linux-kernel - Re: device driver probe return codes

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Message-ID: <20081218101426.GI12431@fluff.org.uk>
Date:	Thu, 18 Dec 2008 10:14:26 +0000
From:	Ben Dooks <ben-linux@...ff.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ben Dooks <ben-linux@...ff.org>, linux-kernel@...r.kernel.org
Subject: Re: device driver probe return codes

On Tue, Dec 16, 2008 at 11:41:28PM -0800, Andrew Morton wrote:
> On Tue, 16 Dec 2008 21:53:31 +0000 Ben Dooks <ben-linux@...ff.org> wrote:
> 
> > I would like some feedback on the following regarding some
> > form of standardising return codes from a device driver probe
> > to try and stop some basic mistakes.
> > 
> > This document is not complete, any additions would be welcone.
> > 
> > 
> > probe() error return codes
> > ==========================
> > 
> > -ENODEV		This should be reserved for the case where there
> > 		really is no device here.
> > 
> > 		If you do not have a necessary resource or other setup
> > 		information this is NOT the error you want to return as
> > 		the driver core will not print any error message to the
> > 		user. Even if you driver prints a warning, this is still
> > 		not the error code to return.
> > 
> > -ENXIO		See -ENODEV, this is taken by the driver core to mean
> > 		there is "No such device or address (POSIX.1)".
> > 
> > -ENOMEM		This is used to signify lack of memory resources, such
> > 		as a failure to kmalloc() device state.
> > 
> > -EBUSY		A resource you need is not avaialable.
> > 
> > 		This is returned if you cannot get exclusive access to a
> > 		resource such as a request_mem_region() has failed.
> > 
> > -EINVAL		An argument to the driver is invalid.
> > 
> > -EIO		An input/output error occured. This could be due to
> > 		the device responding in an unexpected way or that
> > 		the device did not complete a request properly.
> > 
> > 
> 
> Sounds good.  I forsee a lot of patches to fix all this up :)
> 
> > 
> > Generic driver examples
> > =======================
> > 
> > Requesting an memory region
> > ---------------------------
> > 
> > 	if (!request_mem_region(start, len, why))
> > 		return -EBUSY;
> > 
> > Mapping a IO region
> > -------------------
> > 
> > 	regs = ioremap(base, size);
> > 	if (!regs)
> > 		return -EIO;
> > 
> > possibly -EFAULT here? 
> 
> I don't think EFAULT is appropriate here - there was no fault.
> 
> And EIO to me implies some problem with a hardware device.
> 
> > Not -ENOMEM, which a number of drivers do.
> 
> And ENOMEM should be reserved for page allocation exhaustion.
> 
> It's a bit presumptuous, but perhaps EBUSY here?

Ok, that might be ok, but it will get overloaded as -EBUSY is
a possible return from a number of resource reservation calls.
 
> > 
> > Platform driver specific examples
> > =================================
> > 
> > Examples when writing platform drivers.
> > 
> > All examples in this section assume the probe prototype is:
> > 
> > static int my_probe(struct platform_device *pdev)
> > 
> > 
> > Checking platform data
> > ----------------------
> > 
> > 	if (!pdev->dev.platform_data)
> > 		return -EINVAL;
> > 
> > should -ENOENT be returned here?
> 
> Well.. what would cause this to come about?
> 
> Perhaps BUG() is more appropriate ;)

WARN_ON() possibly, BUG() is a bit terminal to the whole kernel
process and if triggered before the console is working can be
rather hard to detect.

Possibly change this to:

	 if (!pdev->dev.platform_data) {
		dev_dbg(&pdev->dev, "no platform data supplied\n");
		WARN();
		return -EINVAL;
	}
 
> > 
> > Finding platform resource(s)
> > ----------------------------
> > 
> > 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > 	if (!res)
> > 		return -ENOENT;
> 
> I don't really like the practice of returning what are clearly
> filesystem-related errnos just because they're kinda sorta conceptually
> similar to what actually happened.
> 
> Again, what could actually cause this to happen?  The device doesn't
> have any memory resources?  So it's busted?  EIO or ENODEV?

ENODEV is part of the problem that we're trying to avoid returning
as it gets silently discarded by the device core. Unfortunately without
creating a new set of error codes specifically for device probes then
we're out of choice.
 
> > 	   
> > Getting a clock
> > ---------------
> > 
> > 	clk = clk_get(&pdev->dev, NULL);
> > 	if (IS_ERR(clk))
> > 		return PTR_ERR(clk);
> 
> yup.
> 
> 
> But really, the drivers should be doing far less of this
> errno-guessing.  In the majority of cases (platform_get_resource,
> ioremap, request_mem_region from your examples) the core kernel
> function should have returned an IS_ERR value and all the driver needs
> to do is to return it.

Yes, although the work to fixup some of these might be large it certainly
shouldn't be too complicated to do.

-- 
Ben (ben@...ff.org, http://www.fluff.org/)

  'a smiley only costs 4 bytes'
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