linux-kernel - [PATCH] staging: pi433: enforce tx_cfg to be set before any message can be sent

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Date:   Sun, 9 Jan 2022 11:26:52 +1300
From:   Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
To:     gregkh@...uxfoundation.org, paulo.miguel.almeida.rodenas@...il.com,
        realwakka@...il.com
Cc:     linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: [PATCH] staging: pi433: enforce tx_cfg to be set before any message
 can be sent

this driver relies on exposing a char device to userspace to tx
messages. Every message can be sent using different trasmitter settings
such so the tx_cfg must be written before sending any messages.
Failing to do so will cause the message to fail silently depending on
printk/dynamic_debug settings which makes it hard to troubleshoot.

This patch add a control variable that will get initialized once tx_cfg
is set for the fd used when interacting with the char device.

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
---
Patch dependency: 
- This patch needs the patch below to be applied first as both tweak the
  same file.
https://lore.kernel.org/lkml/20220108212728.GA7784@mail.google.com/

meta-comments:
- I'm not entirely sure if -EPERM is the best error code to return here,
  I'm taking suggestions
---
 drivers/staging/pi433/pi433_if.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/pi433/pi433_if.c b/drivers/staging/pi433/pi433_if.c
index 051c9052aeeb..3e9913f4bc7d 100644
--- a/drivers/staging/pi433/pi433_if.c
+++ b/drivers/staging/pi433/pi433_if.c
@@ -108,6 +108,9 @@ struct pi433_device {
 struct pi433_instance {
 	struct pi433_device	*device;
 	struct pi433_tx_cfg	tx_cfg;
+
+	/* control flags */
+	bool			tx_cfg_initialized;
 };
 
 /*-------------------------------------------------------------------------*/
@@ -823,6 +826,16 @@ pi433_write(struct file *filp, const char __user *buf,
 	if (count > MAX_MSG_SIZE)
 		return -EMSGSIZE;
 
+	/*
+	 * check if tx_cfg has been initialized otherwise we won't be able to
+	 * config the RF trasmitter correctly due to invalid settings
+	 */
+	if (!instance->tx_cfg_initialized) {
+		dev_dbg(device->dev,
+			"write: failed due to uninitialized tx_cfg");
+		return -EPERM;
+	}
+
 	/*
 	 * write the following sequence into fifo:
 	 * - tx_cfg
@@ -897,6 +910,7 @@ pi433_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 			return -EFAULT;
 		mutex_lock(&device->tx_fifo_lock);
 		memcpy(&instance->tx_cfg, &tx_cfg, sizeof(struct pi433_tx_cfg));
+		instance->tx_cfg_initialized = true;
 		mutex_unlock(&device->tx_fifo_lock);
 		break;
 	case PI433_IOC_RD_RX_CFG:
@@ -950,7 +964,7 @@ static int pi433_open(struct inode *inode, struct file *filp)
 	/* setup instance data*/
 	instance->device = device;
 	instance->tx_cfg.bit_rate = 4711;
-	// TODO: fill instance->tx_cfg;
+	instance->tx_cfg_initialized = false;
 
 	/* instance data as context */
 	filp->private_data = instance;
-- 
2.25.4