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Date:   Thu, 5 Sep 2019 13:14:13 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Qian Cai <cai@lca.pw>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>, davem@davemloft.net,
        netdev@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure
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On Thu, 05 Sep 2019 12:03:13 -0400
Qian Cai <cai@lca.pw> wrote:

> > > and could deal with console hardware that involve irq_exit() anyway.  
> > 
> > printk->console_driver->write() does not involve irq.  
> 
> Hmm, from the article,
> 
> https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter
> 
> "Since transmission of a single or multiple characters may take a long time
> relative to CPU speeds, a UART maintains a flag showing busy status so that the
> host system knows if there is at least one character in the transmit buffer or
> shift register; "ready for next character(s)" may also be signaled with an
> interrupt."

I'm pretty sure all serial consoles do a busy loop on the UART and not
use interrupts to notify when it's available. That would require an
asynchronous implementation of printk() which would be quite complex to
implement.

-- Steve