[PATCH v1] drm/mipi_dbi: Use simple right shift instead of double negation

[Sean Paul]

On Thu, Oct 17, 2019 at 02:49:12PM +0300, Andy Shevchenko wrote:
> GCC complains about dubious bitwise OR operand:
> 
> drivers/gpu/drm/drm_mipi_dbi.c:1024:49: warning: dubious: x | !y
>   CC [M]  drivers/gpu/drm/drm_mipi_dbi.o
> 
> As long as buffer is consist of byte (u8) values, we may use
> simple right shift and satisfy compiler. It also reduces amount of
> operations needed.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
> ---
>  drivers/gpu/drm/drm_mipi_dbi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_mipi_dbi.c b/drivers/gpu/drm/drm_mipi_dbi.c
> index 1961f713aaab..445e88b1fc9a 100644
> --- a/drivers/gpu/drm/drm_mipi_dbi.c
> +++ b/drivers/gpu/drm/drm_mipi_dbi.c
> @@ -1021,7 +1021,7 @@ static int mipi_dbi_typec3_command_read(struct mipi_dbi *dbi, u8 *cmd,
>  		unsigned int i;
>  
>  		for (i = 0; i < len; i++)
> -			data[i] = (buf[i] << 1) | !!(buf[i + 1] & BIT(7));
> +			data[i] = (buf[i] << 1) | (buf[i + 1] >> 7);

You should probably have ((buf[i + 1] >> 7) & 0x1) to be super safe.

Do you know anything about this code? It seems like nothing is protecting us
from overrunning buf in this loop. We're just assuming that len < tr[1].len
through this loop and I'm not sure what's protecting us from looking where we
shouldn't.

Sean

>  	}
>  
>  	MIPI_DBI_DEBUG_COMMAND(*cmd, data, len);
> -- 
> 2.23.0
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS