linux-kernel - Re: [RFC PATCH v1 15/25] printk: print history for new consoles

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Message-ID: <20190304092444.GA21004@jagdpanzerIV>
Date:   Mon, 4 Mar 2019 18:24:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 15/25] printk: print history for new consoles

On (02/26/19 16:22), John Ogness wrote:
> > This looks like an alien. The code is supposed to write one message
> > from the given buffer. And some huge job is well hidden there.
> 
> This is a very simple implementation of a printk kthread. It probably
> makes more sense to have a printk kthread per console. That would allow
> fast consoles to not be penalized by slow consoles. Due to the
> per-console seq tracking, the code would already support it.

I believe we discussed "polling consoles" several times.

printk-kthread is one way to implement polling. Another one might
already be implemented in, probably, all serial drivers and we just
need to extend it a bit - polling from console's IRQ handler.

Serial drivers poll UART xmit buffer and print (usually) up to
`count' bytes:

	static irqreturn_t foo_irq_handler(int irq, void *id)
	{
		int count = 512;

		[...]
		while (count > 0 && !uart_circ_empty(xmit)) {
			wr_regb(port, TX, xmit->buf[xmit->tail]);
			xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
			count--;
		}
		[...]

		return IRQ_HANDLED;
	}

So we can also grub NUM (e.g. max 64 entries) pending logbuf messages
and print them from device's isr.

	-ss