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

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Date: Thu, 15 Feb 2024 10:22:57 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
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 07:54:58PM +0200, Andy Shevchenko wrote:
> 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).

Assuming properly working hardware allows to drop many error checks :-)

> > 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?

If a driver author knows it while writing the code, it's obvious. But if
the driver author looks again in 2 years or someone else (e.g. me with
the PWM maintainer hat on and with little pci experience) that knowledge
might be faded.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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