linux-kernel - Re: [PATCH v2] drm/xe: Fix build error for XE_IOCTL

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Message-ID: <6um3enfackjotwj2ikhvpbnyq3ocro34b5iahx72yhzfxemjsn@xqnovtxzsapb>
Date: Fri, 1 Nov 2024 09:33:01 -0500
From: Lucas De Marchi <lucas.demarchi@...el.com>
To: Gyeyoung Baek <gye976@...il.com>
CC: Oded Gabbay <ogabbay@...nel.org>, Thomas Hellström
	<thomas.hellstrom@...ux.intel.com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>, <intel-xe@...ts.freedesktop.org>,
	<dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] drm/xe: Fix build error for XE_IOCTL_DBG macro

On Tue, Oct 29, 2024 at 05:48:58PM +0900, Gyeyoung Baek wrote:
>if CONFIG_DRM_USE_DYNAMIC_DEBUG is set,
>'drm_dbg' function is replaced with '__dynamic_func_call_cls',
>which is replaced with a do while statement.
>so in the previous code, there are the following build errors.
>
>include/linux/dynamic_debug.h:221:58: error: expected expression before ‘do’
>  221 | #define __dynamic_func_call_cls(id, cls, fmt, func, ...) do {   \
>      |                                                          ^~
>include/linux/dynamic_debug.h:248:9: note: in expansion of macro ‘__dynamic_func_call_cls’
>  248 |         __dynamic_func_call_cls(__UNIQUE_ID(ddebug), cls, fmt, func, ##__VA_ARGS__)
>      |         ^~~~~~~~~~~~~~~~~~~~~~~
>include/drm/drm_print.h:425:9: note: in expansion of macro ‘_dynamic_func_call_cls’
>  425 |         _dynamic_func_call_cls(cat, fmt, __drm_dev_dbg,         \
>      |         ^~~~~~~~~~~~~~~~~~~~~~
>include/drm/drm_print.h:504:9: note: in expansion of macro ‘drm_dev_dbg’
>  504 |         drm_dev_dbg((drm) ? (drm)->dev : NULL, DRM_UT_DRIVER, fmt, ##__VA_ARGS__)
>      |         ^~~~~~~~~~~
>include/drm/drm_print.h:522:33: note: in expansion of macro ‘drm_dbg_driver’
>  522 | #define drm_dbg(drm, fmt, ...)  drm_dbg_driver(drm, fmt, ##__VA_ARGS__)
>      |                                 ^~~~~~~~~~~~~~
>drivers/gpu/drm/xe/xe_macros.h:14:21: note: in expansion of macro ‘drm_dbg’
>   14 |         ((cond) && (drm_dbg(&(xe)->drm, \
>      |                     ^~~~~~~
>drivers/gpu/drm/xe/xe_bo.c:2029:13: note: in expansion of macro ‘XE_IOCTL_DBG’
> 2029 |         if (XE_IOCTL_DBG(xe, !gem_obj))
>
>the problem is that,
>XE_IOCTL_DBG uses this function for conditional expr.
>
>so I fix the expr to be compatible with the do while statement,
>by referring to "https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html".
>
>v2: I modified this to print when only cond is true.
>
>Signed-off-by: Gyeyoung Baek <gye976@...il.com>
>---
> drivers/gpu/drm/xe/xe_macros.h | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
>diff --git a/drivers/gpu/drm/xe/xe_macros.h b/drivers/gpu/drm/xe/xe_macros.h
>index daf56c846d03..ac2bd103bb22 100644
>--- a/drivers/gpu/drm/xe/xe_macros.h
>+++ b/drivers/gpu/drm/xe/xe_macros.h
>@@ -11,8 +11,12 @@
> #define XE_WARN_ON WARN_ON
>
> #define XE_IOCTL_DBG(xe, cond) \
>-	((cond) && (drm_dbg(&(xe)->drm, \
>-			    "Ioctl argument check failed at %s:%d: %s", \
>-			    __FILE__, __LINE__, #cond), 1))
>+({ \
>+	if ((cond)) \
>+		drm_dbg(&(xe)->drm, \
>+			"Ioctl argument check failed at %s:%d: %s", \
>+			__FILE__, __LINE__, #cond); \
>+	(cond); \

there's a double cond evaluation here and given any expression can be
given to XE_IOCTL_DBG(), this doens't look very safe. I think this would
be safer as:

#define XE_IOCTL_DBG(xe, cond) ({                                       \
         int cond__ = !!(cond);                                          \
         if (cond__)                                                     \
                 drm_dbg(&(xe)->drm,                                     \
                         "Ioctl argument check failed at %s:%d: %s",     \
                         __FILE__, __LINE__, #cond);                     \
         cond__;                                                         \
})

as it then evaluates cond just once. Also the generated code seems to be
sane compared to what we had before too.

And I also needed this to build-test:

| diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
| index 08cfea04e22bd..82585d442f017 100644
| --- a/drivers/gpu/drm/drm_print.c
| +++ b/drivers/gpu/drm/drm_print.c
| @@ -215,9 +215,8 @@ void __drm_printfn_dbg(struct drm_printer *p, struct va_format *vaf)
|  {
|         const struct drm_device *drm = p->arg;
|         const struct device *dev = drm ? drm->dev : NULL;
| -       enum drm_debug_category category = p->category;
|  
| -       if (!__drm_debug_enabled(category))
| +       if (!__drm_debug_enabled(p->category))
|                 return;
|  
|         __drm_dev_vprintk(dev, KERN_DEBUG, p->origin, p->prefix, vaf);

as otherwise it complains category is unused.

Lucas De Marchi