[PATCH 09/10] MCDE: Add build files and bus

[Arnd Bergmann]

On Thursday 25 November 2010, Marcus LORENTZON wrote:
> From: Arnd Bergmann [mailto:arnd at arndb.de]
> > On Wednesday 10 November 2010, Jimmy Rubin wrote:
> > > This patch adds support for the MCDE, Memory-to-display controller,
> > > found in the ST-Ericsson ux500 products.
> > >
> > > This patch adds the necessary build files for MCDE and the bus that
> > > all displays are connected to.
> > >
> > 
> > Can you explain why you need a bus for this?
> 
> The idea of the bus is to make it easier to add new panel/display support
> using the normal device/driver model. Boards add devices at init, and
> drivers are selected in config. If we were to model the "real physical"
> bus structure it would be very complex, hard to use. We use our own bus,
> but it's really a virtual bus for adding display devices and drivers to 
> MCDE host. Is there any "generic virtual bus type" we should use instead
> of our own type? If you have another idea of how to add display devices
> and drivers without MCDE code modifications, please let us know. A virtual
> bus just give us a nice framework to do this.

All devices that you cannot probe by asking hardware or firmware are normally
considered platform devices. Then again, a platform device is usally
identified by its resources, i.e. MMIO addresses and interrupts, which
I guess your display does not have.

> > With the code you currently have, there is only a single driver
> > associated
> > with this bus type, and also just a single device that gets registered
> > here!
> 
> And yes, currently there's only one single driver. But there's a lot more
> coming in the pipe. Like some LCD, DPI, DBI, DSI display drivers. And once
> all different u8500 boards are pushed, there will be multiple boards
> registering devices with different setups. But one thing at a time.

Your approach is making it hard to review the code in context. Adding a
device driver that uses existing infrastructure is usually straightforward,
but adding infrastructure is a hard thing and needs to be done with great
care, especially if you add infrastructure before it actually is needed.

Staging it in a way that adds all the display drivers later than the
base driver is a good idea, but it would be helpful to also add the
infrastructure at the later stage. Maybe you can try to simplify the
code for now by hardcoding the single display and remove the dynamic
registration. You still have the the code, so once the base code looks
good for inclusion, we can talk about it in the context of adding
dynamic display support back in, possibly in exactly the way you are
proposing now, but perhaps in an entirely different way if we come up
with a better solution.

On a more abstract level, when you want to add large chunks of code
to the kernel, you often cannot find anyone to review and understand
all of it at once. Splitting a subsystem into ten patches by file
level rarely helps, so instead you should ideally have a series of
patches that each add working code that enable more features.

This is of course more work for you, especially if you did not consider
it while writing the code in the first place. Still, you should
always try hard to make code easy to review as much as possible,
because you need to work with reviewers both to get code in and to
make sure you make the quality ends up as good as possible.

> > >+static int __init mcde_subsystem_init(void)
> > >+{
> > >+       int ret;
> > >+       pr_info("MCDE subsystem init begin\n");
> > >+
> > >+       /* MCDE module init sequence */
> > >+       ret = mcde_init();
> > >+       if (ret)
> > >+               return ret;
> > >+       ret = mcde_display_init();
> > >+       if (ret)
> > >+               goto mcde_display_failed;
> > >+       ret = mcde_dss_init();
> > >+       if (ret)
> > >+               goto mcde_dss_failed;
> > >+       ret = mcde_fb_init();
> > >+       if (ret)
> > >+               goto mcde_fb_failed;
> > >+       pr_info("MCDE subsystem init done\n");
> > >+
> > >+       return 0;
> > >+mcde_fb_failed:
> > >+       mcde_dss_exit();
> > >+mcde_dss_failed:
> > >+       mcde_display_exit();
> > >+mcde_display_failed:
> > >+       mcde_exit();
> > >+       return ret;
> > >+}
> > 
> > Splitting up the module into four sub-modules and then initializing
> > everything from one place indicates that something is done wrong
> > on a global scale.
> > 
> > If you indeed need a bus, that should be a separate module that gets
> > loaded first and then has the other modules build on top of.
> 
> Yes, that's the general idea. We don't completely understand the correct
> way of making sure how one module gets loaded before another, without
> selecting init level, like the fs_initcall below you commented on. I
> guess most of our submodules should be initialized during subsys_init.
> But we have not found how to specify the module init order inside subsys
> init level. Maybe you can shed some light on this? Makefile order seems
> like a fragile way of defining init order dependencies.
> Do you think static init calls from machine subsys init are a better solution?

In general, the idea with loadable modules is that you can only load
them in initialization order because of the symbol dependencies: The bus
gets loaded first and it exports symbols used by the device drivers,
so a device driver can be sure that the bus is initialized entirely.

For the case where all modules are built-in, you can rely in link-order
in the Makefile, e.g.

obj-$(CONFIG_FOO_BASE)		+= foo_base.o
obj-$(CONFIG_FOO_SPECIFIC)	+= foo_specific.o # this comes after foo_base

> > I'm not sure how the other parts layer on top of one another, can you
> > provide some more insight?
> 
> +----------------------------+
> | mcde_fb/mcde_kms/mcde_v4l2 |
> +---------------+------------+
> |    mcde_dss   |
> +   +-----------+
> |   | disp drvs |
> +---+-----------+
> |    mcde hw    |
> +---------------+
> |      HW       |
> +---------------+

Ok. One problem with this is that once you have a multitude of
display drivers, you can no longer layer them below mcde_dss.

Having the kms/fb/v4l2 drivers on top definitely makes sense, so
these should all be able to be standalone loadable modules.
I do not understand why you have a v4l2 driver at all, or why
you need both fb and kms drivers, but that is probably because
of my ignorance of display device drivers.

> > From what I understood so far, you have a single multi-channel display
> > controller (mcde_hw.c) that drives the hardware.
> > Each controller can have multiple frame buffers attached to it, which
> > in turn can have multiple displays attached to each of them, but your
> > current configuration only has one of each, right?
> 
> Correct, channels A/B (crtcs) can have two blended "framebuffers" plus
> background color, channels C0/C1 can have one framebuffer.

We might still be talking about different things here, not sure.
 
> > Right now you have a single top-level bus device for the displays,
> > maybe that can be integrated into the controller so the displays are
> > properly rooted below the hardware that drives them.
> 
> Not sure I understand 100%. Routing is independent of bus structure.
> Routing could change dynamically during runtime reconfiguration using
> future KMS for example. Bus is only for connecting display devices
> with drivers. Of course we could have one bus per port/connector.
> But then the code would be more complex and we would end up with one
> bus/device/driver per connector (or in some rare cases 2-4 on DBI/DSI
> connectors).

I mean 'root' as in the root of your device tree, where the tree
is modeled after the logical layout of your hardware.

Looking at the representation in sysfs, you should probably aim
for something like

/sys/devices/axi/axi0/mcde_controller
				/chnlA
					/dspl_crtc0
						/fb0
						/fb1
						/v4l_0
					/dspl_dbi0
						/fb2
						/v4l_1
				/chnlB
					/dspl_ctrc1
						/fb3
				/chnlC
					/dspl_lcd0
						/fb4
						/v4l_2

Not sure if that is close to what your hardware would really
look like. My point is that all the objects that you are
dealing with as a device driver should be represented hierarchically
according to how you probe them.

Assuming the structure above is correct and you cannot probe
any of this by looking at registers, you would put a description
of it into the a data structure (ideally a flattened device tree
or a section of one) and let the probing happen:

* The axi core registers an mcde controller as device axi0.
* udev matches the device and loads the mcde hw driver from
  user space
* the hw driver creates a device for each channel, and passes
  the channel specific configuration data to the channel device
* the dss driver gets loaded through udev and matches all the
  channels
* the dss driver creates the display devices below each channel,
  according to the configuration data it got passed.
* The various display drivers get loaded through udev as needed
  and match the display devices
* Each display device driver initializes the display and creates
  the high-level devices (fb and v4l) as needed.
* Your fb and v4l highlevel drivers get loaded through udev and
  bind to the devices, creating the user space device nodes
  through their subsystems.

Now this would be the most complex scenerio that hopefully is
not really needed, but I guess it illustrates the concept.
I would guess that you can actually reduce this significantly
if you do not actually need all the indirections.

Some parts could also get simpler if you change the layering,
e.g. by making the v4l and fb drivers library code and having
the display drivers call them, rather than have the display
drivers create the devices that get passed to upper drivers.

> > The frame buffer device also looks weird. Right now you only seem
> > to have a single frame buffer registered to a driver in the same
> > module. Is that frame buffer not dependent on a controller?
> 
> MCDE framebuffers are only depending on MCDE DSS. DSS is the API that
> will be used by all user space APIs so that we don't have to duplicate
> the common code. We are planning mcde_kms and mcde_v4l2 drivers on top
> of MCDE DSS(=Display Sub System).

My impression was that you don't need a frame buffer driver if you have
a kms driver, is this wrong?

What does the v4l2 driver do? In my simple world, displays are for output
and v4l is for input, so I must have missed something here.

	Arnd