Re: [PATCH 2/3] usb: mtu3: add memory barrier before set GPD's HWO - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Chunfeng Yun <chunfeng.yun@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>,
	linux-usb@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org,
	Eddie Hung <eddie.hung@mediatek.com>,
	Yuwen Ng <yuwen.ng@mediatek.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH 2/3] usb: mtu3: add memory barrier before set GPD's HWO
Date: Mon, 13 Dec 2021 15:18:11 +0100	[thread overview]
Message-ID: <YbdWI5PD3e6uFz8U@kroah.com> (raw)
In-Reply-To: <20211209031424.17842-2-chunfeng.yun@mediatek.com>

On Thu, Dec 09, 2021 at 11:14:23AM +0800, Chunfeng Yun wrote:
> There is a seldom issue that the controller access invalid address
> and trigger devapc or emimpu violation. That is due to memory access
> is out of order and cause gpd data is not correct.
> Make sure GPD is fully written before giving it to HW by setting its
> HWO.
> 
> Fixes: 48e0d3735aa5 ("usb: mtu3: supports new QMU format")
> Cc: stable@vger.kernel.org
> Reported-by: Eddie Hung <eddie.hung@mediatek.com>
> Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
> ---
>  drivers/usb/mtu3/mtu3_qmu.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/mtu3/mtu3_qmu.c b/drivers/usb/mtu3/mtu3_qmu.c
> index 3f414f91b589..34bb5ac67efe 100644
> --- a/drivers/usb/mtu3/mtu3_qmu.c
> +++ b/drivers/usb/mtu3/mtu3_qmu.c
> @@ -273,6 +273,8 @@ static int mtu3_prepare_tx_gpd(struct mtu3_ep *mep, struct mtu3_request *mreq)
>  			gpd->dw3_info |= cpu_to_le32(GPD_EXT_FLAG_ZLP);
>  	}
>  
> +	/* make sure GPD is fully written before giving it to HW */
> +	mb();

So this means you are using mmio for this structure?  If so, shouldn't
you be using normal io memory read/write calls as well and not just
"raw" pointers like this:

>  	gpd->dw0_info |= cpu_to_le32(GPD_FLAGS_IOC | GPD_FLAGS_HWO);

Are you sure this is ok?

Sprinkling around mb() calls is almost never the correct solution.

If you need to ensure that a write succeeds, shouldn't you do a read
from it afterward?  Many busses require this, doesn't yours?

>  
>  	mreq->gpd = gpd;
> @@ -306,6 +308,8 @@ static int mtu3_prepare_rx_gpd(struct mtu3_ep *mep, struct mtu3_request *mreq)
>  	gpd->next_gpd = cpu_to_le32(lower_32_bits(enq_dma));
>  	ext_addr |= GPD_EXT_NGP(mtu, upper_32_bits(enq_dma));
>  	gpd->dw3_info = cpu_to_le32(ext_addr);
> +	/* make sure GPD is fully written before giving it to HW */
> +	mb();

Again, mb(); does not ensure that memory-mapped i/o actually hits the
HW.  Or if it does on your platform, how?

mb() is a compiler barrier, not a memory write to a bus barrier.  Please
read Documentation/memory-barriers.txt for more details.

thanks,

greg k-h

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