Re: [PATCH v2 1/4] serial: Add Goldfish TTY driver

Kuan-Wei Chiu
2025-12-27T15:08:44Z

Hi Yao,

On Sat, Dec 27, 2025 at 02:07:47AM +0000, Yao Zi wrote:
> On Fri, Dec 26, 2025 at 05:53:57PM +0000, Kuan-Wei Chiu wrote:
> > Add support for the Google Goldfish TTY serial device. This virtual
> > device is commonly used in QEMU virtual machines (such as the m68k
> > virt machine) and Android emulators.
> >
> > The driver implements basic console output and input polling using the
> > Goldfish MMIO interface.
> >
> > Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> > ---
> > Changes in v2:
> > - Update SPDX license identifier to GPL-2.0-or-later.
> > - Sort header inclusions alphabetically.
> > - Move RX buffer into goldfish_tty_priv instead of using a static buffer.
> > - Make sure getc only read a single byte at a time.
> >
> > MAINTAINERS | 6 ++
> > drivers/serial/Kconfig | 8 +++
> > drivers/serial/Makefile | 1 +
> > drivers/serial/serial_goldfish.c | 104 +++++++++++++++++++++++++++++++
> > include/goldfish_tty.h | 18 ++++++
> > 5 files changed, 137 insertions(+)
> > create mode 100644 drivers/serial/serial_goldfish.c
> > create mode 100644 include/goldfish_tty.h
>
> ...
>
> > diff --git a/drivers/serial/serial_goldfish.c b/drivers/serial/serial_goldfish.c
> > new file mode 100644
> > index 00000000000..ce5bff6bf4c
> > --- /dev/null
> > +++ b/drivers/serial/serial_goldfish.c
> > @@ -0,0 +1,104 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +/*
> > + * Copyright (C) 2025, Kuan-Wei Chiu <visitorckw@gmail.com>
> > + * Goldfish TTY driver for U-Boot
> > + */
> > +
> > +#include <asm/io.h>
> > +#include <dm.h>
> > +#include <goldfish_tty.h>
> > +#include <linux/types.h>
> > +#include <serial.h>
> > +
> > +/* Goldfish TTY Register Offsets */
> > +#define GOLDFISH_TTY_PUT_CHAR 0x00
> > +#define GOLDFISH_TTY_BYTES_READY 0x04
> > +#define GOLDFISH_TTY_CMD 0x08
> > +#define GOLDFISH_TTY_DATA_PTR 0x10
> > +#define GOLDFISH_TTY_DATA_LEN 0x14
> > +#define GOLDFISH_TTY_DATA_PTR_HIGH 0x18
> > +#define GOLDFISH_TTY_VERSION 0x20
> > +
> > +/* Commands */
> > +#define CMD_WRITE_BUFFER 2
> > +#define CMD_READ_BUFFER 3
> > +
> > +struct goldfish_tty_priv {
> > + void __iomem *base;
> > + u8 rx_buf;
> > +};
> > +
> > +static int goldfish_serial_getc(struct udevice *dev)
> > +{
> > + struct goldfish_tty_priv *priv = dev_get_priv(dev);
> > + unsigned long base = (unsigned long)priv->base;
> > + unsigned long paddr;
> > + u32 count;
> > +
> > + count = __raw_readl((void *)(base + GOLDFISH_TTY_BYTES_READY));
>
> I think it's okay to do pointer arithmetic directly against priv->base
> in GNU C, right? In which case void * is treated like char *.
>
> If I'm correct, variable base could be dropped, and we could avoid some
> casts here and below.

Agreed.

While standard C doesn't allow it, GNU C does support arithmetic on
void *. Doing this directly makes the code look much cleaner.
Thanks for the feedback, I will update this in v3.

Regards,
Kuan-Wei

>
> > + if (count == 0)
> > + return -EAGAIN;
> > +
> > + paddr = virt_to_phys((void *)&priv->rx_buf);
> > +
> > + __raw_writel(0, (void *)(base + GOLDFISH_TTY_DATA_PTR_HIGH));
> > + __raw_writel(paddr, (void *)(base + GOLDFISH_TTY_DATA_PTR));
> > + __raw_writel(1, (void *)(base + GOLDFISH_TTY_DATA_LEN));
> > +
> > + __raw_writel(CMD_READ_BUFFER, (void *)(base + GOLDFISH_TTY_CMD));
> > +
> > + return priv->rx_buf;
> > +}
>
> Regards,
> Yao Zi