Re: [PATCH 05/18] tty: serial: samsung_tty: add support for Apple UARTs

[Marc Zyngier <maz@kernel.org>]

On 2021-02-07 09:12, Hector Martin 'marcan' wrote:
> On 06/02/2021 22.15, Marc Zyngier wrote:

[...]

>>> -	spin_lock_irqsave(&port->lock, flags);
>>> +	/* Only lock if called from IRQ context */
>>> +	if (irq != NO_IRQ)
>>> +		spin_lock_irqsave(&port->lock, flags);
>> 
>> Isn't that actually dangerous? What prevents the interrupt from firing
>> right in the middle of this sequence and create havoc when called from
>> enable_tx_pio()? I fail to see what you gain with sidestepping the
>> locking.
> 
> The callpath here is:
> 
> uart_start -> __uart_start -> (uart_ops.start_tx)
> s3c24xx_serial_start_tx -> s3c24xx_serial_start_tx_pio ->
> enable_tx_pio -> s3c24xx_serial_tx_chars
> 
> And uart_start takes the uart_port lock. None of the serial functions
> take the lock because the serial core already does, but obviously the
> IRQ handler needs to, *if* it's called as an IRQ handler only.

Right, that's the part I missed, thanks for pointing this out.
It'd probably be cleaner to move the whole s3c24xx_serial_tx_chars()
into a helper and keep the locking in the interrupt handler proper.

[...]

>>> diff --git a/include/uapi/linux/serial_core.h 
>>> b/include/uapi/linux/serial_core.h
>>> index 62c22045fe65..59d102b674db 100644
>>> --- a/include/uapi/linux/serial_core.h
>>> +++ b/include/uapi/linux/serial_core.h
>>> @@ -277,4 +277,7 @@
>>>   /* Freescale LINFlexD UART */
>>>   #define PORT_LINFLEXUART	122
>>>   +/* Apple Silicon (M1/T8103) UART (Samsung variant) */
>>> +#define PORT_APPLE	123
>>> +
>> 
>> Do you actually need a new port type here? Looking at the driver
>> itself, it is mainly used to work out the IRQ model. Maybe introducing
>> a new irq_type field in the port structure would be better than
>> exposing this to userspace (which should see something that is exactly
>> the same as a S3C UART).
> 
> Well... every S3C variant already has its own port type here.
> 
> #define PORT_S3C2410    55
> #define PORT_S3C2440    61
> #define PORT_S3C2400    67
> #define PORT_S3C2412    73
> #define PORT_S3C6400    84
> 
> If we don't introduce a new one, which one should we pretend to be? :)

Pick one! :D

> I agree that it might make sense to merge all of these into one,
> though; I don't know what the original reason for splitting them out
> is. But now that they're part of the userspace API, this might not be
> a good idea. Though, unsurprisingly, some googling suggests there are
> zero users of these defines in userspace.

I don't think we can do that, but I don't think we should keep adding
to this unless there is a very good reason. Greg would know, I expect.

Thanks,

          M.
-- 
Jazz is not dead. It just smells funny...