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SubjectRe: [PATCH v3 3/3] serial: 8250_dw: add fractional divisor support
Hi Andy,

On Mon, 9 Jul 2018 16:32:41 +0800 Jisheng Zhang wrote:

> Hi Andy,
>
> On Mon, 9 Jul 2018 16:23:15 +0800 Jisheng Zhang wrote:
>
> > For Synopsys DesignWare 8250 uart which version >= 4.00a, there's a
> > valid divisor latch fraction register. The fractional divisor width is
> > 4bits ~ 6bits.
> >
> > Now the preparation is done, it's easy to add the feature support.
> > This patch firstly tries to get the fractional divisor width during
> > probe, then setups dw specific get_divisor() and set_divisor() hook.
> >
> > Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
> > ---
> > drivers/tty/serial/8250/8250_dw.c | 45 +++++++++++++++++++++++++++++++
> > 1 file changed, 45 insertions(+)
> >
> > diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> > index fa8a00e8c9c6..e90c3d229f00 100644
> > --- a/drivers/tty/serial/8250/8250_dw.c
> > +++ b/drivers/tty/serial/8250/8250_dw.c
> > @@ -31,6 +31,7 @@
> >
> > /* Offsets for the DesignWare specific registers */
> > #define DW_UART_USR 0x1f /* UART Status Register */
> > +#define DW_UART_DLF 0xc0 /* Divisor Latch Fraction Register */
> > #define DW_UART_CPR 0xf4 /* Component Parameter Register */
> > #define DW_UART_UCV 0xf8 /* UART Component Version */
> >
> > @@ -55,6 +56,7 @@
> >
> > struct dw8250_data {
> > u8 usr_reg;
> > + u8 dlf_size;
> > int line;
> > int msr_mask_on;
> > int msr_mask_off;
> > @@ -366,6 +368,37 @@ static bool dw8250_idma_filter(struct dma_chan *chan, void *param)
> > return param == chan->device->dev->parent;
> > }
> >
> > +/*
> > + * divisor = clk / (16 * baud) = div(I) + div(F)
> > + * "I" means integer, "F" means fractional
> > + *
> > + * 2^dlf_size * clk / (16 * baud) = 2^dlf_size * (div(I) + div(F))
> > + * so, (clk << (dlf_siz - 4)) / baud = (div(I) + div(F)) << dlf_size
> > + *
> > + * let quot = DIV_ROUND_CLOSEST(clk << (dlf_size - 4), baud), we get
> > + * div(I) = quot >> dlf_size
> > + * div(F) = quot & dlf_mask, where dlf_mask = GENMASK(dlf_size - 1, 0)
> > + */
> > +static unsigned int dw8250_get_divisor(struct uart_port *p,
> > + unsigned int baud,
> > + unsigned int *frac)
> > +{
> > + unsigned int quot;
> > + struct dw8250_data *d = p->private_data;
> > +
> > + quot = DIV_ROUND_CLOSEST(p->uartclk << (d->dlf_size - 4), baud);
> > + *frac = quot & GENMASK(d->dlf_size - 1, 0);
> > +
> > + return quot >> d->dlf_size;
>
> After more consideration, I sent out this version for the following two
> points:
>
> 1. the max frac divisor width is 6bits now, but we dunno whether future IP
> extends it or not. This patch version can still support > 6bits width except
> the overflow concern. But fixing overflow(if there is) is as simple as using\
> the DIV_ROUND_CLOSEST_ULL macro.
>
> 2. this version makes use of well implemented GENMASK to get the dlf_mask,
> the micro is well understood, I think. so the code is simplified as well.
>
> 3. the magic "4" is explained in the comments, I hope it could help the
> code.

I agree with your "making the code simple" concern. So I have a consideration
again. I think I made the code complex unnecessarily, the simplest get_divisor code
could be:

get_divisor()
{
unsigned int quot, rem;

quot = clk / (16 * baud);
rem = clk % (16 *baud);
*frac = DIV_ROUND_CLOSEST(rem << dlf_size, 16*baud);

return quot;
}

Compared with previous version, this one adds one more div instruction,
but remove several "shift, and" instructions, the performance isn't that bad.
From another side, even this version gets a trivial performance regression,
the get_divisor() doesn't sit on the hot code path. Making the code simpler is
more important.

I will send a new version.

Thanks

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