Petr Cvek [off-list ref] writes:

Dne 20.8.2015 v 22:01 Robert Jarzmik napsal(a):

quoted

Petr Cvek [off-list ref] writes:

quoted

Add support for alternative LCD backlight with GPIO (no brightness).

Here, I don't understand, the commit message is too short.

Are there 2 brightness controls on magician, or are these 2 different magicians
(hardware wise), each having a different backlight control ? Or is it there is a
GPIO to light up the screen and a PWM backlight ?

I have to understand first.

I have aimed for a configuration, where user can use PWM xor GPIO backlight.

You probably know that in a backlight case GPIO is just a subcase of PWM, with
frequency equal to either 0 or max_backlight_freq.

As pin mux can set GPIO or PWM (or UART :-D) on PWM pin, you can choose if you
spare few kB (as magician has only 64MB RAM and 64MB flash) on GPIO or will
have smooth backlight with PWM.

I'm in front of the same dilemna for 5 years ago with the mioa701 (64MB RAM, 2 *
32MB flash). I chose PWM, judging the PWM stack was worth it.

Moreover, pwm can be a module, loaded or not : what memory do you spare in that
case if you don't load the module ?

I have tested both (I think I have added the gpio backlight for debugging a regression in pwm_bl).
Only ugly thing is the GPIO definition:

#if IS_ENABLED(CONFIG_PWM_PXA)
	/* PWM 0 - LCD backlight */
	GPIO16_PWM0_OUT,
#else
	/* Ensure static backlight without any driver */
	MFP_CFG_OUT(GPIO16, AF0, DRIVE_LOW),    /* Backlight enabled */
#endif

Hmmm.. I think I can do this better in pwm-backlight init/exit (same way as
pxa_ficp). During init the pin mux will switch to Alternate Function (PWM) and
during exit it will switch to GPIO (which can be used by gpio-backlight). This
will remove ugly ifdef.

I don't like changing the muxing after initialization that much : the pin muxing
should represent what there is in hardware, and that is a backlight frequency
switched input pin. The one exception is for hardware workarounds such as the
AC97 reset pin.

I won't take an init/exit with a manipulation of an mfp. I'll let you choose one
way instead :
 - either the GPIO
 - or the PWM (module or not)
Don't forget that this init/exit thing won't be available for a devicetree
magician (if that happens).

Cheers.

-- 
Robert