Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755845Ab3CZWuP (ORCPT ); Tue, 26 Mar 2013 18:50:15 -0400 Received: from mail-pa0-f44.google.com ([209.85.220.44]:62997 "EHLO mail-pa0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755815Ab3CZWuK (ORCPT ); Tue, 26 Mar 2013 18:50:10 -0400 From: Grant Grundler To: Chris Ball Cc: Doug Anderson , Will Newton , Seungwon Jeon , Bing Zhao , Jaehoon Chung , Ashok Nagarajan , Olof Johansson , linux-mmc@vger.kernel.org, linux-kernel@vger.kernel.org, Grant Grundler Subject: [PATCH] mmc: dw_mmc: rewrite CLKDIV computation Date: Tue, 26 Mar 2013 15:50:02 -0700 Message-Id: <1364338202-10311-1-git-send-email-grundler@chromium.org> X-Mailer: git-send-email 1.8.1.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2060 Lines: 59 Last year Seungwon Jeon (Samsung) fixed a bug in CLKDIV computation. But when debugging a related issue (http://crbug.com/221828) I found the code unreadable. This rewrite simplifies the computation and explains each step. Signed-off-by: Grant Grundler --- Tested on Samsung Series 3 Chromebook (exynos 5250 chipset) using ChromeOS 3.4 kernel (not 3.9-rc3 which this patch is based against). I've written a test program to confirm this patch generates the same correct values and will share that separately. drivers/mmc/host/dw_mmc.c | 22 +++++++++++++++------- 1 file changed, 15 insertions(+), 7 deletions(-) diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c index 9834221..6fdf55b 100644 --- a/drivers/mmc/host/dw_mmc.c +++ b/drivers/mmc/host/dw_mmc.c @@ -631,14 +631,22 @@ static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit) if (slot->clock != host->current_speed || force_clkinit) { div = host->bus_hz / slot->clock; - if (host->bus_hz % slot->clock && host->bus_hz > slot->clock) - /* - * move the + 1 after the divide to prevent - * over-clocking the card. + if (host->bus_hz > slot->clock) { + /* don't overclock due to integer math losses */ + if ((div * slot->clock) < host->bus_hz) + div++; + + /* don't overclock due to resolution of HW */ + if (div & 1) + div++; + + /* See 6.2.3 CLKDIV in "Mobile Storage Host Databook" + * Look for dwc_mobile_storage_db.pdf from Synopsys. + * CLKDIV value 0 means divisor 1, val 1 -> 2, ... */ - div += 1; - - div = (host->bus_hz != slot->clock) ? DIV_ROUND_UP(div, 2) : 0; + div /= 2; + } else + div = 0; /* use bus_hz */ dev_info(&slot->mmc->class_dev, "Bus speed (slot %d) = %dHz (slot req %dHz, actual %dHZ" -- 1.8.1.3 -- 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/