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

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Date:   Wed, 13 Oct 2021 11:35:24 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        Robert Jarzmik <robert.jarzmik@...e.fr>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Rikard Falkeborn <rikard.falkeborn@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] [RFC] usb: gadget: avoid unusual inline assembly

On Mon, Sep 27, 2021 at 5:38 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.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@...db.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