Re: [PATCH] spi: imx: correct handling of MXC_CSPIRXDATA value endianness

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Simon,

On 26.05.23 16:03, Simon Horman wrote:
> The existing code seems be intended to handle MXC_CSPIRXDATA values
> which are in big endian. However, it seems that this is only
> handled correctly in the case where the host is little endian.
> 
> First, consider the read case.
> 
> 	u32 val = be32_to_cpu(readl(...))
> 
> readl() will read a 32bit value and return it after applying le32_to_cpu().
> On a little endian host le32_to_cpu() is a noop. So the raw value is
> returned. This is then converted from big endian to host byte-order -
> the value is byte-swapped - using be32_to_cpu(). Assuming the raw value
> is big endian a host byte-order value is obtained. This seems correct.
> 
> However, on a big endian system, le32_to_cpu() will perform a byte-swap,
> while be32_to_cpu() is a noop. Assuming the underlying value is big
> endian this is incorrect, because it should not be byte-swapped to
> obtain the value in host byte-order - big endian.
> 
> Surveying other kernel code it seems that a correct approach is:
> 
> 	 be32_to_cpu((__force __be32)__raw_readl(...))

How about using ioread32be?

> 
> Here __raw_readl() does returns the raw value, without any calls
> that can alter the byte-order. And be32_to_cpu() is called to correctly
> either a) swaps the byte-order on a little endian host or b) does not
> swap the byte-order on a big endian host.
> 
> Second, let us consider the write case:
> 
> 	val = cpu_to_be32(val);
> 	...
> 	writel(val, ...);
> 
> writel() will write the 32bit value, passed as big endian, after applying
> cpu_to_le32(). On a little endian system cpu_to_le32() is a noop and
> thus the big endian value is stored. This seems correct.
> 
> However, on a big endian system cpu_to_le32() will byte-swap the value.
> That is, converting it from big endian to little endian. The little
> endian value is then stored. This seems incorrect.
> 
> Surveying other kernel code it seems that a correct approach is:
> 
> 	__raw_writel((__force u32)cpu_to_be32(val), ...);
> 
> __raw_writel() will write the value with out applying any endian
> conversion functions. Thus the big endian value is written.
> This seems correct for the case at hand.
> 
> This patch adopts the __raw_readl() and __raw_writel() approaches described
> above. It also avoids the following, which stores a big endian value in
> a host byte-order variable.
> 
> 	u32 val;
> 	...
> 	val = cpu_to_be32(val);
> 
> Reported by Sparse as:
> 
>   .../spi-imx.c:410:19: warning: cast to restricted __be32
>   .../spi-imx.c:439:21: warning: incorrect type in assignment (different base types)
>   .../spi-imx.c:439:21:    expected unsigned int [addressable] [usertype] val
>   .../spi-imx.c:439:21:    got restricted __be32 [usertype]
> 
> Compile tested only.
> 
> Fixes: 71abd29057cb ("spi: imx: Add support for SPI Slave mode")
> Signed-off-by: Simon Horman <horms@kernel.org>
> ---
>  drivers/spi/spi-imx.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/spi/spi-imx.c b/drivers/spi/spi-imx.c
> index bd6ddb142b13..99c1f76e073d 100644
> --- a/drivers/spi/spi-imx.c
> +++ b/drivers/spi/spi-imx.c
> @@ -406,7 +406,10 @@ static void spi_imx_buf_tx_swap(struct spi_imx_data *spi_imx)
>  
>  static void mx53_ecspi_rx_slave(struct spi_imx_data *spi_imx)
>  {
> -	u32 val = be32_to_cpu(readl(spi_imx->base + MXC_CSPIRXDATA));
> +	u32 val;
> +
> +	val = be32_to_cpu((__force __be32)__raw_readl(spi_imx->base +
> +						      MXC_CSPIRXDATA));
>  
>  	if (spi_imx->rx_buf) {
>  		int n_bytes = spi_imx->slave_burst % sizeof(val);
> @@ -435,13 +438,13 @@ static void mx53_ecspi_tx_slave(struct spi_imx_data *spi_imx)
>  	if (spi_imx->tx_buf) {
>  		memcpy(((u8 *)&val) + sizeof(val) - n_bytes,
>  		       spi_imx->tx_buf, n_bytes);
> -		val = cpu_to_be32(val);
>  		spi_imx->tx_buf += n_bytes;
>  	}
>  
>  	spi_imx->count -= n_bytes;
>  
> -	writel(val, spi_imx->base + MXC_CSPITXDATA);
> +	__raw_writel((__force u32)cpu_to_be32(val),
> +		     spi_imx->base + MXC_CSPITXDATA);
>  }
>  
>  /* MX51 eCSPI */
> 
> 
> 

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