linux-kernel - Re: [PATCH v4 char-misc-next] misc: microchip: pci1xxxx: Add OTP/EEPROM driver for the pci1xxxx switch

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Date:   Thu, 9 Feb 2023 09:55:02 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tharun Kumar P <tharunkumar.pasumarthi@...rochip.com>
Cc:     arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-gpio@...r.kernel.org, UNGLinuxDriver@...rochip.com
Subject: Re: [PATCH v4 char-misc-next] misc: microchip: pci1xxxx: Add
 OTP/EEPROM driver for the pci1xxxx switch

On Thu, Feb 09, 2023 at 10:12:37AM +0530, Tharun Kumar P wrote:
> +static int e2p_device_write_byte(struct pci1xxxx_otp_e2p_device *priv,
> +				 unsigned long byte_offset, u8 value)
> +{
> +	u32 data;
> +
> +	/* Write the value into EEPROM_DATA_REG register */
> +	writel(value, priv->reg_base + MMAP_EEPROM_OFFSET(EEPROM_DATA_REG));
> +	data = EEPROM_CMD_EPC_TIMEOUT_BIT | EEPROM_CMD_EPC_WRITE | byte_offset;
> +
> +	/* Write the data into EEPROM_CMD_REG register */
> +	writel(data, priv->reg_base + MMAP_EEPROM_OFFSET(EEPROM_CMD_REG));
> +
> +	/* Set the EPC_BUSY bit of EEPROM_CMD_REG register */
> +	writel(EEPROM_CMD_EPC_BUSY_BIT | data, priv->reg_base +
> +	       MMAP_EEPROM_OFFSET(EEPROM_CMD_REG));
> +
> +	/* Wait for the EPC_BUSY bit to get cleared */
> +	do {
> +		data = readl(priv->reg_base + MMAP_EEPROM_OFFSET(EEPROM_CMD_REG));
> +	} while (data & EEPROM_CMD_EPC_BUSY_BIT);

That's a very busy "sit and spin" loop here, what happens if the read of
the bit never actually succeeds?  You just locked up the system with no
way to interrupt it :(

Please provide some sort of timeout, or way to break out of this.

> +
> +	if (data & EEPROM_CMD_EPC_TIMEOUT_BIT) {
> +		dev_err(&priv->pdev->dev, "EEPROM write timed out\n");

How can the timeout bit happen if the busy bit was still set?

And what can userspace do about this if it is reported?

> +		return -EFAULT;

This return value is ONLY for when we have memory faults from reading
to/from userspace and the kernel.  It's not a valid return value for a
device error, sorry.  -EIO maybe?

You return this error in a number of other places in the driver that
shouldn't, please fix this up.

Also the loop issue is in your read_byte call and the same review
comments apply there.

thanks,

greg k-h