linux-kernel - Re: [PATCH 16/18] tty: serial: samsung: shrink the clock selection to 8 clocks

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Date: Wed, 17 Jan 2024 16:26:46 +0000
From: Tudor Ambarus <tudor.ambarus@...aro.org>
To: Sam Protsenko <semen.protsenko@...aro.org>
Cc: krzysztof.kozlowski@...aro.org, alim.akhtar@...sung.com,
 gregkh@...uxfoundation.org, jirislaby@...nel.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 16/18] tty: serial: samsung: shrink the clock selection to
 8 clocks



On 1/16/24 19:09, Sam Protsenko wrote:
> On Wed, Jan 10, 2024 at 4:25 AM Tudor Ambarus <tudor.ambarus@...aro.org> wrote:
>>
>> <linux/serial_s3c.h> provides a clock selection pool of maximum 4 clocks.
> 
> Then maybe it makes sense to turn those two field into 4-bit bit
> fields? More importantly, what particular problem does this patch
> solve, is this optimization really needed, and why? I'm not saying
> it's not needed, just that commit message might've been more verbose
> about this.
> 

I guess I could have been more verbose in the phrase from below and said
that for arm64 ``struct s3c24xx_uart_info`` spans through 2 cachelines
and contains 2 holes, and with a bit of love it can fit a single
cacheline with no holes. The end goal is to reduce the memory footprint
of that struct.

I chose u8 and allowed a max of 8 clocks simple because it's large
enough to allow more clocks than are supported by the driver now, and
not too big to cause spanning of the structure through 2 cachelines.


>> Update the driver to consider a pool selection of maximum 8 clocks. The
>> final scope is to reduce the memory footprint of
>> ``struct s3c24xx_uart_info``.
>>
>> Signed-off-by: Tudor Ambarus <tudor.ambarus@...aro.org>
>> ---
>>  drivers/tty/serial/samsung_tty.c | 22 +++++++++++-----------
>>  1 file changed, 11 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
>> index 436739cf9225..5df2bcebf9fb 100644
>> --- a/drivers/tty/serial/samsung_tty.c
>> +++ b/drivers/tty/serial/samsung_tty.c
>> @@ -81,11 +81,11 @@ struct s3c24xx_uart_info {
>>         unsigned long           tx_fifomask;
>>         unsigned long           tx_fifoshift;
>>         unsigned long           tx_fifofull;
>> -       unsigned int            def_clk_sel;
>> -       unsigned long           num_clks;
>>         unsigned long           clksel_mask;
>>         unsigned long           clksel_shift;
>>         unsigned long           ucon_mask;
>> +       u8                      def_clk_sel;
>> +       u8                      num_clks;
>>         u8                      iotype;
>>