Re: [PATCH] nvmem: core: Fix msb clearing bits

[Mukesh Ojha]

On 1/31/2020 6:13 PM, Shadab Naseem wrote:
> When clearing the msb bits of the resultant buffer, it is
> masked with the modulo of the number of bits needed with
> respect to the BITS_PER_BYTE. To mask out the buffer,
> it is passed though GENMASK of the remainder of the bits
> starting from zeroth bit. This case is valid if nbits is not
> a multiple of BITS_PER_BYTE and you are actually creating
> a GENMASK. If the nbits coming is a multiple of BITS_PER_BYTE,
> it would pass a negative value to the high bit number of
> GENMASK with zero as the lower bit number.
>
> As per the definition of the GENMASK, the higher bit number (h)
> is right operand for bitwise right shift. If the value of the
> right operand is negative or is greater or equal to the number
> of bits in the promoted left operand, the behavior is undefined.
> So passing a negative value to GENMASK could behave differently
> across architecture, specifically between 64 and 32 bit.
> Also, on passing the hard-coded negative value as GENMASK(-1, 0)
> is giving compiler warning for shift-count-overflow.
> Hence making a check for clearing the MSB if the nbits are not
> a multiple of BITS_PER_BYTE.
>
> Signed-off-by: Shadab Naseem <snaseem@xxxxxxxxxxxxxx>
> ---
>   drivers/nvmem/core.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> index 1e4a798..23c1547 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -926,7 +926,8 @@ static void nvmem_shift_read_buffer_in_place(struct nvmem_cell *cell, void *buf)
>   		*p-- = 0;
>   
>   	/* clear msb bits if any leftover in the last byte */
> -	*p &= GENMASK((cell->nbits%BITS_PER_BYTE) - 1, 0);
> +	if (cell->nbits%BITS_PER_BYTE)
> +		*p &= GENMASK((cell->nbits%BITS_PER_BYTE) - 1, 0);


LGTM..

Reviewed-by: mojha@xxxxxxxxxxxxxx


Thanks
Mukesh

>   }
>   
>   static int __nvmem_cell_read(struct nvmem_device *nvmem,