From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 785202765D7; Sun, 13 Sep 2026 03:00:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789268401; cv=none; b=Q60IZmctLbRolQsNbEKDK5af3UH5YZTEQHJQymF/myZKxq07RIlLtTKhYY34rasHDtDX+5nT5WcD8R9AfGrgUjtkyQr53JPoVq41HFwKQv0k60ExwxppyMRZzkAk1iL/OkV/Mgi4vlm3ehJMtYyafGmRXpzQ7QBb5WshZ2TEN9M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789268401; c=relaxed/simple; bh=nebcldvjasnNIaE8xLDrcml/O7iGT6X4/ykys5IcfuQ=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=H6QuXWcaCbdCYL3vRYAec+nBhqyCHD/vCFEJoArQlSSbH+OJ4WLBuYQSuaneSFsWGNO+3LBe91+rCqWpiWJd8SaIKTGJKyUDE7rSoPqIF9FZ5s5a4jkdANKoWesK3EgPVL8opNarklnUeR3FUpK3Ti1vFQDyqw8t/cD+z9HeiRY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AnNpgQFr; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AnNpgQFr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C9AD01F000FF; Sun, 13 Sep 2026 02:59:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789268400; bh=45E+0rPqi4Q5EA4SZ6KErOV5kKZDAlhBMi5yePEo1ZU=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=AnNpgQFr54Usir0fOZgHzFHteOa78dYAJSQZGu+zwZcxDdPwy5OMReLp+WpoxnvhA VRElaRBM8vEG61jZM6hw4RduYsZAx9ZjvnSjsKLzlhhY48e2iTkOwtO67eFktwM+Q+ xg+9ynaGLsdQoQDlktsi7mZoNE3X1K0HtNDgzD7IKfosSI29OtWeIpkJLowmkv7gLo l+SsEt7Qaw90TR2yl2wUpK7KNNJ+7+Ix/9hR4FS8xrrztqDUDl3HvHO2SyXxNH/zPl pcY2a8wmgfudp8Bg8/oPKIeVpLV9A4dENwIqyq3SYzejppkcHgM0DRqPC3zXNbDd+3 5EYFRc7Eg5c0Q== Date: Sun, 13 Sep 2026 03:59:57 +0100 From: Jonathan Cameron To: Abdelnasser Hussein Cc: gregkh@linuxfoundation.org, nuno.sa@analog.com, Michael.Hennerich@analog.com, dlechner@baylibre.com, andy@kernel.org, linux@analog.com, linux-iio@vger.kernel.org, linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org, joshua.crofts1@gmail.com Subject: Re: [PATCH v6 3/3] staging: iio: adc: ad7816: Use DMA-safe buffer for SPI read Message-ID: <20260913035957.7eb0dd2b@jic23-hlaptop> In-Reply-To: <20260912132517.55686-4-abdelnasserhussein11@gmail.com> References: <20260912132517.55686-1-abdelnasserhussein11@gmail.com> <20260912132517.55686-4-abdelnasserhussein11@gmail.com> X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Sat, 12 Sep 2026 16:25:17 +0300 Abdelnasser Hussein wrote: > Stack buffers are unsafe for DMA with VMAP_STACK enabled because > they are not physically contiguous and can share cache lines. This > causes data corruption. This one has come up a few times as a false or at least misleading explanation. It really doesn't have anything to do with VMAP_STACK as that only a most deals with pages and cachelines which matter here are much smaller. Any stack variable is a problem simply because we don't control the data layout and pretty much anything else can end up in the same cache line. > > Fix this by using a dedicated rx_buf aligned with IIO_DMA_MINALIGN > in the device state structure to ensure cache coherency. This also > corrects the size argument in spi_read() to match the new buffer. Make sure you understand why this alignment marking works to ensure the buffer is in a cache line on it's own. However, don't do this for a single use small buffer. Use spi_write_then_read() (with 0 sized write) but also make sure you understand why that works. Jonathan > > Fixes: 7024425db64a ("staging: iio: adc: new driver for AD7816 devices") > Signed-off-by: Abdelnasser Hussein > --- > drivers/staging/iio/adc/ad7816.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c > index acf26c5a267a..2fd2d6796603 100644 > --- a/drivers/staging/iio/adc/ad7816.c > +++ b/drivers/staging/iio/adc/ad7816.c > @@ -53,6 +53,7 @@ struct ad7816_chip_info { > u8 channel_id; /* 0 always be temperature */ > u8 mode; > struct mutex lock; /* protect device state during SPI transfers */ > + __be16 rx_buf __aligned(IIO_DMA_MINALIGN); > }; > > enum ad7816_type { > @@ -68,7 +69,6 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data) > { > struct spi_device *spi_dev = chip->spi_dev; > int ret; > - __be16 buf; > > guard(mutex)(&chip->lock); > > @@ -96,13 +96,13 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data) > > gpiod_set_value(chip->rdwr_pin, 0); > gpiod_set_value(chip->rdwr_pin, 1); > - ret = spi_read(spi_dev, &buf, sizeof(*data)); > + ret = spi_read(spi_dev, &chip->rx_buf, sizeof(chip->rx_buf)); > if (ret < 0) { > dev_err(&spi_dev->dev, "SPI data read error\n"); > return ret; > } > > - *data = be16_to_cpu(buf); > + *data = be16_to_cpu(chip->rx_buf); > > return ret; > }