to continue using this kind of hackery, I > believe we should indicate that ARM64 delay timer code needs to be > implemented correctly and cope for different timer widths. > > Of course the best way would be to redesign this interface right now to > consider time width (it is not just about 32 or 64 bits, there are > various timers, possibly 48- or 52-bit) and change ARM32 implementation > of it as well, if there are any volunteers. I was arguing that it was a lot of extra complexity to add to the code to support different timer sizes for udelay when 32-bits is enough (and it's trivial for timer implementations to truncate down to 32-bits). > Changing the code to always > use 32-bit type regardless of arch is also an option. Right, that's what I was arguing for. >> My personal vote would be to submit a patch to change "cycles_t" to >> always be 32-bits. Given that 32-bits was fine for udelay() for ARM >> that seems sane and simple. If someone later comes up with a super >> compelling reason why we need 64-bit timers for udelay (really??) then >> they can later add all the complexity needed. > > Yes, this could work. I'm not sure what else cycles_t is used for, though. True, it is a bit questionable to change this since it's a type that's not obviously just for udelay(). Perhaps a better option would be to make a new typedef for the result of read_current_timer(). ...or just change it to return a u32? >> Amit: can you code up such a patch and add it to the series? I know >> it changes code that touches all ARM devices but I still think it's >> the right thing to do and actually only really changes behavior on >> ARM64. >> >> >>> Also use of raw accessors in drivers is discouraged - please use >>> readl_relaxed(). >> >> It doesn't seem like that should happen in the same patch. Perhaps >> Amit can do a cleanup patch first that changes all instances of >> __raw_readl / __raw_writel in this file, then submit his patch atop >> that. > > Why not? The patch adding exynos4_read_current_timer() (and so the call > to __raw_readl() was Amit's patch. I'm not asking to fix this in the > whole driver, just to do things correctly in new code. IMHO consistency within a file is more important. Having a single readl_relaxed() in a file that uses all __raw_readl() doesn't seem beneficial to me. ...but I certainly won't NAK the patch if that's what others want. >>> Btw. I don't even see support for this on ARM64 in mainline, where arch >>> timer is always used for delays and AFAIK this is a platform requirement. >> >> Yeah, I'd vote for not using MCT on ARM64, but it I suppose it doesn't >> hurt to keep it working. > > Right now there is no need for this, so let's just keep things simple. It should at least have a comment saying that it's broken in subtle ways (only on udelays that happen to span the 32-bit wraparound) on anything where cycle_t is not 32-bits. Perhaps at least a: WARN_ON_ONCE(sizeof(cycles_t) != sizeof(u32)); The above should be optimized by the compiler. -Doug -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/