Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752461AbbHPOHE (ORCPT ); Sun, 16 Aug 2015 10:07:04 -0400 Received: from pandora.arm.linux.org.uk ([78.32.30.218]:34992 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751420AbbHPOHB (ORCPT ); Sun, 16 Aug 2015 10:07:01 -0400 Date: Sun, 16 Aug 2015 15:06:39 +0100 From: Russell King - ARM Linux To: Arnd Bergmann Cc: Leilk Liu , Mark Rutland , devicetree@vger.kernel.org, Sascha Hauer , linux-kernel@vger.kernel.org, linux-spi@vger.kernel.org, Mark Brown , linux-mediatek@lists.infradead.org, Matthias Brugger , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] spi: Mediatek: fix endian warnings Message-ID: <20150816140639.GE7557@n2100.arm.linux.org.uk> References: <1439289789-29026-1-git-send-email-leilk.liu@mediatek.com> <1871097.B3YgX5rNy0@wuerfel> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1871097.B3YgX5rNy0@wuerfel> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2384 Lines: 55 On Sat, Aug 15, 2015 at 10:16:03PM +0200, Arnd Bergmann wrote: > On Tuesday 11 August 2015 18:43:09 Leilk Liu wrote: > > @@ -359,9 +359,11 @@ static void mtk_spi_setup_dma_addr(struct spi_master *master, > > struct mtk_spi *mdata = spi_master_get_devdata(master); > > > > if (mdata->tx_sgl) > > - writel(cpu_to_le32(xfer->tx_dma), mdata->base + SPI_TX_SRC_REG); > > + writel((__force u32)cpu_to_le32(xfer->tx_dma), > > + mdata->base + SPI_TX_SRC_REG); > > if (mdata->rx_sgl) > > - writel(cpu_to_le32(xfer->rx_dma), mdata->base + SPI_RX_DST_REG); > > + writel((__force u32)cpu_to_le32(xfer->rx_dma), > > + mdata->base + SPI_RX_DST_REG); > > } > > > > This looks wrong: writel takes a CPU-endian argument, so the value returned > from cpu_to_le32() is not appropriate. > > The warning is correct, and you have to remove the cpu_to_le32() conversion > in order to get the driver to behave correctly when the kernel is built > as big-endian. Indeed, it's about time people started thinking more about the warnings and why we have coded things in the way we have. Look people. cpu_to_le32() takes a value in the CPU endian, and converts it to a little endian 32-bit number. See, the clue is in the name. All writel() implementations take a CPU number and write it in little endian format. Hence, writel() almost always uses cpu_to_le32() internally. Now think about what you're saying with "cpu_to_le32(cpu_to_le32())". It's utter rubbish, total crap. It's wrong no matter which way you look at it. We have le32_to_cpu() which does what it says on the tin. Same with be32_to_cpu() and cpu_to_be32(). Don't hack around this stuff with __force. If you're having to use __force to get rid of a warning here, you _ARE_ doing something wrong, no questions about that. __force in driver code is a definite sign that you are doing something wrong. Don't do it. Ask the question if you think you need it. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- 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/