Re: [PATCH v3 2/5] serial: mvebu-uart: implement UART clock driver for configuring UART base clock

["Pali Rohár" <pali@kernel.org>]

On Saturday 17 July 2021 20:05:40 Pali Rohár wrote:
> On Saturday 17 July 2021 19:26:51 Andrew Lunn wrote:
> > On Sat, Jul 17, 2021 at 02:38:26PM +0200, Pali Rohár wrote:
> > > @@ -445,6 +472,7 @@ static void mvebu_uart_shutdown(struct uart_port *port)
> > >  static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud)
> > >  {
> > >  	unsigned int d_divisor, m_divisor;
> > > +	unsigned long flags;
> > >  	u32 brdv, osamp;
> > >  
> > >  	if (!port->uartclk)
> > > @@ -463,10 +491,12 @@ static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud)
> > >  	m_divisor = OSAMP_DEFAULT_DIVISOR;
> > >  	d_divisor = DIV_ROUND_CLOSEST(port->uartclk, baud * m_divisor);
> > >  
> > > +	spin_lock_irqsave(&mvebu_uart_lock, flags);
> > 
> > Hi Pali
> > 
> > You only need spin_lock_irqsave() if you plan on taking the spinlock
> > in an interrupt handler. It seems unlikely the baud rate will be
> > changed in interrupt context? Please check, and then swap to plain
> > spin_lock().
> 
> Hello! Ok, I will check it.

Well, driver is already using spin_lock_irqsave() in all other
functions.

And in linux/clk-provider.h is documented that drivers can call
clk_enable() from an interrupt, so it means that spin_lock_irqsave() is
really needed for mvebu_uart_lock.

> > >  	brdv = readl(port->membase + UART_BRDV);
> > >  	brdv &= ~BRDV_BAUD_MASK;
> > >  	brdv |= d_divisor;
> > >  	writel(brdv, port->membase + UART_BRDV);
> > > +	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
> > >  
> > >  	osamp = readl(port->membase + UART_OSAMP);
> > >  	osamp &= ~OSAMP_DIVISORS_MASK;
> > 
> > > +	/* Recalculate UART1 divisor so UART1 baudrate does not change */
> > > +	if (prev_clock_rate) {
> > > +		divisor = DIV_U64_ROUND_CLOSEST((u64)(val & BRDV_BAUD_MASK) *
> > > +						parent_clock_rate * prev_d1d2,
> > > +						prev_clock_rate * d1 * d2);
> > > +		if (divisor < 1)
> > > +			divisor = 1;
> > > +		else if (divisor > BRDV_BAUD_MAX)
> > > +			divisor = BRDV_BAUD_MAX;
> > > +		val = (val & ~BRDV_BAUD_MASK) | divisor;
> > > +	}
> > 
> > I don't see any range checks in the patch which verifies the requested
> > baud rate is actually possible. With code like this, it seems like the
> > baud rate change will be successful, but the actual baud rate will not
> > be what is requested.
> 
> This code is in function which changes parent UART clock from one used
> by bootloader to clock which will be used by kernel UART driver.
> 
> Yes, it is possible if you configure something unusual in bootloader
> that that this code breaks it. But I think there is not so much what we
> can done here.
> 
> In other patches is updated function mvebu_uart_set_termios() which
> verifies that you can set particular baudrate.
> 
> > > +	/* Recalculate UART2 divisor so UART2 baudrate does not change */
> > > +	if (prev_clock_rate) {
> > > +		val = readl(uart_clock_base->reg2);
> > > +		divisor = DIV_U64_ROUND_CLOSEST((u64)(val & BRDV_BAUD_MASK) *
> > > +						parent_clock_rate * prev_d1d2,
> > > +						prev_clock_rate * d1 * d2);
> > > +		if (divisor < 1)
> > > +			divisor = 1;
> > > +		else if (divisor > BRDV_BAUD_MAX)
> > > +			divisor = BRDV_BAUD_MAX;
> > > +		val = (val & ~BRDV_BAUD_MASK) | divisor;
> > > +		writel(val, uart_clock_base->reg2);
> > 
> > Here it looks like UART1 could request a baud rate change, which ends
> > up setting the clocks so that UART2 is out of range? Could the change
> > for UART1 be successful, but you end up breaking UART2? I'm thinking
> > when you are at opposite ends of the scale. UART2 is running at
> > 110baud and UART1 at 230400baud.
> 
> This code is also in function which just do one time change of UART
> parent clock. Once clk driver is probed this parent clock (and its d1
> and d2 divisors) are not changed anymore. Parent clock and divisors are
> chosen in way that kernel can always configure minimal baudrate 9600 on
> both UARTs.
> 
> You are right that some combinations are not possible. But with these
> patches it is fixed what is supported at clk driver probe time.
> 
> In v3 patch 5/5 is described how to calculate final baudrate from parent
> clock and divisors d1, d2, d, m1, m2, m3, m4. Note that parent clock and
> divisors d1 and d2 are shared for both UARTs. Other parameters (d, m1,
> m2, m3, m4) can be set differently both UART1 and UART2. Changing shared
> values is not possible during usage of UART.
> 
> If you have any idea how to improve current implementation, please let
> me know.
> 
> Also note that all A3720 boards have disabled UART2 in DTS. And I'm not
> sure if there is somebody who uses UART2 or who uses both UARTs.