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References: <20221214-uvc-status-alloc-v3-0-9a67616cc549@chromium.org>
 <20221214-uvc-status-alloc-v3-2-9a67616cc549@chromium.org> <20221215124837.GA19113@lst.de>
In-Reply-To: <20221215124837.GA19113@lst.de>
From:   Ricardo Ribalda <ribalda@chromium.org>
Date:   Tue, 20 Dec 2022 23:59:16 +0100
Message-ID: <CANiDSCsA-Rq4xSLUsAazv=ZjYDOAJNDqFjbNKDno1VzZmUrCDA@mail.gmail.com>
Subject: Re: [PATCH v3 2/2] media: uvcvideo: Do not alloc dev->status
To:     Christoph Hellwig <hch@lst.de>
Cc:     Max Staudt <mstaudt@chromium.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Ming Lei <tom.leiming@gmail.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Yunke Cao <yunkec@chromium.org>, linux-usb@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org
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Hi Christoph

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