linux-kernel - Re: [RESEND PATCH v6 2/3] leds: Add driver for the TLC5925 LED controller

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Date:   Wed, 24 Aug 2022 11:55:54 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Jean-Jacques Hiblot <jjhiblot@...phandler.com>
Cc:     Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Linux LED Subsystem <linux-leds@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH v6 2/3] leds: Add driver for the TLC5925 LED controller

On Wed, Aug 24, 2022 at 11:39 AM Jean-Jacques Hiblot
<jjhiblot@...phandler.com> wrote:
> On 04/08/2022 23:04, Pavel Machek wrote:
> > On Thu 2022-08-04 22:23:00, Jean-Jacques Hiblot wrote:
> >> On 31/07/2022 21:28, Andy Shevchenko wrote:
> >>> On Fri, Jul 22, 2022 at 10:14 AM Jean-Jacques Hiblot
> >>> <jjhiblot@...phandler.com> wrote:

...

> >>> Sorry for my slowpokeness, but I just realized that this driver may
> >>> not be needed. What is the difference to existing gpio-74x164?
> >> It might work. However it might not be as practical and efficient as the
> >> dedicated LED driver.
> >>
> >> I'll give a try.
> > It is certainly preffered solution. If you decide to re-submit the
> > driver anyway, please mention that we already have GPIO driver for
> > compatible chip, and explain why this is superior.

> sorry for the delay. I tried with the  74x164 gpio driver and it works
> as expected.
>
> The only drawbacks are:
>
> - as-is the 74x164 gpio driver supports only one output-enable gpio.
> However in practice I don't think multiple OE GPIOs will ever be used.

Let's leave it to the case when it will be needed. So, we can skip this point.

> - with this approach, every time a LED status is changed the whole
> register has to be sent on the SPI bus. In other words, changes cannot
> be coalesced.

But isn't it the same as what you do in your driver? To me it looks
like you send the entire range of the values each time you change one
LED's brightness. I don't see any differences with the GPIO driver.

> I don't know if this is enough to make a dedicated TLC5925 driver
> desirable in the kernel.

I don't think you have enough justification for a new driver.

-- 
With Best Regards,
Andy Shevchenko