linux-kernel - Re: [PATCH v2 3/7] tty: serial: samsung: constify s3c24xx_serial_drv

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Date:   Mon, 7 Mar 2022 09:40:26 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Jiri Slaby <jirislaby@...nel.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/7] tty: serial: samsung: constify
 s3c24xx_serial_drv_data

On 07/03/2022 09:33, Jiri Slaby wrote:
> On 07. 03. 22, 9:09, Krzysztof Kozlowski wrote:
>> The driver data (struct s3c24xx_serial_drv_data) is only used to
>> initialize the driver properly and is not modified.  Make it const.
> ...
>> @@ -2755,9 +2755,9 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
>>   	},
>>   	.fifosize = { 256, 64, 16, 16 },
>>   };
>> -#define S5PV210_SERIAL_DRV_DATA ((kernel_ulong_t)&s5pv210_serial_drv_data)
>> +#define S5PV210_SERIAL_DRV_DATA (&s5pv210_serial_drv_data)
>>   #else
>> -#define S5PV210_SERIAL_DRV_DATA	(kernel_ulong_t)NULL
>> +#define S5PV210_SERIAL_DRV_DATA	NULL
> 
> Yet, I still don't see why the switch from ulong->ptr happens in this 
> "constify it" patch?

All these defines S5PV210_SERIAL_DRV_DATA and so on are now const and
are assigned to of_device_id.data (s3c24xx_uart_dt_match). Before, these
were assigned with a cast:

static const struct of_device_id s3c24xx_uart_dt_match[] = {
	{ .compatible = "samsung,s5pv210-uart",

	.data = (void *)S5PV210_SERIAL_DRV_DATA }

but since the actual data structure is const, I want to drop the cast.
Casting const via (void *) might hide some possible issues, e.g. if
of_device_id.data becomes actually non-const. There is no particular
issue here, because of_device_id.data and S5PV210_SERIAL_DRV_DATA are
const. But also because they are both const now, I want to drop the cast
via void *.

When (void *) is dropped, the S5PV210_SERIAL_DRV_DATA cannot be
kernel_ulong_t:

../drivers/tty/serial/samsung_tty.c:2753:33: warning: initialization of
‘const void *’ from ‘long unsigned int’ makes pointer from integer
without a cast [-Wint-conversion]

 2753 | #define S5PV210_SERIAL_DRV_DATA (kernel_ulong_t)NULL



Best regards,
Krzysztof