linux-kernel - Re: [PATCH] spi: Raise limit on number of chip selects

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Message-ID: <CAOiHx==FzSyyqP3NzLTeOSVxUQYy3ZhypZrDLsc-OjGCdSzvUA@mail.gmail.com>
Date: Tue, 23 Jan 2024 17:50:22 +0100
From: Jonas Gorski <jonas.gorski@...il.com>
To: Mark Brown <broonie@...nel.org>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>, Herve Codina <herve.codina@...tlin.com>, 
	Amit Kumar Mahapatra <amit.kumar-mahapatra@....com>, Guenter Roeck <linux@...ck-us.net>, 
	"linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH] spi: Raise limit on number of chip selects

Hi,

On Tue, 23 Jan 2024 at 14:56, Mark Brown <broonie@...nel.org> wrote:
>
> On Tue, Jan 23, 2024 at 01:26:04PM +0000, Christophe Leroy wrote:
> > Le 23/01/2024 à 14:18, Mark Brown a écrit :
> > > On Tue, Jan 23, 2024 at 12:04:30PM +0100, Herve Codina wrote:
>
> > >> Moving the SPI_CS_CNT_MAX value from 4 to 8 is not enough to handle my case.
> > >> Tested moving SPI_CS_CNT_MAX to 16 and it was ok.
>
> > > OK, I've also heard 12 as a number which this would cover.
>
> > By the way the comment in include/linux/spi/spi.h is confusing. This
> > SPI_CS_CNT_MAX is really not the max number of CS supported per SPI
> > device but the max number of CS supported per SPI controller.
>
> Well, it's a combination of the comment being confusing and the
> implementation being a bit broken - we simply shouldn't be limiting the
> number of chip selects per controller, the per device limit is much more
> reasonable.  So ideally the code would be changed to reflect the
> comment.

At a first glance at all places using SPI_CS_CNT_MAX I don't see
anything being broken / reading out of bounds if a controller has more
chipselects than SPI_CS_CNT_MAX.

So I think the check of ctrl->num_chipselect in of_spi_parse_dt() is
bogus/unnecessary and is in the wrong place, as this is for parsing a
spi device node and not a controller node. The following check for the
amount of chip selects defined for the spi device should just check
against SPI_CS_CNT_MAX instead of ctrl->num_chipselects.
__spi_add_device() later will ensure that any chip selects are valid
chip selects, so no need for of_spi_parse_dt() to check that either.

But I didn't do a very thorough read, or even tested it, so I might
have easily missed something.

Best Regards,
Jonas