DT binding review for Armada display subsystem

On 07/13/2013 07:44 PM, Sebastian Hesselbarth wrote:

On 07/13/2013 01:12 PM, Russell King - ARM Linux wrote:

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On Sat, Jul 13, 2013 at 12:56:50PM +0200, Sylwester Nawrocki wrote:

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On 07/13/2013 10:35 AM, Jean-Francois Moine wrote:

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On Fri, 12 Jul 2013 13:00:23 -0600 Daniel Drake[off-list ref] wrote:

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On Fri, Jul 12, 2013 at 12:39 PM, Jean-Francois
Moine[off-list ref] wrote:

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I use my Cubox for daily jobs as a desktop computer. My kernel is a DT
driven 3.10.0. The dove-drm, tda998x and si5351 (clock) are kernel
modules. I set 3 clocks in the DT for the LCD0: lcdclk, axi and extclk0

Hmm, a bit off topic question, does it work when the si5351 module gets
removed ? I can't see anything preventing clock provider module from
being
removed why any of its clocks is used and clk_unregister() function is
currently unimplemented.

When I designed the clk API, I arranged for things like clk_get() to
take a reference on the module if the clock was supplied by a module.
You'll see some evidence of that by looking back in the git history
for arch/arm/mach-integrator/include/mach/clkdev.h.

Last time I checked, clkdev API neither implements referencing nor
unregister. This is on Mike's list and IIRC there already has been
a proposal for unregister. Si5351 was the first clkdev driver ever

I wasn't aware of it, thanks. I've seen a patch from Jiada Wang, it seems
they're working on v4 with clock object reference counting. Presumably we
need both clk_get() to be taking reference on the module and reference
counted clk free, e.g. in cases where clock provider is a hot-pluggable
device. It might be too paranoid though, I haven't seen hardware
configurations where a clock source could be unplugged safely when whole
system is running.

that could possibly be unloaded, so there may be still some features
missing.

I guess there should be a warning in e.g. Kconfig saying that it is safe
to use this driver only with CONFIG_MODULE_UNLOAD or something similar.
Otherwise users might not be happy and might start filling bug reports. :)

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If the common clk API has been designed without any thought to clocks
supplied by modules and making sure that in-use clocks don't go away,
then it's going to be a real pain to sort that out. I don't think
refcounting clocks makes sense (what do you do with an enabled clock
that you then remove the module for but it's still being used? Do you
just shut it down anyway? Do you leave it running? What about
subsequent calls?).

Good point, first I thought always disable on last user dropping it but
that may most likely break some platforms. Second thought, get back to
the state when the driver was loaded.

When last user drops the reference it could be already too late to change
anything in hardware, since the clock provider module is already unloaded.

Anyway I suppose it's best to take reference on the clk provider module
in clk_get(). This can easily lead to circular dependencies though, when
e.g. a module A is a clock consumer of a clock provided by module B, and
module B calls back into module A, so it also takes reference on A.
The only solution I could think of is to call clk_get() only before a clock
is enabled and clk_put() right after it is disabled and stops being used.

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I think this is one case where taking a reference on the module supplying
it makes total sense.

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(si5351). Normally, the external clock is used, but, sometimes, the
si5351 module is not yet initialized when the drm driver starts. So,
for 1920x1080p, it uses the lcdclk which sets the LCD clock to 133333
(400000/3) instead of 148500. As a result, display and sound still work
correctly on my TV set thru HDMI.

So, it would be nice to have 2 usable clocks per LCD, until we find a
good way to initialize the modules in the right order at startup time.

Doesn't deferred probing help here ?

Indeed it does. Just because one TV works with such a wrong clock does
not
mean that all TVs and HDMI compatible monitors will do. It's 11% out.

The reason that audio works is because of the way the HDMI transmitter
works
- it can calculate the CTS value to send (by measuring it) and it
sends that
value over the link so the TV can regenerate the correct audio clock from
the HDMI clock.

True, wrong clock will most likely not work on all monitors or TV. My
impression for TVs is that the the "cheaper" the brand is, the more
likely they accept any mode/clock combination ;)

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Whether that's going to be universally true, I don't know - it depends on
how much audio data gets sent via each frame. As the HDMI clock is
slower,
there would need to be more audio data sent.

Also: audio/video clock relation is sent in AVI packets over HDMI.
Picking a clock with 11% off may not even allow you to send (valid) CTS
information.

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lcd0: lcd-controller at 820000 {
compatible = "marvell,dove-lcd0";

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};

lcd1: lcd-controller at 810000 {
compatible = "marvell,dove-lcd1";

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};

Using different compatible strings to indicate multiple instances of
same
hardware doesn't seem right. Unless LCD0, LCD1 are really different
pieces

They aren't. They're 100% identical in the Armada 510.

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of hardware incompatible with each other I think it would be more
correct
to use same compatible property and DT node aliases, similarly as is now
done with e.g. I2C busses:

aliases {
lcd0 = &lcd_0;
lcd1 = &lcd_1;
};

lcd_0: lcd-controller at 820000 {
compatible = "marvell,dove-lcd";

I'd much prefer marvell,armada-510-lcd rather than using the codenames
for
the devices. Otherwise we're going to run into totally different things
being used for different devices (eg, armada-xp...)

+1 for using marvell,armada-510-lcd. I like the nick-name "dove" but
armada-510 is much easier to find on Marvell's website.

Indeed, I'm not much into Marvell SoCs details but "marvell,armada-510-lcd"
seems more clear.

--
Thanks,
Sylwester