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

Sam Protsenko
2024-01-17T16:32:15Z

On Wed, Jan 17, 2024 at 10:26 AM Tudor Ambarus <tudor.ambarus@linaro.org> wrote:
>
>
>
> On 1/16/24 19:09, Sam Protsenko wrote:
> > On Wed, Jan 10, 2024 at 4:25 AM Tudor Ambarus <tudor.ambarus@linaro.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.
>

Oh yeah, I actually like the cachelines part. Please add that bit to
the commit message if you don't mind.

> 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.
>

Gotcha. Maybe also add that reasoning to the commit message. Just a
thought. With above comments addressed, feel free to add:

Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>

>
> >> 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@linaro.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;
> >>