linux-kernel - Re: [PATCH] spi: dw: use "smp_mb()" to avoid sending spi data error

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Message-ID: <20191227002239.GH27497@sirena.org.uk>
Date:   Fri, 27 Dec 2019 00:22:40 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Xinwei Kong <kong.kongxinwei@...ilicon.com>
Cc:     linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, fengsheng <fengsheng5@...wei.com>
Subject: Re: [PATCH] spi: dw: use "smp_mb()" to avoid sending spi data error

On Thu, Dec 26, 2019 at 05:21:28PM +0800, Xinwei Kong wrote:
> this patch will add memory barrier to ensure this "struct dw_spi *dws"
> to complete data setting before enabling this SPI hardware interrupt.
> eg:
> it will fix to this following low possibility error in testing environment
> which using SPI control to connect TPM Modules

> --- a/drivers/spi/spi-dw.c
> +++ b/drivers/spi/spi-dw.c
> @@ -288,6 +288,8 @@ static int dw_spi_transfer_one(struct spi_controller *master,
>  	dws->rx_end = dws->rx + transfer->len;
>  	dws->len = transfer->len;
>  
> +	smp_mb();
> +
>  	spi_enable_chip(dws, 0);

I'd be much more comfortable here if I understood what this was
supposed to be syncing - what exactly gets flushed here and why
is a memory barrier enough to ensure it's synced?  A comment in
the code would be especially good so anyone modifying the code
understands this in future.

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