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From:   hammer hsieh <hammerh0314@gmail.com>
Date:   Thu, 13 Jan 2022 18:56:44 +0800
Message-ID: <CAOX-t57KZb0hNDuhPsabkmkf_qOOLqyH3yuvkHP6UNwhLodWDg@mail.gmail.com>
Subject: Re: [PATCH v6 2/2] serial:sunplus-uart:Add Sunplus SoC UART Driver
To:     Jiri Slaby <jirislaby@kernel.org>
Cc:     Greg KH <gregkh@linuxfoundation.org>, robh+dt@kernel.org,
        linux-serial@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, p.zabel@pengutronix.de,
        wells.lu@sunplus.com, "hammer.hsieh" <hammer.hsieh@sunplus.com>
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Jiri Slaby <jirislaby@kernel.org> =E6=96=BC 2022=E5=B9=B41=E6=9C=8813=E6=97=
=A5 =E9=80=B1=E5=9B=9B =E4=B8=8B=E5=8D=885:08=E5=AF=AB=E9=81=93=EF=BC=9A
>
> 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 =3D 0x9C00-0000 , isc_addr =3D 0x9C00-001C
> > dev/ttySUP1 base_adr =3D 0x9C00-0080 , isc_addr =3D 0x9C00-009C
> > dev/ttySUP2 base_adr =3D 0x9C00-0100 , isc_addr =3D 0x9C00-011C
> > dev/ttySUP3 base_adr =3D 0x9C00-0180 , isc_addr =3D 0x9C00-019C
> > dev/ttySUP4 base_adr =3D 0x9C00-0200 , isc_addr =3D 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?
>

Each UART ISC register contains

Bit7 MSM(Modem Status) INT enable / disable (Access type RW) not use
now (0: default)
Bit6 LSM(Line Status) INT  enable / disable  (Access type RW) not use
now(0: default)
Bit5 RXM INT enable / disable  (Access type RW) set this
Bit4 TXM INT enable / disable  (Access type RW) set this

Bit3 MS(Modem Status) INT flag (Access type Read only) not use now (0: defa=
ult)
Bit2 LS(Line Status) INT flag (Access type Read only) not use now (0: defau=
lt)
Bit1 RX INT flag (Access type Read only) read this
Bit0 TX INT flag (Access type Read only) read this

sunplus_shutdown()
main purpose is to turn off TX INT(bit4) and RX INT(bit5)
bit7 and bit6 not used, should be 0.
bit3 ~ bit0 read only, no effect while writing 0 to them.

> thanks,
> --
> js
> suse labs