Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753597AbbFDMTx (ORCPT ); Thu, 4 Jun 2015 08:19:53 -0400 Received: from protonic.xs4all.nl ([83.163.252.89]:24494 "EHLO protonic.xs4all.nl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752816AbbFDMTv (ORCPT ); Thu, 4 Jun 2015 08:19:51 -0400 Date: Thu, 4 Jun 2015 14:19:49 +0200 From: David Jander To: Adrian Hunter Cc: Ulf Hansson , Sascha Hauer , Johan Rudholm , Javier Martinez Canillas , linux-mmc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 2/2] mmc: core.c: Add comment to clarify special cases of ERASE/TRIM Message-ID: <20150604141949.01641d76@archvile> In-Reply-To: <55703387.8060509@intel.com> References: <1433413214-21614-1-git-send-email-david@protonic.nl> <55703387.8060509@intel.com> Organization: Protonic Holland X-Mailer: Claws Mail 3.11.1 (GTK+ 2.24.27; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5371 Lines: 146 Dear Adrian, Thanks for reacting. On Thu, 04 Jun 2015 14:16:23 +0300 Adrian Hunter wrote: > On 04/06/15 13:20, David Jander wrote: > > Signed-off-by: David Jander > > Please never send delta patches. Always send a new version of the whole > patch. Sorry for that. This was meant as a separate patch though... the original would be part 1/2 and this is part 2/2 (as noted in the subject), and I did it only to get an idea if I understood Ulf correctly (RFC). > > --- > > drivers/mmc/core/core.c | 28 +++++++++++++++++++++++++++- > > 1 file changed, 27 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c > > index 6c9611b..b6aa9ad 100644 > > --- a/drivers/mmc/core/core.c > > +++ b/drivers/mmc/core/core.c > > @@ -2109,11 +2109,20 @@ int mmc_erase(struct mmc_card *card, unsigned int > > from, unsigned int nr, !(card->ext_csd.sec_feature_support & > > EXT_CSD_SEC_GB_CL_EN)) return -EOPNOTSUPP; > > > > + /* > > + * Sanity check: If we do not erase aligned, whole erase-groups, > > return > > + * an error, since we intended a "secure" erase, silently not > > erasing > > + * something would be unacceptable. > > + */ > > I am not sure the value of a comment that can anyway be inferred from the > code. Neither am I, but Ulf suggested to put some more comments to the code in this function. At least that's what I understood... I figured it was not so obvious why we take different approaches to ERASE and SECURE_ERASE, so some explaining might have been desirable. I even took the time to go through the eMMC specification to see if there was some recommendation about this... > > if (arg == MMC_SECURE_ERASE_ARG) { > > if (from % card->erase_size || nr % card->erase_size) > > return -EINVAL; > > } > > > > + /* > > + * Make sure only erase-groups that are fully contained in the > > erase > > + * region are erased. Silently ignore the rest. > > + */ > > Ditto > > > if (arg == MMC_ERASE_ARG) { > > rem = from % card->erase_size; > > if (rem) { > > @@ -2140,6 +2149,14 @@ int mmc_erase(struct mmc_card *card, unsigned int > > from, unsigned int nr, /* 'from' and 'to' are inclusive */ > > to -= 1; > > > > + /* > > + * Special case where only one erase-group fits in the timout > > budget: > > timout -> timeout Oops. Thanks. > > + * If the region crosses an erase-group boundary on this > > particular > > + * case, we will be trimming more than one erase-group which, > > does not > > + * fit in the timeout budget of the controller, so we need to > > split it > > + * and call mmc_do_erase() twice if necessary. This special case > > is > > + * identified by the card->eg_boundary flag. > > + */ > > if ((arg & MMC_TRIM_ARGS) && (card->eg_boundary) && > > (from % card->erase_size)) { > > rem = card->erase_size - (from % card->erase_size); > > @@ -2244,7 +2261,16 @@ static unsigned int mmc_do_calc_max_discard(struct > > mmc_card *card, if (!qty) > > return 0; > > > > - /* We can only erase one erase group special case */ > > + /* > > + * When specifying a sector range to trim, chances are we might > > cross > > + * an erase-group boundary even if the amount of sectors is less > > than > > + * one erase-group. > > + * If we can only fit one erase-group in the controller timeout > > budget, > > + * we have to care that erase-group boundaries are not crossed by > > a > > + * single trim operation. We flag that special case with > > "eg_boundary". > > + * In all other cases we can just decrement qty and pretend that > > we > > + * always touch (qty + 1) erase-groups as a simple optimization. > > The language seems a little odd here. We are setting the max_discard limit > which does not involve "pretending" or "optimization", it is just a > calculation. The important point is that the calculation has to count the > maximum number of erase blocks affected not the size in erase blocks. You > could give an example e.g. if a 2 sector trim crosses an erase block > boundary then that counts as 2 erase blocks affected. Sorry to disagree here. Strictly speaking we are not only calculating max_discard, because max_discard is useless for a function that takes sectors as arguments, when this value depends on erase-groups and not sectors. There is no valid function converting from one to the other, so we _need_ to pretend something. That's what the somewhat obscure "if (qty==1) return 1" trickery does, together with the magical "--qty" afterwards. The original code pretends that we always cross an erase-group boundary, hence the --qty. This needs explaining, because strictly speaking it is not correct because max_discard can be higher. It just doesn't produce wrong results because we "are on the safe side". And doing something different for the case qty==1 is definitely an optimization... which is what the first patch intends to do. Maybe the name of the function is misleading...? > > + */ > > if (qty == 1) > > card->eg_boundary = 1; > > else > > > Best regards, -- David Jander Protonic Holland. -- 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/