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Date: Mon, 22 Apr 2024 21:28:58 +0200
From: "Arnd Bergmann" <arnd@kernel.org>
To: "Niklas Schnelle" <schnelle@linux.ibm.com>,
 "Helge Deller" <deller@kernel.org>
Cc: "Helge Deller" <deller@gmx.de>, linux-fbdev@vger.kernel.org,
 dri-devel@lists.freedesktop.org, "Heiko Carstens" <hca@linux.ibm.com>,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] video: Handle HAS_IOPORT dependencies
Content-Type: text/plain

On Mon, Apr 22, 2024, at 10:34, Niklas Schnelle wrote:
> On Thu, 2024-04-11 at 16:00 +0200, Helge Deller wrote:
>> * Niklas Schnelle <schnelle@linux.ibm.com>:
>> > In a future patch HAS_IOPORT=n will disable inb()/outb() and friends at
>> > compile time. We thus need to #ifdef functions and their callsites which
>> > unconditionally use these I/O accessors. In the include/video/vga.h
>> > these are conveniently all those functions with the vga_io_* prefix.
>> 
>> Why don't you code it like in the patch below?
>> inb_p(), outb_p() and outw() would then need to be defined externally
>> without an implementation so that they would generate link time errors
>> (instead of compile time errors).
>
> This may be personal preference but I feel like link time errors would
> be very late to catch a configuration that can't work. Also this would
> bypass the __compiletime_error("inb()) requires CONFIG_HAS_IOPORT");
> added instead of the in*()/out*() helpers to make it easy to spot the
> problem.
>
> I'm not a fan of #ifdeffery either but I think in this case it is
> simple, well enough contained and overall there aren't that many spots
> where we need to exclude just some sections of code vs entire drivers
> with vga.h probably being the worst of them all.

Agreed. I also tried to see if we can move stuff out of vga.h
to have it included in fewer places, as almost everything that
uses this header already has a HAS_IOPORT dependency, but that
would be a lot more work.

The other one that gains a few ugly #ifdefs is the 8250 driver,
everything else is already merged in linux-next or needs a simple
Kconfig dependency.

I think we can make the vga.h file a little more readable
by duplicating the functions and still keep the __compiletime_error()
version in asm/io.h, see below.

    Arnd


diff --git a/include/video/vga.h b/include/video/vga.h
index 947c0abd04ef..7e1d8252b732 100644
--- a/include/video/vga.h
+++ b/include/video/vga.h
@@ -197,6 +197,23 @@ struct vgastate {
 extern int save_vga(struct vgastate *state);
 extern int restore_vga(struct vgastate *state);
 
+static inline unsigned char vga_mm_r (void __iomem *regbase, unsigned short port)
+{
+	return readb (regbase + port);
+}
+
+static inline void vga_mm_w (void __iomem *regbase, unsigned short port, unsigned char val)
+{
+	writeb (val, regbase + port);
+}
+
+static inline void vga_mm_w_fast (void __iomem *regbase, unsigned short port,
+				  unsigned char reg, unsigned char val)
+{
+	writew (VGA_OUT16VAL (val, reg), regbase + port);
+}
+
+#ifdef CONFIG_HAS_IOPORT
 /*
  * generic VGA port read/write
  */
@@ -217,22 +234,6 @@ static inline void vga_io_w_fast (unsigned short port, unsigned char reg,
 	outw(VGA_OUT16VAL (val, reg), port);
 }
 
-static inline unsigned char vga_mm_r (void __iomem *regbase, unsigned short port)
-{
-	return readb (regbase + port);
-}
-
-static inline void vga_mm_w (void __iomem *regbase, unsigned short port, unsigned char val)
-{
-	writeb (val, regbase + port);
-}
-
-static inline void vga_mm_w_fast (void __iomem *regbase, unsigned short port,
-				  unsigned char reg, unsigned char val)
-{
-	writew (VGA_OUT16VAL (val, reg), regbase + port);
-}
-
 static inline unsigned char vga_r (void __iomem *regbase, unsigned short port)
 {
 	if (regbase)
@@ -258,7 +259,25 @@ static inline void vga_w_fast (void __iomem *regbase, unsigned short port,
 	else
 		vga_io_w_fast (port, reg, val);
 }
+#else
 
+static inline unsigned char vga_r (void __iomem *regbase, unsigned short port)
+{
+	return vga_mm_r(regbase, port);
+}
+
+static inline void vga_w(void __iomem *regbase, unsigned short port, unsigned char val)
+{
+	vga_mm_w (regbase, port, val);
+}
+
+static inline void vga_w_fast (void __iomem *regbase, unsigned short port,
+			       unsigned char reg, unsigned char val)
+{
+	vga_mm_w_fast(regbase, port, reg, val);
+}
+
+#endif
 
 /*
  * VGA CRTC register read/write