[PATCH v4] clocksource: timer-sp804: Fix read_current_timer() issue when clock source is not registered

[Stephen Eta Zhou <stephen.eta.zhou@gmail.com>]

Register a valid read_current_timer() function for the
SP804 timer on ARM32.

On ARM32 platforms, when the SP804 timer is selected as the clocksource,
the driver does not register a valid read_current_timer() function.
As a result, features that rely on this API—such as rdseed—consistently
return incorrect values.

To fix this, a delay_timer structure is registered during the SP804
driver's initialization. The read_current_timer() function is implemented
using the existing sp804_read() logic, and the timer frequency is reused
from the already-initialized clocksource.

Signed-off-by: Stephen Eta Zhou <stephen.eta.zhou@gmail.com>
---
Changes in v4:
- Dropped redundant `delay.freq = rate;` assignment in `sp804_clocksource_and_sched_clock_init()`
- Dropped redundant `delay.read_current_timer` and `register_current_timer_delay()` lines in `sp804_of_init()`
- No functional changes to the driver logic; these lines were unnecessary as per Daniel's feedback.
- Link to v3: https://lore.kernel.org/all/20250414-sp804-fix-read_current_timer-v3-1-53b3e80d7183@gmail.com

Changes in v3:
- Updated the commit message for clarity and structure
- Link to v2: https://lore.kernel.org/all/BYAPR12MB3205D7A2BAA2712C89E03C4FD5D42@BYAPR12MB3205.namprd12.prod.outlook.com

Changes in v2:
- Added static keyword to struct delay_timer delay
- Integrate sp804_read_delay_timer_read and
  struct delay_timer delay together
- I moved the acquisition of delay.freq to
  sp804_clocksource_and_sched_clock_init.
  sp804_clocksource_and_sched_clock_init has already
  acquired and judged freq, so I can use it directly,
  and in this way I don’t need to consider whether to
  use clk1 or clk2, which can ensure that the clock source
  is available and reliable.
- Added detailed description information in Commit
- Link to v1: https://lore.kernel.org/all/BYAPR12MB3205C9C87EB560CA0CC4984BD5FB2@BYAPR12MB3205.namprd12.prod.outlook.com
---
 drivers/clocksource/timer-sp804.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/clocksource/timer-sp804.c b/drivers/clocksource/timer-sp804.c
index cd1916c0532507fb3ce7a11bfab4815906e326d5..e82a95ea472478ae096b2bf7abea0d65a7bca480 100644
--- a/drivers/clocksource/timer-sp804.c
+++ b/drivers/clocksource/timer-sp804.c
@@ -21,6 +21,10 @@
 #include <linux/of_irq.h>
 #include <linux/sched_clock.h>
 
+#ifdef CONFIG_ARM
+#include <linux/delay.h>
+#endif
+
 #include "timer-sp.h"
 
 /* Hisilicon 64-bit timer(a variant of ARM SP804) */
@@ -102,6 +106,23 @@ static u64 notrace sp804_read(void)
 	return ~readl_relaxed(sched_clkevt->value);
 }
 
+#ifdef CONFIG_ARM
+static struct delay_timer delay;
+static unsigned long sp804_read_delay_timer_read(void)
+{
+	return sp804_read();
+}
+
+static void sp804_register_delay_timer(int freq)
+{
+	delay.freq = freq;
+	delay.read_current_timer = sp804_read_delay_timer_read;
+	register_current_timer_delay(&delay);
+}
+#else
+static inline void sp804_register_delay_timer(int freq) {}
+#endif
+
 static int __init sp804_clocksource_and_sched_clock_init(void __iomem *base,
 							 const char *name,
 							 struct clk *clk,
@@ -114,6 +135,8 @@ static int __init sp804_clocksource_and_sched_clock_init(void __iomem *base,
 	if (rate < 0)
 		return -EINVAL;
 
+	sp804_register_delay_timer(rate);
+
 	clkevt = sp804_clkevt_get(base);
 
 	writel(0, clkevt->ctrl);
@@ -318,6 +341,7 @@ static int __init sp804_of_init(struct device_node *np, struct sp804_timer *time
 		if (ret)
 			goto err;
 	}
+
 	initialized = true;
 
 	return 0;

---
base-commit: 7ee983c850b40043ac4751836fbd9a2b4d0c5937
change-id: 20250411-sp804-fix-read_current_timer-1c7c5a448c6c

Best regards,
-- 
Stephen Eta Zhou <stephen.eta.zhou@gmail.com>