linux-kernel - Re: [PATCH v6 2/2] serial:sunplus-uart:Add Sunplus SoC UART Driver

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Date:   Thu, 13 Jan 2022 10:08:11 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     hammer hsieh <hammerh0314@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, robh+dt@...nel.org,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, p.zabel@...gutronix.de,
        wells.lu@...plus.com, "hammer.hsieh" <hammer.hsieh@...plus.com>
Subject: Re: [PATCH v6 2/2] serial:sunplus-uart:Add Sunplus SoC UART Driver

On 13. 01. 22, 9:54, hammer hsieh wrote:
>>> +static void sunplus_shutdown(struct uart_port *port)
>>> +{
>>> +     unsigned long flags;
>>> +
>>> +     spin_lock_irqsave(&port->lock, flags);
>>> +     writel(0, port->membase + SUP_UART_ISC);
>>> +     spin_unlock_irqrestore(&port->lock, flags);
>>
>> I asked last time:
>> * What bus is this -- posting?
>>
>> You replied:
>> * Here just clear interrupt.
>> * Not really understand your comment?
>>
>> So I am asking again:
>> What bus is this? Isn't a posted write a problem here? I mean, shouldn't
>> you read from the register so that the write hits the device? That
>> depends on the bus this sits on, so just asking.
>>
> 
> Each UART has its own ISC register.
> Ex.
> dev/ttySUP0 base_adr = 0x9C00-0000 , isc_addr = 0x9C00-001C
> dev/ttySUP1 base_adr = 0x9C00-0080 , isc_addr = 0x9C00-009C
> dev/ttySUP2 base_adr = 0x9C00-0100 , isc_addr = 0x9C00-011C
> dev/ttySUP3 base_adr = 0x9C00-0180 , isc_addr = 0x9C00-019C
> dev/ttySUP4 base_adr = 0x9C00-0200 , isc_addr = 0x9C00-021C
> So sunplus_shutdown() just simply turn off its own device isc only.
> That's why I didn't read register value, just write 0 for it.

Could you explain me what posted write is and how does it not matter in 
this case?

thanks,
-- 
js
suse labs