linux-kernel - Re: [PATCH v2 1/1] tty: serial: owl: Add support for kernel debugger

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Message-ID: <02c664f5-8107-7757-2e20-c446a0458539@kernel.org>
Date:   Fri, 8 Jan 2021 08:58:38 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Cristian Ciocaltea <cristian.ciocaltea@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andreas Färber <afaerber@...e.de>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-actions@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] tty: serial: owl: Add support for kernel debugger

On 07. 01. 21, 19:16, Cristian Ciocaltea wrote:
> Hi Greg,
> 
> Thank you for the review!
> 
> On Thu, Jan 07, 2021 at 04:20:55PM +0100, Greg Kroah-Hartman wrote:
>> On Tue, Jan 05, 2021 at 07:02:02PM +0200, Cristian Ciocaltea wrote:
>>> Implement 'poll_put_char' and 'poll_get_char' callbacks in struct
>>> 'owl_uart_ops' that enables OWL UART to be used for kernel debugging
>>> over serial line.
>>>
>>> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@...il.com>
> 
> [...]
> 
>>> +
>>> +static void owl_uart_poll_put_char(struct uart_port *port, unsigned char ch)
>>> +{
>>> +	while (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_TFFU)
>>> +		cpu_relax();
>>
>> Unbounded loops?  What could possibly go wrong?
>>
>> :(
>>
>> Please don't do that in the kernel, put a max bound on this.
> 
> I didn't realize the issue since I had encountered this pattern in many
> other serial drivers, as well: altera_uart, arc_uart, atmel_serial, etc.
> 
>> And are you _SURE_ that cpu_relax() is what you want to call here?
> 
> I'm thinking of replacing the loop with 'readl_poll_timeout_atomic()',
> if that would be a better approach.

It might be better, yes. Either way, if you add a bound to the loop, you 
definitely need a more precise timing, so ndelay/udelay instead of 
cpu_relax.

thanks,
-- 
js