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

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Message-ID: <CANiDSCsA-Rq4xSLUsAazv=ZjYDOAJNDqFjbNKDno1VzZmUrCDA@mail.gmail.com>
Date:   Tue, 20 Dec 2022 23:59:16 +0100
From:   Ricardo Ribalda <ribalda@...omium.org>
To:     Christoph Hellwig <hch@....de>
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>, 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

Hi Christoph

On Thu, 15 Dec 2022 at 13:48, Christoph Hellwig <hch@....de> wrote:
>
> 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.

Thanks a lot for the explanation. I agree, we should keep the
allocation as it is :). Sorry for the noise

Best regards!



-- 
Ricardo Ribalda