On Thu, 2012-03-08 at 14:52 +0530, Archit Taneja wrote:

On Thursday 08 March 2012 02:16 PM, Tomi Valkeinen wrote:

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On Thu, 2012-03-08 at 14:04 +0530, Archit Taneja wrote:

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On Wednesday 07 March 2012 06:14 PM, Tomi Valkeinen wrote:

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-	r = hdmi_init_platform_driver();
-	if (r) {
-		DSSERR("Failed to initialize hdmi\n");
-		goto err_hdmi;
+	/*
+	 * It's ok if the output-driver register fails. It happens, for example,
+	 * when there is no output-device (e.g. SDI for OMAP4).
+	 */

Suppose we do a omap2plus_defconfig, CONFIG_OMAP2_DSS_SDI would be
selected, and sdi.c would be built, if we boot on OMAP4, why would a sdi
driver register cause a failure? Wouldn't the sdi driver just get
registered, and wait till eternity for the corresponding sdi platform
device to get registered?

No. Well, yes.

Currently we use platform_driver_register() to register the drivers, and
it does just what you described. But a few patches later I change
platform_driver_register() to platform_driver_probe(), which will return
ENODEV if there are no matching devices for the driver.

I originally had the platform_driver_probe() patch before this patch,
and thus the comment above made sense. Now the patch is after this
patch, so the comment is not exactly right until the probe patch is also
applied.

Oh okay. But the comment after the patch set still says "It's ok if the 
output-driver register fails.", we could change it to "It's ok if the 
output-driver probe fails."

Well, I guess this goes into nitpicking area, but if there are no
devices, probe is not called at all. So I think it's the driver register
that fails in that case. If there is a device, and it is probed, and
that fails, then it's probe which fails.

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The point with platform_driver_probe() is that it can be used with
non-removable devices which are created at boot time, like the DSS
components. With platform_driver_probe() the probe function is called
only at that one time, and never afterwards. So probe can be in __init
section, and thrown away after init.

So platform_driver_probe() is like a driver_register() + probe().

Yes. Well, when platform_driver_register() is called, and the devices
are already present, it will call the probe also. So in that sense they
are similar. The difference is that for platform_driver_register() the
probe pointer must be in the driver struct, and it stays there even
after init. For platform_driver_probe(), the probe pointer is given as
an argument to the function, and thus it's not stored anywhere and can
be thrown away afterwards.

Okay, in our case, all the devices are created at boot time, and if 
omapdss were a module, the probes would have been thrown away after 
module_init(), right?

Yes. If omapdss is a module, the functions marked with __init are
discarded after the module_init is done. If omapdss is built-in, the
__init funcs are thrown away after the kernel's init done.

quoted

One side effect of using platform_driver_probe() is that it returns
ENODEV is there are no devices. In a simple module, the error can be
then returned from module_init, thus causing the whole module to be
unloaded. Our case is a bit more complex as we have multiple drivers in
the same module.

A downside with that is that we don't really know if the ENODEV error
happened because there were no devices (which is ok), or if it came from
probe function (which is not so ok). However, I thought that it doesn't
matter if an output driver has failed. We can still continue with the
other output drivers just fine.

If we ensure that none of our probes return ENODEV(even though it may 
make sense to return it if a func within probe fails), we could 
differentiate between the 2 cases, right?

True, I thought about that. But we can never be sure that the functions
called by the probe (clk_get, or whatever) won't return ENODEV. Of
course, we could check what they return, and change the error to
something else, but I'm not sure if that's good either.

quoted

Actually, there is a small problem. If, for example, DSI driver fails to
load, and DPI driver tries to use DSI PLL...

If we could differentiate between an error occuring because the device 
doesn't exist and an error occuring because the probe failed, we could 
bail out if any of the probes fail, right?

Yes, but it feels a bit hackish to try to use the error as I pointed out
above. So I'd rather go the other way: the drivers should somehow
register the stuff they offer, so for example when the DSI1 is probed,
it should register the DSI1 PLL to dss core. And DPI would have to ask
for the DSI1 PLL from dss core.

That, of course, is not a trivial change, so for the moment this stuff
is slightly broken in error cases. Perhaps we could figure out some kind
of clean hack for that...

Alternatively, if the platform driver code was changed to tell us
clearly if it was the probe that failed or if there just weren't any
devices, we could also use that.

 Tomi