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

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Date: Fri, 19 Jan 2024 10:54:26 +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

Hi,

On 19. 01. 24, 10:43, Tudor Ambarus wrote:
>>> 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?

bool :).

-- 
js
suse labs