linux-kernel - Re: [PATCH v3 2/2] media: uvcvideo: Do not alloc dev->status

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Message-ID: <20221215124837.GA19113@lst.de>
Date:   Thu, 15 Dec 2022 13:48:37 +0100
From:   Christoph Hellwig <hch@....de>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Max Staudt <mstaudt@...omium.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Ming Lei <tom.leiming@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Yunke Cao <yunkec@...omium.org>,
        Christoph Hellwig <hch@....de>, linux-usb@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH v3 2/2] media: uvcvideo: Do not alloc dev->status

On Thu, Dec 15, 2022 at 11:57:19AM +0100, Ricardo Ribalda wrote:
> +	/*
> +	 * Ensure that status is aligned, making it safe to use with
> +	 * non-coherent DMA.
> +	 */
> +	struct uvc_status status __aligned(ARCH_KMALLOC_MINALIGN);

This should be ARCH_DMA_MINALIGN, not ARCH_KMALLOC_MINALIGN.

Note that without an __aligned tag on the next member as well, those
next members might get cache corrupted.

>  
>  	struct input_dev *input;

.. and without also aligning the next member you'll might still
corrupt everything adter the DMAed member.

That's the reason why I generall advocate against playing these
__aligned games as they can easily go wrong if someone reorders
the structure.