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SubjectRe: [PATCH 1/5] power: RFC: introduce a new power API
On Mon, 17 Dec 2007 08:51:23 +0300
Anton Vorontsov <> wrote:

> Hello Andres, David,
> Firstly, Andres, thank you for the efforts.
> I quite foreseen what exactly you had in mind when we were
> discussing the idea. With patches it's indeed easier to show
> flaws of this approach.
> On Sun, Dec 16, 2007 at 09:36:24PM -0500, David Woodhouse wrote:
> > On Sun, 2007-12-16 at 21:24 -0500, Andres Salomon wrote:
> > > This API has the power_supply drivers device their own device_attribute
> > > list; I find this to be a lot more flexible and cleaner.
> I don't see how this is more flexible and cleaner. See below.
> > > For example,
> > > rather than having a function with a huge switch statement (as olpc_battery
> > > currently has), we have separate callback functions.
> Is this an improvement? Look into ds2760_battery.c. I scared to
> imagine what it will look like after conversion.

Why? It would not look bad after conversion. Basically:

static ssize_t ds2760_battery_get_status(struct device *dev,
struct device_attribute *attr, char *buf)
struct ds2760_device_info *di = to_ds2760_device_info(psy);
return power_supply_status_str(di->charge_status, buf);
static ssize_t ds2760_battery_get_voltage_now(struct device *dev,
struct device_attribute *attr, char *buf)
struct ds2760_device_info *di = to_ds2760_device_info(psy);
return sprintf(buf, "%d\n", di->voltage_uV);
} so on.

If I wanted to get really clever, I could do:

#define DS2760_CALLBACK(name, fmt, var) \
static ssize_t ds2760_battery_get_##name(struct device *dev, \
struct device_attribute *attr, char *buf) \
{ \
struct ds2760_device_info *di = to_ds2760_device_info(psy); \
ds2760_battery_read_status(di); \
return sprintf(buf, fmt, var); \

DS2760_CALLBACK(voltage_now, "%d\n", di->voltage_uV)
DS2760_CALLBACK(current_now, "%d\n", di->current_uA)

etc.. but, I'm not trying to compress lines of code, I'm trying
to ensure things are readable.

> As for olpc's "huge switch statement", it could be split into
> functions _without_ drastic changes to PSY class. As the bonus,
> you will get _inlining_ of these functions by gcc, because
> there will be just single user of these functions. With
> "exported-via-pointers" functions you can't do that.
> You have tons of similar functions with similar functionality, that
> only differs by the data source. That scheme was in the early PSY
> class I posted here this summer. And I turned it down, fortunately.
> On a bet, I can convert "huge switch statement" to nicely look switch
> statement. It will as nice as ds2760's.
> The problem isn't in the PSY class.

We're still going to be stuck with a huge switch statement. Yes, it
wouldn't be *as* big, but ds2760_battery.c has a decently sized switch
statement, and olpc_battery.c has even more properties.

The huge switch statement is the least of my worries, though. Getting
rid of it is just a bonus.

> > > We're not limited
> > > to drivers only being able to pass 'int' and 'char*'s in sysfs,
> You're not limited to "int" and "char *". Anything more than that
> is unnecessary, so far.

See below about the eeprom dump. Originally, it was desired for this
to be a hex string; after that, binary. Of course, once I actually
started adding device_attributes to olpc_battery.c, I started wondering;
why not just make *all* the properties device_attributes? And, what if
I want to show something larger than a signed int? What if I have a
value that I want to pad with 0's?

> > > we're
> > > not forced to keep a global string around in memory (as is again the
> > > case for olpc_battery's serial number code),
> If battery chip can report strings, then you anyway must keep it in
> the memory. The question is when to allocate memory and when to free
> it. Side question is for how long to keep it.
> Given that that string is small enough (dozen bytes), it's doesn't
> matter for how long we'll allocate it. So, in most cases it's easy
> to answer: allocate at probe, free at remove, so keep it for whole
> battery lifetime. (In contrast, adding tons of functions will waste
> _much more_ space than these dozen bytes!)
> IIRC this is the main difficulty you're facing with current properties
> approach. You've converted whole class to the something different..
> but you didn't show a single user of that change. Sorry, olpc still
> using hard-coded manufacturer string:

Well, no, I was talking about the serial number string. It's not
upstream yet, it's just in OLPC's repo.;a=commitdiff;h=f9b4313060ab9047942707da6d3084f7792e714c

Note bat_serial, 17 bytes sitting around. That's actually not that
bad, merely awkward. Worse (and what caused me to start reworking
the API) was a dump of the EEPROM contents in hex; basically, (0x80-0x20)*2.
So, a buffer of 192 bytes was required to be kept around in memory. As
we add more features, we end up w/ more and more wasted space.

> +static ssize_t olpc_bat_manufacturer(struct device *dev,
> + struct device_attribute *attr, char *buf)
> +{
> + int ret;
> + uint8_t ec_byte;
> +
> + ret = olpc_bat_get_status(&ec_byte);
> + if (ret)
> + return ret;
> +
> + ec_byte = BAT_ADDR_MFR_TYPE;
> + ret = olpc_ec_cmd(EC_BAT_EEPROM, &ec_byte, 1, &ec_byte, 1);
> + if (ret)
> + return ret;
> + switch (ec_byte >> 4) {
> + case 1:
> + strcpy(buf, "Gold Peak");
> break;
> + case 2:
> + strcpy(buf, "BYD");
> break;
> + default:
> + strcpy(buf, "Unknown");
> break;
> + }
> +
> + return ret;
> +}
> In other words: all these strings can and should be static. Why
> spend cpu cycles on strcpy'ing things that can be not strcpy'ed?

Sure, except we have to spend the CPU cycles copying them into the
buffer *anyways*. After all, that's how power_supply_sysfs gets
the returned char*'s into the sysfs buffer..

> I don't see S/N function. I'm sure it could be implemented easily
> using today's properties approach.
> > > we don't have ordering
> > > restrictions w/ the return value being interpreted based upon where it's
> > > located in the array... etc.
> What exact "restrictions" you're talking about? There are no
> restrictions per se.

The power_supply_properties enum; 'int's are listed first, 'char*'s are
listed second. power_supply_show_property() assumes that anything
after POWER_SUPPLY_PROP_MODEL_NAME in that list is a char*, and anything
before it is an integer. No other options are available, and if you
accidentally put an int property somewhere in the strings section; well,
too bad.

> > > The other API seems to encourage driver
> > > authors to get their custom sysfs knobs into the list of sysfs knobs, and
> > > this one doesn't.
> Yes, API is encouraging to add knobs, but not just any knobs. Only
> ones that make sense as a property of a PSY (opposite to some kind
> property of PSY driver itself). The count of such properties is
> limited, physically.

I'll reference;a=commitdiff;h=f9b4313060ab9047942707da6d3084f7792e714c
again, because it's a good example of the problems w/ the current API.

Basically, we need to add POWER_SUPPLY_PROP_ACCUM_CURRENT to the core
driver. I don't see any other power_supply driver actually needing
that; however, the alternative is having magical #defines for those
that are in olpc_battery.c.. and we hope that the core enum list never
accidentally clobbers our internal defines. ie,


Inside of olpc_bat_get_property:


And, we hope that include/linux/power_supply.h never grows to include 48
entries inside the power_supply_properties enum (otherwise, we could end
up with power_supply_show_property() mangling OLPC_PROP_ACCUM_CURRENT).
Realistic? Not overly. But, having to choose arbitrary numbers is
*not* a desirable way to do things, nor is requiring every single
property from every single driver to end up in the power_supply_properties
enum list.

The power_supply_class docs say, "Power supply class is extensible, and
allows to define drivers own attributes."; however, they don't say *how*
to define their own attributes, and I don't see any power_supply drivers
that are doing that. This leads me to believe that the API is not
suited for this sort of thing.

> I'm recalling question about raw data. No, PSY class isn't for raw
> data you're getting from the firmware. Implement driver-specific
> binary attribute, that will contain device-specific raw data.
> Ideally, you should not export raw data at all (though, good idea
> is to export them into the debugfs).

Sure, debugfs is an option; however, for a quick and dirty tests, one
of our hardware people wanted to be able to simply view the contents
of the eeprom (and it wasn't clear what format was best. My assumption
was that hex would be okay, but it turned out to be fairly unreadable.
Binary piped to od was much nicer).

> > > If there is interest in this API, I'll convert the rest of the power_supply
> > > drivers over to it and resubmit patches.
> >
> > Looks sane enough to me.
> Heh..
> > If Anton has no objections, I'll merge it.
> Sorry, lots of objections.

Yes, the callback API is slightly more verbose. I see that as a good
thing; I don't think the power_supply core should be magically turning
results into what it thinks they should be. I also think that moving
code out of the power_supply core, and into specific power_supply
drivers is a good idea. If I'm using pda_power.ko, I don't want all
the excess baggage that olpc_battery.ko might need (and therefore
had to include in power_supply_core).

Finally, to compare code size.. The current API:

-rw-r--r-- 1 dilinger dilinger 56768 2007-12-17 02:21 drivers/power/pda_power.o
-rw-r--r-- 1 dilinger dilinger 31830 2007-12-17 02:21 drivers/power/power_supply_core.o
-rw-r--r-- 1 dilinger dilinger 25120 2007-12-17 02:21 drivers/power/power_supply_leds.o
-rw-r--r-- 1 dilinger dilinger 30220 2007-12-17 02:21 drivers/power/power_supply_sysfs.o

The API I'm suggesting:

-rw-r--r-- 1 dilinger dilinger 55208 2007-12-17 02:19 drivers/power/pda_power.o
-rw-r--r-- 1 dilinger dilinger 30832 2007-12-17 02:19 drivers/power/power_supply_core.o
-rw-r--r-- 1 dilinger dilinger 24232 2007-12-17 02:19 drivers/power/power_supply_leds.o
-rw-r--r-- 1 dilinger dilinger 26680 2007-12-17 02:19 drivers/power/power_supply_sysfs.o

I think I can actually do even better, but I'm not going to bother unless
folks like the new API.

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