linux-kernel - Re: [PATCH v1 1/6] clk: rockchip: Add supprot to limit input rate for fractional divider

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Message-ID: <3395739.hcHkC3K37q@diego>
Date:   Fri, 12 Apr 2019 14:21:22 +0200
From:   Heiko Stübner <heiko@...ech.de>
To:     Christoph Müllner 
        <christoph.muellner@...obroma-systems.com>
Cc:     Elaine Zhang <zhangqing@...k-chips.com>, mturquette@...libre.com,
        sboyd@...nel.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org,
        xxx@...k-chips.com, xf@...k-chips.com, huangtao@...k-chips.com,
        Finley Xiao <finley.xiao@...k-chips.com>
Subject: Re: [PATCH v1 1/6] clk: rockchip: Add supprot to limit input rate for fractional divider

Hi Christoph,

Am Freitag, 12. April 2019, 14:12:52 CEST schrieb Christoph Müllner:
> On 12.04.19 13:52, Heiko Stübner wrote:
> > Am Mittwoch, 3. April 2019, 11:42:24 CEST schrieb Elaine Zhang:
> >> From: Finley Xiao <finley.xiao@...k-chips.com>
> >>
> >> From Rockchips fractional divider usage, some clocks can be generated
> >> by fractional divider, but the input clock frequency of fractional
> >> divider should be less than a specified value.
> >>
> >> Signed-off-by: Finley Xiao <finley.xiao@...k-chips.com>
> >> Signed-off-by: Elaine Zhang <zhangqing@...k-chips.com>
> > 
> > can you tell me where these maximum input values come from?
> > 
> > I talked to Christoph from Theobroma (Cc'ed) last week and he mentioned
> > that they're using the fractional divider with a higher input frequency
> > to create a very specific frequency [some details are gone from my memory
> > though] they can't get otherwise.
> > 
> > So I really don't want to break their working setup by introducing barriers
> > that are not strictly necessary.
> > 
> > @Christoph: can you describe the bits from your fractional setup that
> > I've forgotten please?
> 
> We need to set the I2S0 clock to 24.56 MHz.
> 
> When restricting the input frequency to a maximum of 600 Mhz,
> we could use the integer divider to get 400 Mhz (dividing by 2).
> However, with the 400 Mhz as input to the frac divider,
> we run into the problem, that the maximum possible output frequency
> is 20 MHz (there is another restriction which states that the
> fraction input : output frequency must be >= 20).

just for clarification, what is the current input frequency you
already use sucessfully?


Heiko

> > 
> >> diff --git a/drivers/clk/rockchip/clk-px30.c b/drivers/clk/rockchip/clk-px30.c
> >> index 601a77f1af78..ccabce35580b 100644
> >> --- a/drivers/clk/rockchip/clk-px30.c
> >> +++ b/drivers/clk/rockchip/clk-px30.c
> >> @@ -21,6 +21,7 @@
> >>  #include "clk.h"
> >>  
> >>  #define PX30_GRF_SOC_STATUS0		0x480
> >> +#define PX30_FRAC_MAX_PRATE		600000000
> > 
> > 
> >> diff --git a/drivers/clk/rockchip/clk-rk3368.c b/drivers/clk/rockchip/clk-rk3368.c
> >> index 7c4d242f19c1..67c2da5e7d61 100644
> >> --- a/drivers/clk/rockchip/clk-rk3368.c
> >> +++ b/drivers/clk/rockchip/clk-rk3368.c
> >> @@ -20,6 +20,9 @@
> >>  #include "clk.h"
> >>  
> >>  #define RK3368_GRF_SOC_STATUS0	0x480
> >> +#define RK3368_I2S_FRAC_MAX_PRATE       600000000
> >> +#define RK3368_UART_FRAC_MAX_PRATE	600000000
> >> +#define RK3368_SPDIF_FRAC_MAX_PRATE	600000000
> > 
> >> diff --git a/drivers/clk/rockchip/clk-rk3399.c b/drivers/clk/rockchip/clk-rk3399.c
> >> index 5a628148f3f0..1d81382bd3e0 100644
> >> --- a/drivers/clk/rockchip/clk-rk3399.c
> >> +++ b/drivers/clk/rockchip/clk-rk3399.c
> >> @@ -21,6 +21,12 @@
> >>  #include <dt-bindings/clock/rk3399-cru.h>
> >>  #include "clk.h"
> >>  
> >> +#define RK3399_I2S_FRAC_MAX_PRATE       600000000
> >> +#define RK3399_UART_FRAC_MAX_PRATE	600000000
> >> +#define RK3399_SPDIF_FRAC_MAX_PRATE	600000000
> >> +#define RK3399_VOP_FRAC_MAX_PRATE	600000000
> >> +#define RK3399_WIFI_FRAC_MAX_PRATE	600000000
> >> +
> > 
> > 
> > 
>