linux-kernel - Re: [PATCH] serial: stm32: use mapbase instead of membase for DMA

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Date:   Mon, 26 Sep 2016 15:21:30 +0200
From:   Gerald Baeza <gerald.baeza@...com>
To:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Jiri Slaby <jslaby@...e.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] serial: stm32: use mapbase instead of membase for DMA

Hi Arnd

On 09/23/2016 09:38 PM, Arnd Bergmann wrote:
> Building this driver with a 64-bit dma_addr_t type results in
> a compiler warning:
>
> drivers/tty/serial/stm32-usart.c: In function 'stm32_of_dma_rx_probe':
> drivers/tty/serial/stm32-usart.c:746:20: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
> drivers/tty/serial/stm32-usart.c: In function 'stm32_of_dma_tx_probe':
> drivers/tty/serial/stm32-usart.c:818:20: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
>
> While the type conversion here is harmless, this hints at a different
> problem: we pass an __iomem pointer into a DMA engine, which expects
> a phys_addr_t. This happens to work because stm32 has no MMU and
> ioremap() is an identity mapping here, but it's still an incorrect
> API use. Using dma_addr_t is doubly wrong here, because that would
> be the result of dma_map_single() rather than the physical address.
>
> Using the mapbase instead fixes multiple issues:
>
> - the warning is gone
> - we don't go through ioremap in error
> - the cast is gone, making it use the correct resource_size_t/phys_addr_t
>   type in the process.
>
> Fixes: 3489187204eb ("serial: stm32: adding dma support")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/tty/serial/stm32-usart.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
> index 4d3001b77e7e..2adb678a863b 100644
> --- a/drivers/tty/serial/stm32-usart.c
> +++ b/drivers/tty/serial/stm32-usart.c
> @@ -743,7 +743,7 @@ static int stm32_of_dma_rx_probe(struct stm32_port *stm32port,
>
>  	/* Configure DMA channel */
>  	memset(&config, 0, sizeof(config));
> -	config.src_addr = (dma_addr_t)port->membase + ofs->rdr;
> +	config.src_addr = port->mapbase + ofs->rdr;
>  	config.src_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
>
>  	ret = dmaengine_slave_config(stm32port->rx_ch, &config);
> @@ -815,7 +815,7 @@ static int stm32_of_dma_tx_probe(struct stm32_port *stm32port,
>
>  	/* Configure DMA channel */
>  	memset(&config, 0, sizeof(config));
> -	config.dst_addr = (dma_addr_t)port->membase + ofs->tdr;
> +	config.dst_addr = port->mapbase + ofs->tdr;
>  	config.dst_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
>
>  	ret = dmaengine_slave_config(stm32port->tx_ch, &config);
>

Indeed, thanks for this finding.

Reviewed-by: Gerald Baeza <gerald.baeza@...com>