[PATCH v4 6/8] wifi: brcmfmac: pcie: replace msleep polling with usleep_range and backoff

[Shivesh <chanelshivesh@gmail.com>]

Two latency improvements:

1. H2D mailbox polling
   brcmf_pcie_send_mb_data() polled for the dongle to consume the
   previous H2D mailbox message with msleep(10) per iteration. As the
   dongle typically clears the register within a few hundred
   microseconds, each call incurred at least 10ms of unnecessary
   latency (often 20ms+ due to jiffy rounding).

   Replace with usleep_range() using exponential backoff: start at
   ~50us and double each iteration up to 5ms, with a 1-second absolute
   timeout matching the original budget. Add an early-exit check for
   BRCMFMAC_PCIE_STATE_DOWN so a dead dongle does not hold the caller
   for a full second.

2. IRQ teardown polling
   brcmf_pcie_release_irq() waited for in_irq to clear using
   msleep(50) in a 20-iteration loop (up to 1 second). Replace with
   usleep_range(1000, 2000) in a 1000-iteration loop, preserving the
   same ~1 second maximum while allowing the function to return in
   microseconds when the IRQ handler finishes quickly.

Fixes: 9e37f045d5e7 ("brcmfmac: Adding PCIe bus layer support.")
Signed-off-by: Shivesh <chanelshivesh@gmail.com>
---
 .../broadcom/brcm80211/brcmfmac/pcie.c        | 66 ++++++++++++++++---
 1 file changed, 58 insertions(+), 8 deletions(-)

diff --git a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/pcie.c b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/pcie.c
index 13662aa4b4ea..9338a5faa260 100644
--- a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/pcie.c
+++ b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/pcie.c
@@ -268,6 +268,27 @@ static const struct brcmf_firmware_mapping brcmf_pcie_fwnames[] = {
 
 #define BRCMF_PCIE_MBDATA_TIMEOUT		msecs_to_jiffies(2000)
 
+/*
+ * H2D mailbox poll timing parameters.
+ *
+ * The dongle typically clears the H2D mailbox register within a few
+ * hundred microseconds after the doorbell interrupt fires.  The
+ * original code used msleep(10) * 100 iterations, meaning the
+ * minimum observable latency was 10ms even when the dongle was fast.
+ *
+ * We instead start with a short sleep and double it each iteration
+ * (exponential backoff) up to BRCMF_PCIE_MB_POLL_MAX_US, staying
+ * within the same 1-second absolute timeout.
+ *
+ * MIN_US / INITIAL_MAX_US : usleep_range bounds for the first iteration.
+ * MAX_US     : cap on the per-iteration sleep (µs).
+ * TIMEOUT_US : total budget before giving up (1 second).
+ */
+#define BRCMF_PCIE_MB_POLL_MIN_US		40
+#define BRCMF_PCIE_MB_POLL_INITIAL_MAX_US	50
+#define BRCMF_PCIE_MB_POLL_MAX_US		5000
+#define BRCMF_PCIE_MB_POLL_TIMEOUT_US		1000000
+
 #define BRCMF_PCIE_CFGREG_STATUS_CMD		0x4
 #define BRCMF_PCIE_CFGREG_PM_CSR		0x4C
 #define BRCMF_PCIE_CFGREG_MSI_CAP		0x58
@@ -766,7 +787,8 @@ brcmf_pcie_send_mb_data(struct brcmf_pciedev_info *devinfo, u32 htod_mb_data)
 	struct brcmf_core *core;
 	u32 addr;
 	u32 cur_htod_mb_data;
-	u32 i;
+	u32 elapsed_us = 0;
+	u32 sleep_us = BRCMF_PCIE_MB_POLL_INITIAL_MAX_US;
 
 	shared = &devinfo->shared;
 	addr = shared->htod_mb_data_addr;
@@ -776,12 +798,40 @@ brcmf_pcie_send_mb_data(struct brcmf_pciedev_info *devinfo, u32 htod_mb_data)
 		brcmf_dbg(PCIE, "MB transaction is already pending 0x%04x\n",
 			  cur_htod_mb_data);
 
-	i = 0;
+	/*
+	 * Wait for the dongle to consume the previous H2D mailbox message.
+	 *
+	 * There is no interrupt that signals when the dongle clears this
+	 * register, so polling is unavoidable.  The original code used
+	 * msleep(10) per iteration, incurring at least 10ms of latency
+	 * even when the dongle responded in microseconds.
+	 *
+	 * We use usleep_range() with exponential backoff instead:
+	 *   - First iteration sleeps ~50µs (fast path for responsive dongle).
+	 *   - Each subsequent iteration doubles the sleep, capped at 5ms,
+	 *     so long waits still yield the CPU without busy-spinning.
+	 *   - Total timeout matches the original 1-second limit.
+	 *   - We bail early if the device has gone down so that a dead
+	 *     dongle does not hold the caller for a full second.
+	 */
 	while (cur_htod_mb_data != 0) {
-		msleep(10);
-		i++;
-		if (i > 100)
+		if (devinfo->state == BRCMFMAC_PCIE_STATE_DOWN) {
+			brcmf_dbg(PCIE, "Device down, aborting MB send\n");
 			return -EIO;
+		}
+
+		if (elapsed_us >= BRCMF_PCIE_MB_POLL_TIMEOUT_US) {
+			brcmf_err("Timeout waiting for H2D MB slot after %u us\n",
+				  elapsed_us);
+			return -EIO;
+		}
+
+		usleep_range(BRCMF_PCIE_MB_POLL_MIN_US, sleep_us);
+		elapsed_us += sleep_us;
+
+		/* Exponential backoff, capped at BRCMF_PCIE_MB_POLL_MAX_US */
+		sleep_us = min(sleep_us * 2, (u32)BRCMF_PCIE_MB_POLL_MAX_US);
+
 		cur_htod_mb_data = brcmf_pcie_read_tcm32(devinfo, addr);
 	}
 
@@ -1001,10 +1051,10 @@ static void brcmf_pcie_release_irq(struct brcmf_pciedev_info *devinfo)
 	free_irq(pdev->irq, devinfo);
 	pci_disable_msi(pdev);
 
-	msleep(50);
+	usleep_range(1000, 2000);
 	count = 0;
-	while ((devinfo->in_irq) && (count < 20)) {
-		msleep(50);
+	while ((devinfo->in_irq) && (count < 1000)) {
+		usleep_range(1000, 2000);
 		count++;
 	}
 	if (devinfo->in_irq)
-- 
2.53.0