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

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Message-ID: <842d36c7-9452-431f-95c4-ff114484d201@linaro.org>
Date: Fri, 19 Jan 2024 09:43:25 +0000
From: Tudor Ambarus <tudor.ambarus@...aro.org>
To: Jiri Slaby <jirislaby@...nel.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 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@...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.
> 

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