linux-kernel - Re: [PATCH v2 1/5] pwm: dwc: drop redundant error check

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Message-ID: <Zcz-csPY5x29DP7v@smile.fi.intel.com>
Date: Wed, 14 Feb 2024 19:54:58 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc: Raag Jadav <raag.jadav@...el.com>, jarkko.nikula@...ux.intel.com,
	mika.westerberg@...ux.intel.com, lakshmi.sowjanya.d@...el.com,
	linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] pwm: dwc: drop redundant error check

On Wed, Feb 14, 2024 at 06:45:48PM +0100, Uwe Kleine-König wrote:
> On Thu, Feb 08, 2024 at 07:04:33PM +0200, Andy Shevchenko wrote:
> > On Thu, Feb 08, 2024 at 08:46:44AM +0100, Uwe Kleine-König wrote:
> > > On Thu, Feb 08, 2024 at 12:35:25PM +0530, Raag Jadav wrote:
> > > > pcim_iomap_table() fails only if pcim_iomap_regions() fails. No need to
> > > > check for failure if the latter is already successful.
> > > 
> > > Is this really true? pcim_iomap_table() calls devres_alloc_node() which
> > > might fail if the allocation fails. (Yes, I know
> > > https://lwn.net/Articles/627419/, but the rule is still to check for
> > > errors, right?)
> > 
> > We do not add a dead code to the kernel, right?
> > 
> > > What am I missing?
> > 
> > Mysterious ways of the twisted PCI devres code.
> > Read the above commit message again :-)
> > 
> > For your convenience I can elaborate. pcim_iomap_table() calls _first_
> > devres_find() which _will_ succeed if the pcim_iomap_regions() previously
> > succeeded. Does it help to understand how it designed?
> 
> I assume you're saying that after pcim_iomap_regions() succeeded it's
> already known that pcim_iomap_table() succeeds (because the former
> already called the latter).
> 
> I'm still concerned here. I agree that error checking might be skipped
> if it's clear that no error can happen (the device cannot disappear
> between these two calls, can it?), 

It depends. If you call it in some asynchronous callbacks which may be run
after PCI device disappears, then indeed, it's problematic. But you probably
will have much bigger issue at that point already.

In ->probe() it's guaranteed to work as I suggested (assuming properly working
hardware).

> but for me as an uninitiated pci code
> reader, I wonder about
> 
> 	dwc->base = pcim_iomap_table(pci)[0];
> 
> without error checking. (OTOH, if pcim_iomap_table() returned NULL, the
> "[0]" part is already problematic.)

Seems it's your problem, many drivers use the way I suggested.

> I'd like to have a code comment here saying that pcim_iomap_table()
> won't return NULL.

Why? It's redundant. If you use it, you should know this API.
So, the bottom line, does this API needs better documentation?

-- 
With Best Regards,
Andy Shevchenko