lkml.org 
[lkml]   [2020]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 2/2] clocksource: Add driver for the Ingenic JZ47xx OST
Hi Thomas,


Le jeu., janv. 9, 2020 at 15:28, Thomas Gleixner <tglx@linutronix.de> a
écrit :
> Paul Cercueil <paul@crapouillou.net> writes:
>> +static u64 notrace ingenic_ost_clocksource_read64(struct
>> clocksource *cs)
>> +{
>> + u32 val1, val2;
>> + u64 count, recount;
>> + s64 diff;
>> +
>> + /*
>> + * The buffering of the upper 32 bits of the timer prevents wrong
>> + * results from the bottom 32 bits overflowing due to the timer
>> ticking
>> + * along. However, it does not prevent wrong results from
>> simultaneous
>> + * reads of the timer, which could reset the buffer mid-read.
>> + * Since this kind of wrong read can happen only when the bottom
>> bits
>> + * overflow, there will be minutes between wrong reads, so if we
>> read
>> + * twice in succession, at least one of the reads will be correct.
>> + */
>> +
>> + /* Bypass the regmap here as we must return as soon as possible */
>
> I have a hard time to understand this comment. "Bypass the regmap ..."
> and then use a regmap function?

Ah, sorry, it's a leftover from a previous version of the patch. It
used to bypass the regmap in order to complete as fast as possible.

>
>> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTL, &val1);
>> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTHBUF, &val2);
>> + count = (u64)val1 | (u64)val2 << 32;
>> +
>> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTL, &val1);
>> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTHBUF, &val2);
>> + recount = (u64)val1 | (u64)val2 << 32;
>> +
>> + /*
>> + * A wrong read will produce a result that is 1<<32 too high: the
>> bottom
>> + * part from before overflow and the upper part from after
>> overflow.
>> + * Therefore, the lower value of the two reads is the correct
>> value.
>> + */
>> +
>> + diff = (s64)(recount - count);
>> + if (unlikely(diff < 0))
>> + count = recount;
>
> Is this really the right approach here? What is the 64bit readout
> buying
> you?
>
> The timekeeping code can handle a 32bit counter perfectly fine and the
> only advantage you get is that your maximum possible idle time will be
> longer with a 64bit counter.
>
> But is that really worth the overhead of four MMIO reads versus one
> in a
> hotpath?

The timer is 64-bit so I thought it made sense to register it as such.
Using it as just a 32-bit counter sounds better indeed.

Thanks,
-Paul



\
 
 \ /
  Last update: 2020-01-09 19:27    [W:0.031 / U:2.968 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site