[PATCH] mmc: mtk-sd: increase the minimum bus frequency

[Weijie Gao <weijie.gao@mediatek.com>]

On Fri, 2021-03-05 at 11:44 +0900, Jaehoon Chung wrote:
> Dear Waijie,
> 
> On 3/5/21 11:30 AM, Weijie Gao wrote:
> > With a 48MHz input clock, the lowest bus frequency can be as low as
> > 48000000 / (4 * 4095) = 2930Hz. Such an extremely low frequency will cause
> > the mmc framework take seconds to finish the initialization.
> > 
> > Limiting the minimum bus frequency to a slightly higher value can solve the
> > issue without any side effects.
> > 
> > Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
> > ---
> >  drivers/mmc/mtk-sd.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/mmc/mtk-sd.c b/drivers/mmc/mtk-sd.c
> > index 3b9c12266a..1e1726ef31 100644
> > --- a/drivers/mmc/mtk-sd.c
> > +++ b/drivers/mmc/mtk-sd.c
> > @@ -232,6 +232,8 @@
> >  
> >  #define SCLK_CYCLES_SHIFT		20
> >  
> > +#define MIN_BUS_CLK			260000
> 
> Is there any reason to define 260KHz? 
> According to specification, clock frequency should be chosen one of {100000, 200000, 300000, 400000}.
> 
> How about defined one of them?

This value is picked up from an old version driver (named msdc) [1].
Of course it's completely OK to change it to a standard value from the
specification.

[1]
https://github.com/MediaTek-Labs/linkit-smart-7688-uboot/blob/master/drivers/msdc/msdc.h#L94

> 
> Best Regards,
> Jaehoon Chung
> 
> > +
> >  #define CMD_INTS_MASK	\
> >  	(MSDC_INT_CMDRDY | MSDC_INT_RSPCRCERR | MSDC_INT_CMDTMO)
> >  
> > @@ -1639,6 +1641,9 @@ static int msdc_drv_probe(struct udevice *dev)
> >  	else
> >  		cfg->f_min = host->src_clk_freq / (4 * 4095);
> >  
> > +	if (cfg->f_min < MIN_BUS_CLK)
> > +		cfg->f_min = MIN_BUS_CLK;
> > +
> >  	cfg->f_max = host->src_clk_freq;
> >  
> >  	cfg->b_max = 1024;
> > 
>