Message-ID: <20140917170150.GB2464@xora-yoga-13>
Date:	Wed, 17 Sep 2014 10:01:50 -0700
From:	Graeme Gregory <graeme.gregory@...aro.org>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	suravee.suthikulpanit@....com, catalin.marinas@....com,
	will.deacon@....com, linux-arm-kernel@...ts.infradead.org,
	linux-apci@...r.kernel.org, linux-kernel@...r.kernel.org,
	astone@...hat.com, hanjun.guo@...aro.org, rjw@...ysocki.net,
	marc.zyngier@....com, mark.rutland@....com, Sudeep.Holla@....com,
	olof@...om.net, grant.likely@...aro.org, arnd@...db.de,
	jason@...edaemon.net
Subject: Re: [PATCH 4/4] [RFC PATCH for Juno 2/2] tty: SBSA compatible UART

On Wed, Sep 17, 2014 at 11:40:29AM +0100, One Thousand Gnomes wrote:
> 
> Firstly provide some useful information about the hardware. It's no good
> wavng your arms at a document that requires agreeing to a giant ARM T&C
> to get access to. Most of don't work for ARM and we'd have to get our own
> corporate legal to approve the legal garbage involved.
> 
> Secondly explain why you can't use PL011 given that it already supports
> non DMA accesses. What would it take to tweak it further for this ?
> 
> 

As the original author of this driver it is only meant for internal use
inside Linaro. It only ever reached the level of "good enough" for
internal testing.

There is a discussion on a more complete driver in this thread.

http://www.spinics.net/lists/arm-kernel/msg358083.html

Graeme

> > +static void sbsa_tty_do_write(const char *buf, unsigned count)
> > +{
> > +	unsigned long irq_flags;
> > +	struct sbsa_tty *qtty = sbsa_tty;
> > +	void __iomem *base = qtty->base;
> > +	unsigned n;
> > +
> > +	spin_lock_irqsave(&qtty->lock, irq_flags);
> > +	for (n = 0; n < count; n++) {
> > +		while (readw(base + UART01x_FR) & UART01x_FR_TXFF)
> > +			mdelay(10);
> > +		writew(buf[n], base + UART01x_DR);
> 
> serious - you are going to sit and spin in kernel space with interrupts
> off for an indefinite period ?
> 
> No. Even if your hardware is so completely brain dead and broken that it
> hasn't got an interrupt for 'write room' that's not acceptable. You need
> error handling and some kind of sensible timer based solution.
> 
> To put it simply. You have a queue (or you should - your driver is broken
> in that respect too), you have a baud rate, you have a bit time. From
> that you can compute sensible wakeup points to try and refill the
> hardware FIFO. Assuming the hardware fifo is not tiny you don't even have
> to be that good an aim.
> 
> It's acceptable for the printk console code to spin, if you think getting
> the message out on a failure or error outweighs the pain. It's not
> acceptable for the tty layer.
> 
> > +static void sbsauart_fifo_to_tty(struct sbsa_tty *qtty)
> > +{
> > +	void __iomem *base = qtty->base;
> > +	unsigned int flag, max_count = 32;
> > +	u16 status, ch;
> > +
> > +	while (max_count--) {
> > +		status = readw(base + UART01x_FR);
> > +		if (status & UART01x_FR_RXFE)
> > +			break;
> > +
> > +		/* Take chars from the FIFO and update status */
> > +		ch = readw(base + UART01x_DR);
> > +		flag = TTY_NORMAL;
> > +
> > +		if (ch & UART011_DR_BE)
> > +			flag = TTY_BREAK;
> > +		else if (ch & UART011_DR_PE)
> > +			flag = TTY_PARITY;
> > +		else if (ch & UART011_DR_FE)
> > +			flag = TTY_FRAME;
> > +		else if (ch & UART011_DR_OE)
> > +			flag = TTY_OVERRUN;
> > +
> > +		ch &= SBSAUART_CHAR_MASK;
> > +
> > +		tty_insert_flip_char(&qtty->port, ch, flag);
> 
> If its a console you ought to support the sysrq interfaces.
> 
> > +static int sbsa_tty_write_room(struct tty_struct *tty)
> > +{
> > +	return 32;
> > +}
> 
> You can't do this. You need a proper queue and queueing mechanism or
> you'll break in some situations (aside from sitting spinning in your
> write code trashing your system performance entirely). We have a kfifo
> object in the kernel which is really trivial to use and should do what
> you need without any effort.
> 
> > +
> > +static void sbsa_tty_console_write(struct console *co, const char *b,
> > +								unsigned count)
> > +{
> > +	sbsa_tty_do_write(b, count);
> > +
> > +	if(b[count - 1] == '\n');
> > +		sbsa_tty_do_write("\r", 1);
> 
> I would expect \r\n to be the order ?
> 
> > +static struct tty_port_operations sbsa_port_ops = {
> > +};
> 
> No power management ?
> 
> > +
> > +static const struct tty_operations sbsa_tty_ops = {
> > +	.open = sbsa_tty_open,
> > +	.close = sbsa_tty_close,
> > +	.hangup = sbsa_tty_hangup,
> > +	.write = sbsa_tty_write,
> > +	.write_room = sbsa_tty_write_room,
> 
> No termios handling ?
> 
> > +static int sbsa_tty_probe(struct platform_device *pdev)
> > +{
> > +	struct sbsa_tty *qtty;
> > +	int ret = -EINVAL;
> > +	int i;
> > +	struct resource *r;
> > +	struct device *ttydev;
> > +	void __iomem *base;
> > +	u32 irq;
> > +
> > +	r = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > +	if (r == NULL)
> > +		return -EINVAL;
> > +
> > +	base = ioremap(r->start, r->end - r->start);
> > +	if (base == NULL)
> > +		pr_err("sbsa_tty: unable to remap base\n");
> 
> So you are then going to continue and randomly crash  ???
> 
> Also you've got a device so use dev_err() and friends on it
> 
> > +	if (pdev->id > 0)
> > +		goto err_unmap;
> 
> Why not test this before you do all the mapping ??
> 
> 
> It's clean code, it's easy to understand it just doesn't seem to be very
> complete ?
> 
> Alan
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