From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753697AbeDTHfb (ORCPT ); Fri, 20 Apr 2018 03:35:31 -0400 Received: from mail-io0-f176.google.com ([209.85.223.176]:32893 "EHLO mail-io0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753597AbeDTHf3 (ORCPT ); Fri, 20 Apr 2018 03:35:29 -0400 X-Google-Smtp-Source: AIpwx48nQy1k0Rtg/ZXGMeuzSVEIxI27svTzF5kVZloONV/HN2XVm4wQT8j2/fGL0pSQsEr5h4KY2Dee4r0egVmvT98= MIME-Version: 1.0 In-Reply-To: <20180417121130.25281-1-Eugeniy.Paltsev@synopsys.com> References: <20180417121130.25281-1-Eugeniy.Paltsev@synopsys.com> From: Ulf Hansson Date: Fri, 20 Apr 2018 09:35:28 +0200 Message-ID: Subject: Re: [RFC 0/2] dw_mmc: add multislot support To: Eugeniy Paltsev Cc: "linux-mmc@vger.kernel.org" , Linux Kernel Mailing List , linux-snps-arc@lists.infradead.org, linux-samsung-soc , Linux ARM , Jaehoon Chung , Shawn Lin , Kukjin Kim , Krzysztof Kozlowski , Alexey Brodkin Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [...] > > 2. Add missing stuff to support multislot mode in DesignWare MMC driver. > * Add missing slot switch to __dw_mci_start_request() function. > * Refactor set_ios function: > a) Calculate common clock which is > suitable for all slots instead of directly use clock value > provided by mmc core. We calculate common clock as the minimum > among each used slot clocks. This clock is calculated in > dw_mci_calc_common_clock() function which is called > from set_ios() > b) Disable clock only if no other slots are ON. > c) Setup clock directly in set_ios() only if no other slots > are ON. Otherwise adjust clock in __dw_mci_start_request() > function before slot switch. > d) Move timings and bus_width setup to separate funcions. > * Use timing field in each slot structure instead of common field in > host structure. > * Add locks to serialize access to registers. Sorry, but this is a hack to *try* to make multi-slot work and this isn't sufficient. There were good reasons to why the earlier non-working multi slot support was removed from dw_mmc. Let me elaborate a bit for your understanding. The core uses a host lock (mmc_claim|release_host()) to serialize operations and commands, as to confirm to the SD/SDIO/(e)MMC specs. The above changes gives no guarantees for this. To make that work, we would need a "mmc bus lock" to be managed by the core. However, inventing a "mmc bus lock" would lead to other problems related to I/O scheduling for upper layers - it simply breaks. For example, I/O requests for one card/slot can then starve I/O requests reaching another card/slot. [...] Kind regards Uffe