linux-kernel - Re: [PATCH] tty: serial: samsung: Correct clock selection logic

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Date:   Mon, 11 May 2020 12:40:18 -0700
From:   Jonathan Bakker <xc-racer2@...e.ca>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     kgene@...nel.org, gregkh@...uxfoundation.org, jslaby@...e.com,
        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] tty: serial: samsung: Correct clock selection logic

Hi Krzysztof,

On 2020-05-11 3:08 a.m., Krzysztof Kozlowski wrote:
> On Fri, May 08, 2020 at 06:34:33PM -0700, Jonathan Bakker wrote:
>> Some variants of the samsung tty driver can pick which clock
>> to use for their baud rate generation.  In the DT conversion,
>> a default clock was selected to be used if a specific one wasn't
>> assigned and then a comparison of which clock rate worked better
>> was done.  Unfortunately, the comparison was implemented in such
>> a way that only the default clock was ever actually compared.
>> Fix this by iterating through all possible clocks, except when a
>> specific clock has already been picked via clk_sel (which is
>> only possible via board files).
>>
>> Signed-off-by: Jonathan Bakker <xc-racer2@...e.ca>
>> ---
>>  drivers/tty/serial/samsung_tty.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
>> index 73f951d65b93..9d2b4be44209 100644
>> --- a/drivers/tty/serial/samsung_tty.c
>> +++ b/drivers/tty/serial/samsung_tty.c
>> @@ -1281,14 +1281,14 @@ static unsigned int s3c24xx_serial_getclk(struct s3c24xx_uart_port *ourport,
>>  	struct s3c24xx_uart_info *info = ourport->info;
>>  	struct clk *clk;
>>  	unsigned long rate;
>> -	unsigned int cnt, baud, quot, clk_sel, best_quot = 0;
>> +	unsigned int cnt, baud, quot, best_quot = 0;
>>  	char clkname[MAX_CLK_NAME_LENGTH];
>>  	int calc_deviation, deviation = (1 << 30) - 1;
>>  
>> -	clk_sel = (ourport->cfg->clk_sel) ? ourport->cfg->clk_sel :
>> -			ourport->info->def_clk_sel;
>>  	for (cnt = 0; cnt < info->num_clks; cnt++) {
>> -		if (!(clk_sel & (1 << cnt)))
>> +		/* Keep selected clock if provided */
> 
> Makes sense and good catch.
> 
> Reviewed-by: Krzysztof Kozlowski <krzk@...nel.org>
> 
> I wonder about the s3c24xx_serial_enable_baudclk() which has similar
> pattern - is there
> testing only def_clk_sel on purpose?

Yeah, I saw this instance too.  5086e0a409a0c ("tty: serial: samsung: Enable
baud clock during initialisation") introduced it, which was just to make sure
that some clock was enabled during initialization.  Since it doesn't appear to
be critical which clock it is, I left it as it was.

Thanks,
Jonathan

> 
> Best regards,
> Krzysztof
> 
>> +		if (ourport->cfg->clk_sel &&
>> +			!(ourport->cfg->clk_sel & (1 << cnt)))
>>  			continue;
>>  
>>  		sprintf(clkname, "clk_uart_baud%d", cnt);
>> -- 
>> 2.20.1
>>