Re: [PATCH 4/4] media: cxd2820r: use DIV_ROUND_CLOSEST_ULL()

[Alex Elder <elder@ieee.org>]

On 03/20/2015 12:27 PM, Javi Merino wrote:
> On Fri, Mar 20, 2015 at 01:51:36PM +0000, Alex Elder wrote:
>> On 03/20/2015 06:14 AM, Javi Merino wrote:
>>> Now that the kernel provides DIV_ROUND_CLOSEST_ULL(), drop the internal
>>> implementation and use the kernel one.
>>>
>>> Cc: Antti Palosaari <crope@iki.fi>
>>> Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
>>> Signed-off-by: Javi Merino <javi.merino@arm.com>
>>> ---
>>> I've only compile-tested it, I don't have the hardware to run it.
>>>
>>>  drivers/media/dvb-frontends/cxd2820r_c.c    | 2 +-
>>>  drivers/media/dvb-frontends/cxd2820r_core.c | 6 ------
>>>  drivers/media/dvb-frontends/cxd2820r_priv.h | 2 --
>>>  drivers/media/dvb-frontends/cxd2820r_t.c    | 2 +-
>>>  drivers/media/dvb-frontends/cxd2820r_t2.c   | 2 +-
>>>  5 files changed, 3 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/media/dvb-frontends/cxd2820r_c.c b/drivers/media/dvb-frontends/cxd2820r_c.c
>>> index 149fdca3fb44..72b0e2db3aab 100644
>>> --- a/drivers/media/dvb-frontends/cxd2820r_c.c
>>> +++ b/drivers/media/dvb-frontends/cxd2820r_c.c
>>> @@ -79,7 +79,7 @@ int cxd2820r_set_frontend_c(struct dvb_frontend *fe)
>>>  
>>>  	num = if_freq / 1000; /* Hz => kHz */
>>>  	num *= 0x4000;
>>> -	if_ctl = 0x4000 - cxd2820r_div_u64_round_closest(num, 41000);
>>> +	if_ctl = 0x4000 - DIV_ROUND_CLOSEST_ULL(num, 41000);
>>>  	buf[0] = (if_ctl >> 8) & 0x3f;
>>>  	buf[1] = (if_ctl >> 0) & 0xff;
>>>  
>>> diff --git a/drivers/media/dvb-frontends/cxd2820r_core.c b/drivers/media/dvb-frontends/cxd2820r_core.c
>>> index 422e84bbb008..490e090048ef 100644
>>> --- a/drivers/media/dvb-frontends/cxd2820r_core.c
>>> +++ b/drivers/media/dvb-frontends/cxd2820r_core.c
>>> @@ -244,12 +244,6 @@ error:
>>>  	return ret;
>>>  }
>>>  
>>> -/* 64 bit div with round closest, like DIV_ROUND_CLOSEST but 64 bit */
>>> -u32 cxd2820r_div_u64_round_closest(u64 dividend, u32 divisor)
>>> -{
>>> -	return div_u64(dividend + (divisor / 2), divisor);
>>> -}
>>
>> Technically, I'd say this has a bug, because the result
>> needs to be 64 bits wide or your results might be much
>> different from what might be desired.
>>
>> Practically though, I'm pretty sure all callers provide
>> values that ensure the result is valid.
> 
> All the callers are substituted in this patch so we can make sure that
> they are all correct.
> 
>> I only call attention because this patch changes the return
>> type of the function that gets called to do the calculation.
> 
> I'm not sure I follow.  Do you mean that this:
> 
> 	if_ctl = 0x4000 - DIV_ROUND_CLOSEST_ULL(num, 41000);
> 
> Should actually be:
> 
> 	if_ctl = 0x4000 - (u32)DIV_ROUND_CLOSEST_ULL(num, 41000);

I'm merely stating that your change includes changing the
type of the value returned (cxd2820r_div_u64_round_closest()
returns u32, DIV_ROUND_CLOSEST_ULL() returns U64).  So
in that respect, your patch is not completely trivial.

That said, as you point out (and I also did a quick check)
it does not seem to be an issue here.

The bug (if you call it that) existed before your patch, and
I am not suggesting you change anything.  Just hoping for
a verification that the different return type does not
lead to any problems.  I'm convinced.

					-Alex



> ?
> 
> if_ctl is a u16 so I don't think you gain anything by doing that.
> 
>>> -
>>>  static int cxd2820r_set_frontend(struct dvb_frontend *fe)
>>>  {
>>>  	struct cxd2820r_priv *priv = fe->demodulator_priv;
>>> diff --git a/drivers/media/dvb-frontends/cxd2820r_priv.h b/drivers/media/dvb-frontends/cxd2820r_priv.h
>>> index 7ff5f60c83e1..4b428959b16e 100644
>>> --- a/drivers/media/dvb-frontends/cxd2820r_priv.h
>>> +++ b/drivers/media/dvb-frontends/cxd2820r_priv.h
>>> @@ -64,8 +64,6 @@ int cxd2820r_wr_reg_mask(struct cxd2820r_priv *priv, u32 reg, u8 val,
>>>  int cxd2820r_wr_regs(struct cxd2820r_priv *priv, u32 reginfo, u8 *val,
>>>  	int len);
>>>  
>>> -u32 cxd2820r_div_u64_round_closest(u64 dividend, u32 divisor);
>>> -
>>>  int cxd2820r_wr_regs(struct cxd2820r_priv *priv, u32 reginfo, u8 *val,
>>>  	int len);
>>>  
>>> diff --git a/drivers/media/dvb-frontends/cxd2820r_t.c b/drivers/media/dvb-frontends/cxd2820r_t.c
>>> index 51401d036530..008cb2ac8480 100644
>>> --- a/drivers/media/dvb-frontends/cxd2820r_t.c
>>> +++ b/drivers/media/dvb-frontends/cxd2820r_t.c
>>> @@ -103,7 +103,7 @@ int cxd2820r_set_frontend_t(struct dvb_frontend *fe)
>>>  
>>>  	num = if_freq / 1000; /* Hz => kHz */
>>>  	num *= 0x1000000;
>>> -	if_ctl = cxd2820r_div_u64_round_closest(num, 41000);
>>> +	if_ctl = DIV_ROUND_CLOSEST_ULL(num, 41000);
> 
> if_ctl is a u32, so you get the same behavior that you were getting
> before: the downcasting of u64 to u32 happened in
> cxd2820r_div_u64_round_closest(), now it happens here.
> 
>>>  	buf[0] = ((if_ctl >> 16) & 0xff);
>>>  	buf[1] = ((if_ctl >>  8) & 0xff);
>>>  	buf[2] = ((if_ctl >>  0) & 0xff);
>>> diff --git a/drivers/media/dvb-frontends/cxd2820r_t2.c b/drivers/media/dvb-frontends/cxd2820r_t2.c
>>> index 9c0c4f42175c..35fe364c7182 100644
>>> --- a/drivers/media/dvb-frontends/cxd2820r_t2.c
>>> +++ b/drivers/media/dvb-frontends/cxd2820r_t2.c
>>> @@ -120,7 +120,7 @@ int cxd2820r_set_frontend_t2(struct dvb_frontend *fe)
>>>  
>>>  	num = if_freq / 1000; /* Hz => kHz */
>>>  	num *= 0x1000000;
>>> -	if_ctl = cxd2820r_div_u64_round_closest(num, 41000);
>>> +	if_ctl = DIV_ROUND_CLOSEST_ULL(num, 41000);
> 
> Same here if_ctl is a u32.
> 
> Cheers,
> Javi
>