linux-kernel - Re: [PATCH] spi: spi-fsl-lpspi: fix watermark truncation caused by type cast

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Message-ID: <aSXHB_CWYZYnQ6ui@black.igk.intel.com>
Date: Tue, 25 Nov 2025 16:11:03 +0100
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: david laight <david.laight@...box.com>
Cc: Frank Li <Frank.li@....com>, carlos.song@....com, broonie@...nel.org,
	rongqianfeng@...o.com, linux-spi@...r.kernel.org,
	imx@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: spi-fsl-lpspi: fix watermark truncation caused by
 type cast

On Sat, Nov 22, 2025 at 10:57:16AM +0000, david laight wrote:
> On Fri, 21 Nov 2025 11:19:34 -0500
> Frank Li <Frank.li@....com> wrote:
> > On Mon, Nov 17, 2025 at 11:03:55AM +0800, carlos.song@....com wrote:

...

> > > +	/*
> > > +	 * t->len is 'unsigned' and txfifosize and watermrk is 'u8', force
> > > +	 * type cast is inevitable. When len > 255, len will be truncated in min_t(),
> > > +	 * it caused wrong watermark set. 'unsigned int' is as the designated type
> > > +	 * for min_t() to avoid truncation.
> > > +	 */
> > > +	fsl_lpspi->watermark = min_t(unsigned int,
> > >  				     fsl_lpspi->txfifosize,
> > >  				     t->len);  
> > 
> > There are thread discussion about min() and min_t()
> > 
> > https://lore.kernel.org/all/20251119224140.8616-1-david.laight.linux@gmail.com/
> 
> The big comment even carefully explains that the two types are unsigned ones.
> So a simple min() is absolutely fine (and the comment can go away).
> 
> The old typecheck in min was just so stupid.
> In this case the 'u8' variable is promoted to 'int' (they always are)
> and then converted to 'unsigned int' to match the other type.
> Even though there is an implicit 'int' => 'unsigned int' cast it is
> impossible for a negative value to become a large positive on
> (which is the only justification for the type check).
> 
> I'd check the file for other uses on min_t() as well.

Just came to this thread to echoing what David said. +1 to the above, please
convert to simple min(). The use cases for min_t() and max_t() should be rare
really.

-- 
With Best Regards,
Andy Shevchenko