linux-kernel - Re: [PATCH v2 2/4] usb: typec: tps6598x: Add trace event for status register

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Message-ID: <YCpbtV8TyIuCmy+4@bogon.m.sigxcpu.org>
Date:   Mon, 15 Feb 2021 12:32:05 +0100
From:   Guido Günther <agx@...xcpu.org>
To:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ramsay Jones <ramsay@...sayjones.plus.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-usb@...r.kernel.org,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] usb: typec: tps6598x: Add trace event for status
 register

Hi,
On Sun, Feb 14, 2021 at 09:41:27PM +0100, Luc Van Oostenryck wrote:
> On Sun, Feb 14, 2021 at 11:00:48AM -0800, Linus Torvalds wrote:
> > On Sun, Feb 14, 2021 at 10:42 AM Ramsay Jones
> > <ramsay@...sayjones.plus.com> wrote:
> > >
> > > >
> > > > I looked around but didn't find any hints how to fix this. Any pointers
> > > > I missed (added the sparse list to cc:)?
> > >
> > > This is a limitation of sparse; when using the 'stringize' pre-processor
> > > operator #, the maximum size of the resulting string is about 8k (if I
> > > remember correctly).
> > 
> > Well, yes and no.
> > 
> > The C89 standard actually says that a string literal can be at most
> > 509 characters to be portable. C99 increased it to 4095 characters.
> > 
> > Sparse makes the limit higher, and the limit could easily be expanded
> > way past 8kB - but the point is that large string literals are
> > actually not guaranteed to be valid C.
> > 
> > So honestly, it really sounds like that TRACE_EVENT() thing is doing
> > something it shouldn't be doing.
> 
> In itself, it's OKish but it does a lot of macro expansions and most
> arguments are macros of macros of ... but the problem seems to be
> limited to TP_printk().
> 
> In the current case, the offender is the string 'print_fmt_tps6598x_status'
> which is just under 26K long especially because it expand
> TPS6598X_STATUS_FLAGS_MASK but also because the arguments use FIELD_GET()
> and thus __BF_FIELD_CHECK().

That was a great hint! Using a custom FIELD_GET() that drops the
__BF_FIELD_CHECK() makes things fit.
Cheers,
 -- Guido

> > 
> > I don't think there's any fundamental limit why sparse does 8kB as a
> > limit (just a few random buffers). Making sparse accept larger ones
> > should be as simple as just increasing MAX_STRING, but I really don't
> > think the kernel should encourage that kind of excessive string sizes.
> 
> Like you noted, there are just a few cases in the kernel and IIRC
> there is or was one case in it too.
> I would tend to increase MAX_STRING to something like 32 or 64K,
> in order to keep it reasonable but let sparse to continue its processing,
> but add a warning when the string/token is bigger than the current 8K.
> 
> -- Luc
>