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SubjectRe: [PATCH 09/10] MCDE: Add build files and bus
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[dri people: please have a look at the KMS discussion way below]

On Thursday 25 November 2010 19:00:26 Marcus LORENTZON wrote:
> > -----Original Message-----
> > From: Arnd Bergmann [mailto:arnd@arndb.de]
> > Sent: den 25 november 2010 17:48
> > To: Marcus LORENTZON
> > Cc: linux-arm-kernel@lists.infradead.org; Jimmy RUBIN; linux-
> > media@vger.kernel.org; Dan JOHANSSON; Linus WALLEIJ
> > Subject: Re: [PATCH 09/10] MCDE: Add build files and bus
> >
> > On Thursday 25 November 2010, Marcus LORENTZON wrote:
> > > From: Arnd Bergmann [mailto:arnd@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.
> > > > >

[note: please configure your email client properly so it keeps
proper attribution of text and and does not rewrap the citations
incorrectly. Wrap your own text after 70 characters]

> >
> > 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.
>
> Then we might be on right track to model them as devices on a
> platform bus. Since most displays/panels can't be "plug-n-play"
> probed, instead devices has to be statically declared in
> board-xx.c files in mach-ux500 folder. Or is platform bus a
> "singleton"? Or can we define a new platform bus device?
> Displays like HDMI TV-sets are not considered for device/driver
> in mcde. Instead there will be a hdmi-chip-device/driver on the
> mcde bus. So all devices and drivers on this bus are static.

I think I need to clarify to things:

* When I talk about a bus, I mean 'struct bus_type', which identifies
all devices with a uniform bus interface to their parent device
(think: PCI, USB, I2C). You seem to think of a bus as a specific
instance of that bus type, i.e. the device that is the parent of
all the connected devices. If you have only one instance of a bus
in any system, and they are all using the same driver, do not add
a bus_type for it.
A good reason to add a bus_type would be e.g. if the "display"
driver uses interfaces to the dss that are common among multiple
dss drivers from different vendors, but the actual display drivers
are identical. This does not seem to be the case.

* When you say that the devices are static, I hope you do not mean
static in the C language sense. We used to allow devices to be
declared as "static struct" and registered using
platform_device_register (or other bus specific functions). This
is no longer valid and we are removing the existing users, do not
add new ones. When creating a platform device, use
platform_device_register_simple or platform_device_register_resndata.

I'm not sure what you mean with drivers being static. Predefining
the association between displays and drivers in per-machine files is
fine, but since this is really board specific, it would be better
to eventually do this through data passed from the boot loader, so
you don't have to have a machine file for every combination of displays
that is in the field.

> > 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.
>
> What about starting with MCDE HW, which is the core HW driver doing
> all real work? And then continue with the infrastructure + some displays
> + drivers ...

This is already the order in which you submitted them, I don't see a
difference here. I was not asking to delay any of the code, just to put
them in a logical order.

> Only problem is that we then have a driver that can't be used from user
> space, because I don't think I can find anyone with enough time to write
> a display driver + framebuffer on top of mcde_hw (which is what the
> existing code does).

Well, developer time does not appear to be one of your problems, you
already wasted tons of it by developing a huge chunk of code that isn't
going anywhere because you wrote it without consulting the upstream
community ;-)

There is no need to develop anything from scratch here, you already have
the code you want to end up with. What I would do here is to start with
a single git commit that adds the full driver. Then take out bits you
don't absolutely need to keep the driver from showing text on your
screen (not necessarily in this order):

* Take out display drivers one by one, until there is only one left.
Do a git commit after each driver
* Take out the register definitions that are not actually used in your
code
* Remove the infrastructure for dynamic displays and hardcode the one
you use
* Take out the frame buffer code
* Take out the infrastructure for multiple user-interfaces, hardcoding KMS
to the DSS
* Anything else you don't absolutely need.

Finally, you should end up with a very lean driver that only does a
single thing and only works on one very specific board. Remove that, too,
in a final commit. Now use git to reverse the patch order and you have
a nice series that you can use for patch submission, one feature at a
time. Then we can discuss the individual merits of each patch.

In the future, best plan for how you want to submit the code while
you're writing it, instead of as an afterthought. Quite often, the
first patch to submit is also one of the early stages of the driver,
so there is no need to wait for the big picture before you start
submitting. This way, we can work out conceptual mistakes early on,
saving a lot of your time, and the reviewer's time as well.

> > 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
>
> Ok, we will do this for the mcde stuff. How do we handle stuff that span
> different kernel folders? Like drivers/misc and drivers/video/mcde etc.
> We can't just change the order of top level makefiles, that would break
> other drivers I guess.

Right, you have to find a different solution for those. Most importantly,
a module in one directory should not have intimate knowledge of data
structures in a different module in another directory.

In your example, drivers/misc is probably wrong anyway. Try ignoring this
problem at first by forcing all the drivers loadable modules, which will
naturally fix the initialization order. When you still have link order
problems by building all the drivers into the kernel after this, we can
have another look to find the least ugly solution.

> > > > 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.
>
> Not sure what you mean, we have plenty of drivers and devices already.
> Maybe I should try to clarify picture.

I mean the layering of loadable modules: you cannot make a high-level
module link against multiple low-level modules that export the
same interface. If you have multiple modules that implement the same
interface like you diplay drivers, they need to be on top!

> DSS give access to all display devices probed on the virtual mcde
> dss bus, or platform bus with specific type of devices if you like.
> All calls to DSS operate on a display device, like create an
> overlay(=framebuffer), request an update, set power mode, etc.
> All calls to DSS related to display itself and not only framebuffer
> scanout, will be passed on to the display driver of the display
> device in question. All calls DSS only related to overlays, like
> buffer pointers, position, rotation etc is handled directly by DSS
> calling mcde_hw.
>
> You could see mcde_hw as a physical level driver and mcde_dss closer
> to a logical driver, delegating display specific decisions to the
> display driver. Another analogy is mcde_hw is host driver and display
> drivers are client device drivers. And DSS is a collection of logic
> to manage the interaction between host and client devices.

The way you describe it, I would picture it differently:

+----------+ +----+-----+-----+ +-------+
| mcde_hw | | fb | kms | v4l | | displ |
+----+----------------------------------+
| HW | mcde_dss |
+----+----------------------------------+

In this model, the dss is the core module that everything else
links to. The hw driver talks to the actual hardware and to the
dss. The three front-ends only talk to the dss, but not to the
individual display drivers or to the hw code directly (i.e. they
don't use their exported symbols or internal data structures.
The display drivers only talk to the dss, but not to the front-ends
or the hw drivers.

Would this be a correct representation of your modules?

> > 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.
>
> All APIs have to be provided, these are user space API requirements.
> KMS has a generic FB implementation. But most of KMS is modeled by
> desktop/PC graphics cards. And while we might squeeze MCDE in to look
> like a PC card, it might also just make things more complex and
> restrict us to do things not possible in PC architecture.

Ok, so you have identified a flaw with the existing KMS code. You should
most certainly not try to make your driver fit into the flawed model by
making it look like a PC. Instead, you are encouraged to fix the problems
with KMS to make sure it can also meet your requirements. The reason
why it doesn't do that today is that all the existing users are PC
hardware and we don't build infrastructure that we expect to be used
in the future but don't need yet. It would be incorrect anyway.

Can you describe the shortcomings of the KSM code? I've added the dri-devel
list to Cc, to get the attention of the right people.

> Alex Deucher noted in a previous post that we also have the option of
> implementing the KMS ioctls. We are looking at both options. And having
> our own framebuffer driver might make sense since it is a very basic
> driver, and it will allow us to easily extend support for things like
> partial updates for display panels with on board memory. These panels
> with memory (like DSI command mode displays) is one of the reasons why
> KMS is not the perfect match. Since we want to expose features available
> for these types of displays.

Ok.

> > > > 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.
>
> In short,
> KMS connector = MCDE port
> KMS encoder = MCDE channel
> KMS crtc = MCDE overlay

Any chance you could change the identifiers in the code for this
without confusing other people?

> > 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.
>
> Yes, mcde_bus should be connected to mcde, this is a bug. The display
> drivers will placed in this bus, since this is where they are probed
> like platform devices, by name (unless driver can do MIPI standard
> probing or something). Framebuffers/V4L2 overlay devices can't be
> put in same hierarchy, since they have multiple "parents" in case
> the same framebuffer is cloned to multiple displays for example.
> But I think I understand your more general point of sysfs representing
> the "real" probe hierarchy. And this is something we will look at.

Ok. If your frame buffers are not children of the displays, they should
however be children of the controller:

.../mcde_controller/
/chnlA/
/displ_crtc0
/displ_dbi0
/chnlB/
dspl_crtc1
/fb0
/fb1
/fb2
/v4l_0
/v4l_1

Does this fit better?

> > 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
>
> We are trying to avoid dynamic driver loading and udev for platform
> devices to be able to show application graphics within a few seconds
> after boot.

That is fine, you don't need to do that for products. However, it
is valuable to be able to do it and to think of it in this way.
When you are able to have everything modular, it is much easier to
spot layering violations and you can much easier define the object
life time rules.

Also, for the general case of building a cross-platform kernel,
you want to be able to use modules for everything. Remember that
we are targetting a single kernel binary that can run on multiple
SoC families, potentially with hundreds of different boards.

> > * the hw driver creates a device for each channel, and passes
> > the channel specific configuration data to the channel device
>
> We have to migrate displays in runtime between different channels
> (different use cases and different channel features), we don't want
> to model displays as probed beneath the channel. Maybe the
> port/connector could be a device. But that code is so small, so it
> might just add complexity to visualize sysfs hierarchy.
> What do you think?

This makes it pretty obvious that the channel should not be a
device, but rather something internal to the dss or hw module.

What is the relation between a port/connector and a display?
If it's 1:1, it should be the same 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.
>
> "All" display devices need static platform_data from
> mach-ux500/board-xx.c. This is why we have the bus,
> to bind display dev and driver.

You don't need to instantiate the device from the board though,
just provide the data. When you add the display specific data
to the dss data, the dss can create the display devices:

static struct mcde_display_data mcde_displays[2] = {
{
...
}, {
...
},
};

static struct mcde_dss_data {
int num_displays;
struct mcde_display_data *displays;
} my_dss = {
.num_displays = 2,
.displays = &mcde_displays;
};

The mcde_dss probe function then takes the dss_data and iterates
the displays, creating a new child device for each.

> > * 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.
>
> This is setup by board/product specific code. Display drivers
> just enable use of the HW, not defining how the displays are
> used from user space.

Right, this also gets obsolete, since as you said an fb cannot be
the child of a display.

> > * 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.
>
> Devices are static from mach-ux500/board-xx. And v4l2/fb setup
> is board/product specific and could change dynamically.

Not sure how the fb setup can be both board specific and dynamic.
If it's statically defined per board, it should be part of the
dss data, and dss can then create the fb devices. If it's completely
dynamic, it gets created through user space interaction anyway.

> > > > 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?
>
> No, see above. Just that we have mcde dss to support multiple user
> space apis by customer request. Then doing our own fb on top of
> that is very simple and adds flexibility.

This sounds like an odd thing for a customer to ask for ;-)

In my experience customers want to solve specific problems like
everyone else, they have little interest in adding complexity
for the sake of it. Is there something wrong with one of the
interfaces? If so, it would be better to fix that than to add
an indirection to allow more of them!

> > 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.
>
> Currently nothing, since it is not finished. But the idea (and
> requirement) is that normal graphics will use framebuffer and
> video/camera overlays will use v4l2 overlays. Both using same
> mcde channel and display. Some users might also configure their
> board to use two framebuffers instead. Or maybe only use KMS etc ...

I still don't understand, sorry for being slow. Why does a camera
use a display?

Arnd


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