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 10:39:19 +0200
From:   Jean-Jacques Hiblot <jjhiblot@...phandler.com>
To:     Pavel Machek <pavel@....cz>
CC:     Andy Shevchenko <andy.shevchenko@...il.com>,
        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 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:
>>>> The TLC5925 is a 16-channels constant-current LED sink driver.
>>>> It is controlled via SPI but doesn't offer a register-based interface.
>>>> Instead it contains a shift register and latches that convert the
>>>> serial input into a parallel output.
>>>>
>>>> Datasheet: https://www.ti.com/lit/ds/symlink/tlc5925.pdf
>>>> Signed-off-by: Jean-Jacques Hiblot <jjhiblot@...phandler.com>
>>>> Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
>>> 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.

Hi all,

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.

- 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.


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

JJ

>
> Thanks,
> 								Pavel
>