Re: [PATCH v3 3/3] console/fbcon: Add support for deferred console takeover

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Hi,

On 28-06-18 10:37, Daniel Vetter wrote:
On Thu, Jun 28, 2018 at 10:20 AM, Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
Hi,

On 28-06-18 09:55, Daniel Vetter wrote:

On Tue, Jun 26, 2018 at 08:36:12PM +0200, Hans de Goede wrote:

Currently fbcon claims fbdevs as soon as they are registered and takes
over
the console as soon as the first fbdev gets registered.

This behavior is undesirable in cases where a smooth graphical bootup is
desired, in such cases we typically want the contents of the framebuffer
(typically a vendor logo) to stay in place as is.

The current solution for this problem (on embedded systems) is to not
enable fbcon.

This commit adds a new FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER config
option,
which when enabled defers fbcon taking over the console from the dummy
console until the first text is displayed on the console. Together with
the
"quiet" kernel commandline option, this allows fbcon to still be used
together with a smooth graphical bootup, having it take over the console
as
soon as e.g. an error message is logged.

Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>


A bit a late comment, and feel free to reject: Have you considered
registering the fbcon driver, but not doing any modesets until the very
first real message shows up?


I have tried something like that, but it did not work out, this patch-set
actually is my 3th attempt at this (the other 2 were never posted
because they did not work).

The fbcon code is woven quite tightly into the console code, so this was
the only clean way I could find to achieve what I want.

I assumed/feared as much. Would be good to cover that in the commit
message, to justify your approach better.

Ok, v5 with an updated commit message (and without any code changes)
coming up.

Regards,

Hans



---
Changes in v2:
-Check the whole string when checking for erases in putcs, instead of
just
   the first char
-Make dummycon_blank return 1, so that a redraw gets triggered and any
text
   rendered while blanked gets output so that it can trigger a deferred
   takeover if one is pending

Changes in v3:
-Call WARN_CONSOLE_UNLOCKED() from fbcon_output_notifier()
-Unregister the notifier on fbcon_exit()
-Document the fbcon=nodefer commandline option in
Documentation/fb/fbcon.txt
---
   Documentation/fb/fbcon.txt       |  7 ++++
   drivers/video/console/Kconfig    | 11 +++++
   drivers/video/console/dummycon.c | 67 +++++++++++++++++++++++++----
   drivers/video/fbdev/core/fbcon.c | 72 ++++++++++++++++++++++++++++++++
   include/linux/console.h          |  5 +++
   5 files changed, 154 insertions(+), 8 deletions(-)

diff --git a/Documentation/fb/fbcon.txt b/Documentation/fb/fbcon.txt
index 79c22d096bbc..d4d642e1ce9c 100644
--- a/Documentation/fb/fbcon.txt
+++ b/Documentation/fb/fbcon.txt
@@ -155,6 +155,13 @@ C. Boot options
         used by text. By default, this area will be black. The 'color'
value
         is an integer number that depends on the framebuffer driver being
used.
   +6. fbcon=nodefer
+
+       If the kernel is compiled with deferred fbcon takeover support,
normally
+       the framebuffer contents, left in place by the
firmware/bootloader, will
+       be preserved until there actually is some text is output to the
console.
+       This option causes fbcon to bind immediately to the fbdev device.
+
   C. Attaching, Detaching and Unloading
     Before going on how to attach, detach and unload the framebuffer
console, an
diff --git a/drivers/video/console/Kconfig
b/drivers/video/console/Kconfig
index 4110ba7d7ca9..e91edef98633 100644
--- a/drivers/video/console/Kconfig
+++ b/drivers/video/console/Kconfig
@@ -150,6 +150,17 @@ config FRAMEBUFFER_CONSOLE_ROTATION
            such that other users of the framebuffer will remain normally
            oriented.
   +config FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+       bool "Framebuffer Console Deferred Takeover"
+       depends on FRAMEBUFFER_CONSOLE=y && DUMMY_CONSOLE=y
+       help
+         If enabled this defers the framebuffer console taking over the
+         console from the dummy console until the first text is
displayed on
+         the console. This is useful in combination with the "quiet"
kernel
+         commandline option to keep the framebuffer contents initially
put up
+         by the firmware in place, rather then replacing the contents
with a
+         black screen as soon as fbcon loads.
+
   config STI_CONSOLE
           bool "STI text console"
         depends on PARISC && HAS_IOMEM
diff --git a/drivers/video/console/dummycon.c
b/drivers/video/console/dummycon.c
index f2eafe2ed980..45ad925ad5f8 100644
--- a/drivers/video/console/dummycon.c
+++ b/drivers/video/console/dummycon.c
@@ -26,6 +26,65 @@
   #define DUMMY_ROWS    CONFIG_DUMMY_CONSOLE_ROWS
   #endif
   +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+/* These are both protected by the console_lock */
+static RAW_NOTIFIER_HEAD(dummycon_output_nh);
+static bool dummycon_putc_called;
+
+void dummycon_register_output_notifier(struct notifier_block *nb)
+{
+       raw_notifier_chain_register(&dummycon_output_nh, nb);
+
+       if (dummycon_putc_called)
+               nb->notifier_call(nb, 0, NULL);
+}
+
+void dummycon_unregister_output_notifier(struct notifier_block *nb)
+{
+       raw_notifier_chain_unregister(&dummycon_output_nh, nb);
+}
+
+static void dummycon_putc(struct vc_data *vc, int c, int ypos, int xpos)
+{
+       dummycon_putc_called = true;
+       raw_notifier_call_chain(&dummycon_output_nh, 0, NULL);
+}
+
+static void dummycon_putcs(struct vc_data *vc, const unsigned short *s,
+                          int count, int ypos, int xpos)
+{
+       int i;
+
+       if (!dummycon_putc_called) {
+               /* Ignore erases */
+               for (i = 0 ; i < count; i++) {
+                       if (s[i] != vc->vc_video_erase_char)
+                               break;
+               }
+               if (i == count)
+                       return;
+
+               dummycon_putc_called = true;
+       }
+
+       raw_notifier_call_chain(&dummycon_output_nh, 0, NULL);
+}
+
+static int dummycon_blank(struct vc_data *vc, int blank, int
mode_switch)
+{
+       /* Redraw, so that we get putc(s) for output done while blanked
*/
+       return 1;
+}
+#else
+static void dummycon_putc(struct vc_data *vc, int c, int ypos, int xpos)
{ }
+static void dummycon_putcs(struct vc_data *vc, const unsigned short *s,
+                          int count, int ypos, int xpos) { }
+static int dummycon_blank(struct vc_data *vc, int blank, int
mode_switch)
+{
+       return 0;
+}
+#endif
+
   static const char *dummycon_startup(void)
   {
       return "dummy device";
@@ -44,9 +103,6 @@ static void dummycon_init(struct vc_data *vc, int
init)
   static void dummycon_deinit(struct vc_data *vc) { }
   static void dummycon_clear(struct vc_data *vc, int sy, int sx, int
height,
                            int width) { }
-static void dummycon_putc(struct vc_data *vc, int c, int ypos, int xpos)
{ }
-static void dummycon_putcs(struct vc_data *vc, const unsigned short *s,
-                          int count, int ypos, int xpos) { }
   static void dummycon_cursor(struct vc_data *vc, int mode) { }
     static bool dummycon_scroll(struct vc_data *vc, unsigned int top,
@@ -61,11 +117,6 @@ static int dummycon_switch(struct vc_data *vc)
         return 0;
   }
   -static int dummycon_blank(struct vc_data *vc, int blank, int
mode_switch)
-{
-       return 0;
-}
-
   static int dummycon_font_set(struct vc_data *vc, struct console_font
*font,
                              unsigned int flags)
   {
diff --git a/drivers/video/fbdev/core/fbcon.c
b/drivers/video/fbdev/core/fbcon.c
index cd8d52a967aa..5fb156bdcf4e 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -129,6 +129,12 @@ static inline void fbcon_map_override(void)
   }
   #endif /* CONFIG_FRAMEBUFFER_CONSOLE_DETECT_PRIMARY */
   +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+static bool deferred_takeover = true;
+#else
+#define deferred_takeover false
+#endif
+
   /* font data */
   static char fontname[40];
   @@ -499,6 +505,12 @@ static int __init fb_console_setup(char *this_opt)
                                 margin_color = simple_strtoul(options,
&options, 0);
                         continue;
                 }
+#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+               if (!strcmp(options, "nodefer")) {
+                       deferred_takeover = false;
+                       continue;
+               }
+#endif
         }
         return 1;
   }
@@ -3100,6 +3112,9 @@ static int fbcon_fb_unregistered(struct fb_info
*info)
         WARN_CONSOLE_UNLOCKED();
   +     if (deferred_takeover)
+               return 0;
+
         idx = info->node;
         for (i = first_fb_vc; i <= last_fb_vc; i++) {
                 if (con2fb_map[i] == idx)
@@ -3140,6 +3155,13 @@ static void fbcon_remap_all(int idx)
         WARN_CONSOLE_UNLOCKED();
   +     if (deferred_takeover) {
+               for (i = first_fb_vc; i <= last_fb_vc; i++)
+                       con2fb_map_boot[i] = idx;
+               fbcon_map_override();
+               return;
+       }
+
         for (i = first_fb_vc; i <= last_fb_vc; i++)
                 set_con2fb_map(i, idx, 0);
   @@ -3191,6 +3213,11 @@ static int fbcon_fb_registered(struct fb_info
*info)
         idx = info->node;
         fbcon_select_primary(info);
   +     if (deferred_takeover) {
+               pr_info("fbcon: Deferring console take-over\n");
+               return 0;
+       }
+
         if (info_idx == -1) {
                 for (i = first_fb_vc; i <= last_fb_vc; i++) {
                         if (con2fb_map_boot[i] == idx) {
@@ -3566,8 +3593,46 @@ static int fbcon_init_device(void)
         return 0;
   }
   +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+static struct notifier_block fbcon_output_nb;
+
+static int fbcon_output_notifier(struct notifier_block *nb,
+                                unsigned long action, void *data)
+{
+       int i;
+
+       WARN_CONSOLE_UNLOCKED();
+
+       pr_info("fbcon: Taking over console\n");
+
+       dummycon_unregister_output_notifier(&fbcon_output_nb);
+       deferred_takeover = false;
+       logo_shown = FBCON_LOGO_DONTSHOW;
+
+       for (i = 0; i < FB_MAX; i++) {
+               if (registered_fb[i])
+                       fbcon_fb_registered(registered_fb[i]);
+       }
+
+       return NOTIFY_OK;
+}
+
+static void fbcon_register_output_notifier(void)
+{
+       fbcon_output_nb.notifier_call = fbcon_output_notifier;
+       dummycon_register_output_notifier(&fbcon_output_nb);
+}
+#else
+static inline void fbcon_register_output_notifier(void) {}
+#endif
+
   static void fbcon_start(void)
   {
+       if (deferred_takeover) {
+               fbcon_register_output_notifier();
+               return;
+       }
+
         if (num_registered_fb) {
                 int i;
   @@ -3594,6 +3659,13 @@ static void fbcon_exit(void)
         if (fbcon_has_exited)
                 return;
   +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
+       if (deferred_takeover) {
+               dummycon_unregister_output_notifier(&fbcon_output_nb);
+               deferred_takeover = false;
+       }
+#endif
+
         kfree((void *)softback_buf);
         softback_buf = 0UL;
   diff --git a/include/linux/console.h b/include/linux/console.h
index dfd6b0e97855..f59f3dbca65c 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -21,6 +21,7 @@ struct console_font_op;
   struct console_font;
   struct module;
   struct tty_struct;
+struct notifier_block;
     /*
    * this is what the terminal answers to a ESC-Z or csi0c query.
@@ -220,4 +221,8 @@ static inline bool vgacon_text_force(void) { return
false; }
     extern void console_init(void);
   +/* For deferred console takeover */
+void dummycon_register_output_notifier(struct notifier_block *nb);
+void dummycon_unregister_output_notifier(struct notifier_block *nb);
+
   #endif /* _LINUX_CONSOLE_H */
--
2.17.1

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