Re: [PATCH] [RFC] usb: gadget: avoid unusual inline assembly

From: Nick Desaulniers <ndesaulniers@google.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Daniel Mack <daniel@zonque.org>,
	Haojian Zhuang <haojian.zhuang@gmail.com>,
	Robert Jarzmik <robert.jarzmik@free.fr>,
	Felipe Balbi <balbi@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Rikard Falkeborn <rikard.falkeborn@gmail.com>,
	linux-arm-kernel@lists.infradead.org, linux-usb@vger.kernel.org,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH] [RFC] usb: gadget: avoid unusual inline assembly
Date: Wed, 13 Oct 2021 11:35:24 -0700	[thread overview]
Message-ID: <CAKwvOd=rFUfuTgK14y__3OQdgwzBCLp+ZVUUqsXfOmSCe1M8Mg@mail.gmail.com> (raw)
In-Reply-To: <20210927123830.1278953-1-arnd@kernel.org>

On Mon, Sep 27, 2021 at 5:38 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> clang does not understand the "mrc%?" syntax:
>
> drivers/usb/gadget/udc/pxa25x_udc.c:2330:11: error: invalid % escape in inline assembly string
>
> I don't understand it either, but removing the %? here gets it to build.
> This is probably wrong and someone else should do a proper patch.
>
> Any suggestions?

https://gcc.gnu.org/onlinedocs/gccint/Output-Template.html says:
>> ‘%’ followed by a punctuation character specifies a substitution that does not use an operand. Only one case is standard: ‘%%’ outputs a ‘%’ into the assembler code.

I don't think the intention was to have `mrc?` in the emitted
assembler (is that even valid?), so it's not clear to me what this
point of `%?` was. Patch LGTM.

>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/usb/gadget/udc/pxa25x_udc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/usb/gadget/udc/pxa25x_udc.c b/drivers/usb/gadget/udc/pxa25x_udc.c
> index a09ec1d826b2..52cdfd8212d6 100644
> --- a/drivers/usb/gadget/udc/pxa25x_udc.c
> +++ b/drivers/usb/gadget/udc/pxa25x_udc.c
> @@ -2325,7 +2325,7 @@ static int pxa25x_udc_probe(struct platform_device *pdev)
>         pr_info("%s: version %s\n", driver_name, DRIVER_VERSION);
>
>         /* insist on Intel/ARM/XScale */
> -       asm("mrc%? p15, 0, %0, c0, c0" : "=r" (chiprev));
> +       asm("mrc p15, 0, %0, c0, c0" : "=r" (chiprev));
>         if ((chiprev & CP15R0_VENDOR_MASK) != CP15R0_XSCALE_VALUE) {
>                 pr_err("%s: not XScale!\n", driver_name);
>                 return -ENODEV;
> --
> 2.29.2
>

-- 
Thanks,
~Nick Desaulniers