Re: [PATCH 1/3] serial: Add Goldfish TTY driver

[Kuan-Wei Chiu <visitorckw@gmail.com>]

Hi Yao,

On Fri, Dec 19, 2025 at 03:14:03AM +0000, Yao Zi wrote:
> On Thu, Dec 18, 2025 at 06:52:50PM +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>
> > ---
> >  MAINTAINERS                      |   6 ++
> >  drivers/serial/Kconfig           |   8 +++
> >  drivers/serial/Makefile          |   1 +
> >  drivers/serial/serial_goldfish.c | 112 +++++++++++++++++++++++++++++++
> >  include/goldfish_tty.h           |  18 +++++
> >  5 files changed, 145 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..85d2a93b6ff
> > --- /dev/null
> > +++ b/drivers/serial/serial_goldfish.c
> > @@ -0,0 +1,112 @@
> > +// SPDX-License-Identifier: GPL-2.0+
> 
> GPL-2.0+ has been deprecated as a SPDX license identifier.
> GPL-2.0-or-later should probably be used instead[1].

Thanks for the pointer. I wasn't aware of that.
I will update it in all the other files as well in v2.

> 
> > +/*
> > + * Copyright (C) 2025, Kuan-Wei Chiu <visitorckw@gmail.com>
> > + * Goldfish TTY driver for U-Boot
> > + */
> > +
> > +#include <dm.h>
> > +#include <serial.h>
> > +#include <goldfish_tty.h>
> > +#include <asm/io.h>
> > +#include <linux/types.h>
> 
> It looks better if you sort the headers :)

Sure. Will do. :)

> 
> ...
> 
> > +static int goldfish_serial_getc(struct udevice *dev)
> > +{
> > +	static u8 rx_buf[4];
> > +	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));
> > +	if (count == 0)
> > +		return -EAGAIN;
> > +
> > +	if (count > sizeof(rx_buf))
> > +		count = sizeof(rx_buf);
> > +
> > +	paddr = virt_to_phys((void *)rx_buf);
> > +
> > +	rx_buf[0] = 0xAA;
> > +
> > +	__raw_writel(0, (void *)(base + GOLDFISH_TTY_DATA_PTR_HIGH));
> > +	__raw_writel(paddr, (void *)(base + GOLDFISH_TTY_DATA_PTR));
> > +	__raw_writel(count, (void *)(base + GOLDFISH_TTY_DATA_LEN));
> 
> You send a command that reads count bytes, where count represents the
> maximum of bytes available in the buffer and sizeof(rx_buf), so it's
> possible to read more than one character...
> 
> > +	__raw_writel(CMD_READ_BUFFER, (void *)(base + GOLDFISH_TTY_CMD));
> > +
> > +	if (rx_buf[0] == 0xAA)
> > +		return -EAGAIN;
> > +
> > +	return rx_buf[0];
> 
> But only the first character in the buffer is finally returned. Isn't
> the following characters (if present) are incorrectly discarded?

You're right. Requesting multiple bytes causes the hardware to drop
subsequent characters since getc only consumes one.

I will set the read length to 1 in v2.

> 
> rx_buf is declared as static in this case. Are you originally intended
> to do some type of buffering here?

Originally, I used static to avoid performing DMA on the stack.
However, I realize that this introduces re-entrancy issues, which
prevents supporting multiple device instances properly.

In v2, I will move the buffer into struct goldfish_tty_priv.

> 
> Additionally, I don't get the point of setting rx_buf[0] to 0xaa then
> checks whether it changes after sending CMD_READ_BUFFER. Is there a case
> that CMD_READ_BUFFER would fail even when reading TTY_BYTES_READY
> returns non-zero? I had a brief look at qemu's goldfish tty
> implementation, but didn't find such situation.

TBH, this is debug code used during development that I should have
removed before submitting. I will remove it in v2.

For reference, here is the planned update for v2:

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));
	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,
Kuan-Wei

> 
> > +}
> 
> Best regards,
> Yao Zi
> 
> [1]: https://lore.kernel.org/u-boot/20251212142859.GQ303283@bill-the-cat