linux-kernel - Re: [PATCHv1 0/6] leds: pca9653x: support inverted outputs and cleanups

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Date:	Wed, 20 Apr 2016 10:01:28 +0200
From:	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
To:	Olliver Schinagl <oliver@...inagl.nl>
Cc:	Jacek Anaszewski <j.anaszewski@...sung.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Richard Purdie <rpurdie@...ys.net>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux LED Subsystem <linux-leds@...r.kernel.org>,
	Peter Meerwald <pmeerw@...erw.net>
Subject: Re: [PATCHv1 0/6] leds: pca9653x: support inverted outputs and cleanups

Hi Ollivier

On Wed, Apr 20, 2016 at 9:21 AM, Olliver Schinagl <oliver@...inagl.nl> wrote:

What I am propossing is at probe():

replace:

if (pdata) {
/* Configure output: open-drain or totem pole (push-pull) */
if (pdata->outdrv == PCA963X_OPEN_DRAIN)
i2c_smbus_write_byte_data(client, PCA963X_MODE2, 0x01);
else
i2c_smbus_write_byte_data(client, PCA963X_MODE2, 0x05);
}

with something like (forgive the spacing):

u8 mode2 = PCA963X_MODE2_DMBLNK | 0x1;
if (pdata && pdata->outdrv == PCA963X_OPEN_DRAIN)
   mode2 |= 0x4;
i2c_smbus_write_byte_data(pca963x->chip->client, PCA963X_MODE2, mode2);

and then remove from pca963x_blink() these lines:

u8 mode2 = i2c_smbus_read_byte_data(pca963x->chip->client,
PCA963X_MODE2);

if (!(mode2 & PCA963X_MODE2_DMBLNK))
i2c_smbus_write_byte_data(pca963x->chip->client, PCA963X_MODE2,
mode2 | PCA963X_MODE2_DMBLNK);

As I said before, the reason for this proposal is that the code NEVER
clears PCA963X_MODE2_DMBLNK, only sets it.
Unfortunately I do not have the HW to test this change.

>
> Now I understand your concern, the i2c operations are slow and time
> consuming making the mutex very expensive.
>
> The thing is, to be able to write the blink bit, we need to read the whole
> mode2 register, to do a proper read-modify-write. We don't know what's in
> the mode2 register and we only want to write the bit if it is actually not
> set to begin with, to save a i2c write operation.

As I said earlier, nowhere in the code clears that bit. The bit is
only set. so no reason to read/modify/write. We can set that bit at
probe time and assume that it will not be changed.

>
> We start this function already however with with two write calls of
> sequential registers, the grp and pwm enable registers. There is even a call
> to automatically update these registers, which I think we'd use
> i2c_master_send() to set the address via the auto-increment register and
> enable auto increment of these two registers. Now we reduced the 2 seperate
> calls into one bigger 'faster' call. So 1 win there. But! it will require us
> however to change the other calls to disable auto increment via de mode1
> register. Since this is an extra i2c_write operation, it makes the other i2c
> writes more expensive, which may happen much more often (enable blink only
> happens occasionally, changing the brightness may change a lot (fade in fade
> out).

Be careful with that. Sometimes this chips are connected to the smbus,
wich has a limited number of operations/lengths. We should keep the
i2c_smbus_write_byte_data() call, because that makes the driver
compatible with more hardware.

>
> So unless i'm totally misunderstanding something, I don't think we can safe
> much here at all.

To be clear: My proposal is that (after being tested), move the set of
DMBLINK to probe, remove the read/modify/write from blink() and keep
the locking as it is now, only protecting ledout.

Also you need to fix the patch that breaks bisect, but I believe that
you are already working on that.

I have reviewed a bit Documentation/device-tree and it seems that
there is already a binding for active-low. Instead of nxp,active-low
you should call it just active-low. But I am not a device-tree expert.

Finally, you mention that you are fixing some checkpatch errors, but I
cannot replicate those in my side :S

ricardo@...ix:~/curro/linux$ scripts/checkpatch.pl -f
drivers/leds/leds-pca963x.c
total: 0 errors, 0 warnings, 439 lines checked

drivers/leds/leds-pca963x.c has no obvious style problems and is ready
for submission.

So maybe the errors you are fixing are introduced by your patches.
About the other style patches, I do not know what is the policy of the
Maintainer in that matter, especially when the driver did not break
originally checkpatch.

Regards!

---
Ricardo