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

From: Gerald Baeza
Date: Mon Sep 26 2016 - 09:22:30 EST

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@xxxxxxxx>
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@xxxxxx>