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

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Message-ID: <8f3f85d0-866e-4e5a-8177-05c26c08b278@kernel.org>
Date: Fri, 19 Jan 2024 10:07:01 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: Tudor Ambarus <tudor.ambarus@...aro.org>,
 Sam Protsenko <semen.protsenko@...aro.org>
Cc: krzysztof.kozlowski@...aro.org, alim.akhtar@...sung.com,
 gregkh@...uxfoundation.org, linux-arm-kernel@...ts.infradead.org,
 linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-serial@...r.kernel.org, andre.draszik@...aro.org,
 peter.griffin@...aro.org, kernel-team@...roid.com, willmcvicker@...gle.com
Subject: Re: [PATCH 17/18] tty: serial: samsung: shrink port feature flags to
 u8

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@...aro.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@...aro.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.

-- 
js
suse labs