From mboxrd@z Thu Jan 1 00:00:00 1970 From: Adrian Hunter Subject: Re: [PATCH] mmc: core: Force a "detect" to handle non-properly removed cards Date: Mon, 16 Jan 2012 09:45:31 +0200 Message-ID: <4F13D59B.2060607@intel.com> References: <1325586798-16276-1-git-send-email-ulf.hansson@stericsson.com> <4F04C412.1030604@intel.com> <4F0AC942.4060404@stericsson.com> <4F0AD879.10801@intel.com> <4F0AE82C.10000@stericsson.com> <4F0AF157.7090101@intel.com> <4F0AF96B.4050500@stericsson.com> <4F0C035D.7070705@intel.com> <4F0C1A1C.8070007@stericsson.com> <4F0C2ACD.4090002@intel.com> <4F100196.8010104@stericsson.com> <4F100AE5.3040304@intel.com> <4F10161E.2080107@stericsson.com> <4F101ED5.9090007@intel.com> <4F102E34.6030704@stericsson.com> <4F1034F5.1020305@intel.com> <4F104130.8030906@stericsson.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from mga14.intel.com ([143.182.124.37]:27707 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752864Ab2APHpb (ORCPT ); Mon, 16 Jan 2012 02:45:31 -0500 In-Reply-To: <4F104130.8030906@stericsson.com> Sender: linux-mmc-owner@vger.kernel.org List-Id: linux-mmc@vger.kernel.org To: Ulf Hansson Cc: "linux-mmc@vger.kernel.org" , Chris Ball , Per FORLIN , Johan RUDHOLM , Lee Jones On 13/01/12 16:35, Ulf Hansson wrote: > Adrian Hunter wrote: >> On 13/01/12 15:14, Ulf Hansson wrote: >>>>>>> In principles this means the following sequence: >>>>>>> >>>>>>> We will rely on that the get_cd function will return 0 (indicating >>>>>>> card is >>>>>>> removed) when the card is "slowly" removed at the point when the rescan >>>>>>> function is calling it through the bus_ops->detect --> >>>>>>> _mmc_detect_card_removed function. >>>>>>> >>>>>>> This then becomes a race, meaning that the rescan function must be >>>>>>> executing >>>>>>> at the same time the get_cd function will returns 0. Otherwise the >>>>>>> rescan >>>>>>> function will not remove the card. >>>>>>> >>>>>>> Thus my conclusion is that "[PATCH 2/4 v4] MMC/SD: Add callback >>>>>>> function to >>>>>>> detect card" will likely improve behavior but is not the safe >>>>>>> solution to >>>>>>> handle "slowly" removed cards. >>>>>>> >>>>>>> Again, to be sure, we must let the mmc_detect_card_remove function >>>>>>> trigger a >>>>>>> rescan when _mmc_detect_card_removed has detected that the card is >>>>>>> removed. >>>>>>> This should be safe in all circumstances. >>>>>> sdhci has no problem because it does this: >>>>>> >>>>>> - the host controller debounces the card detect line >>>>>> - the host controller records whether or not the card is present >>>>>> - the sdhci driver prevents (errors out) requests when the card is >>>>>> not present >>>>> Debouncing will just be a way of triggering the problem more seldom. Or in >>>>> worst case, state the card has been removed even if it has not. >>>> If a delay is used with mmc_detect_change, debouncing is not necessary. >>>> >>>>> Just because you get a GPIO irq on the detect line does not mean the >>>>> card is >>>>> removed, debouncing or not. I consider this as pure mechanical switch >>>>> which >>>>> likely has glitches and I don't see that we should trust it fully. We only >>>>> want to trigger a detect work, which is exactly what is done in the patch >>>>> from Guennadi Liakhovetski "mmc: add a generic GPIO card-detect helper". >>>> The original problem was "slow card removal". "Unreliable card detect" >>>> is a separate problem. Currently there is polling (MMC_CAP_NEEDS_POLL) >>>> for that. Alternatively there is MMC_CAP2_RESCAN_ON_ERROR as we have >>>> discussed. >>> I do not understand why you mention "Unreliable card detect"? That has >>> nothing to do with this patch. >>> >>> So to conclude the discussion, do you believe that this patch is acceptable >>> as long as we add a CAPS2 option "MMC_CAP2_RESCAN_ON_ERROR", which if not >>> set will prevent the detect work from being scheduled from >>> mmc_detect_card_removed? >> >> Yes >> > > OK, but.. :-) > > I were just about to update the patch according to your recommendation when > I realized the following: > > Once _mmc_detect_card_removed has set the card state as removed > ("mmc_card_set_removed"), the card will never be accessible for I/O requests > any more, all I/O will "silently" be thrown away in the block layer. This > leads to that there should definitely be no reason for _not_ letting a > scheduled rescan remove the card as soon as possible. In other words the > CAP2 should not be needed. > > Did I miss something? > > Agree? No. mmc_detect_card_removed() will not check/set the card removed unless there has been a call to mmc_detect_change() to set the host->detect_change flag. MMC_CAP2_RESCAN_ON_ERROR is definitely needed. Do not confuse mmc_detect_card_removed() with _mmc_detect_card_removed(). The former is called by block.c. The latter is only called by mmc_rescan() via the ->detect method.