Re: [PATCH 17/18] tty: serial: samsung: shrink port feature flags to u8

Tudor Ambarus
2024-01-19T09:44:01Z

On 1/19/24 09:07, Jiri Slaby wrote:

Hi, Jiri!

> On 19. 01. 24, 9:56, Tudor Ambarus wrote:
>>
>>
>> On 1/16/24 19:03, Sam Protsenko wrote:
>>> On Wed, Jan 10, 2024 at 4:25 AM Tudor Ambarus
>>> <tudor.ambarus@linaro.org> wrote:
>>>>
>>>> There's a single flag defined as of now. Shrink the feature flags to u8
>>>> and aim for a better memory footprint for ``struct s3c24xx_uart_info``.
>>>>
>>>> Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
>>>> ---
>>>>   drivers/tty/serial/samsung_tty.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/tty/serial/samsung_tty.c
>>>> b/drivers/tty/serial/samsung_tty.c
>>>> index 5df2bcebf9fb..598d9fe7a492 100644
>>>> --- a/drivers/tty/serial/samsung_tty.c
>>>> +++ b/drivers/tty/serial/samsung_tty.c
>>>> @@ -90,7 +90,7 @@ struct s3c24xx_uart_info {
>>>>
>>>>          /* uart port features */
>>>>
>>>> -       unsigned int            has_divslot:1;
>>>> +       u8                      has_divslot:1;
>>>
>>> But that's already a bit field. Why does it matter which type it is?
>>>
>>
>> If using unsigned int the bitfied is combined with the previous u8
>> fields, whereas if using u8 the bitfield will be independently defined.
>> So no benefit in terms of memory footprint, it's just a cosmetic change
>> to align the bitfield with the previous u8 fields. Allowing u32 for just
>> a bit can be misleading as one would ask itself where are the other
>> bits. Between a u32 bitfield and a bool a u8 bitfield seems like a good
>> compromise.
>
> Why? What's wrong with bool? bitfields have terrible semantics wrt
> atomic writes for example.
>

Bool occupies a byte and if more port features will ever be added we'll
occupy more bytes. Here's how the structure will look like with a bool:

struct s3c24xx_uart_info {
const char * name; /* 0 8 */
enum s3c24xx_port_type type; /* 8 4 */
unsigned int port_type; /* 12 4 */
unsigned int fifosize; /* 16 4 */
u32 rx_fifomask; /* 20 4 */
u32 rx_fifoshift; /* 24 4 */
u32 rx_fifofull; /* 28 4 */
u32 tx_fifomask; /* 32 4 */
u32 tx_fifoshift; /* 36 4 */
u32 tx_fifofull; /* 40 4 */
u32 clksel_mask; /* 44 4 */
u32 clksel_shift; /* 48 4 */
u32 ucon_mask; /* 52 4 */
u8 def_clk_sel; /* 56 1 */
u8 num_clks; /* 57 1 */
u8 iotype; /* 58 1 */
bool has_divslot; /* 59 1 */

/* size: 64, cachelines: 1, members: 17 */
/* padding: 4 */
};

What's your preference?
Thanks,
ta