Re: [PATCH 3/6] drivers/auxdisplay: sensehat: Raspberry Pi Sense HAT display driver

[Nicolas Saenz Julienne]

On Wed, 2022-02-02 at 19:25 -0500, Charles Mirabile wrote:
> This patch adds the driver for the 8x8 RGB LED matrix display
> on the Sense HAT. It appears as a character device named sense-hat
> in /dev/. That special file is 192 bytes large and contains one byte
> for each color channel of each pixel. The overall layout is:
> 
>     col 1       col 8
>       |           |
>       v           v
> 0x00: R   . . .   R \
> 0x08: G   . . .   G |> row 1
> 0x10: B   . . .   B /
>       .   .       .
> ...   .     .     .
>       .       .   .
> 0xA8: R   . . .   R \
> 0xB0: G   . . .   G |> row 8
> 0xB8: G   . . .   G /
> 
> Each channel may have any value between 0 and 31 (larger values are
> wrapped) and are translated by a gamma table before being send to the
> device to provide a linear experience of brightness (the gamma table
> can be modified or reset using an ioctl call on the file). The constants
> for the ioctl are in the sensehat.h header also added in this patch.
> 
> Co-developed-by: Mwesigwa Guma <mguma@xxxxxxxxxx>
> Signed-off-by: Mwesigwa Guma <mguma@xxxxxxxxxx>
> Co-developed-by: Joel Savitz <jsavitz@xxxxxxxxxx>
> Signed-off-by: Joel Savitz <jsavitz@xxxxxxxxxx>
> Co-developed-by: Daniel Bauman <dbauman@xxxxxxxxxx>
> Signed-off-by: Daniel Bauman <dbauman@xxxxxxxxxx>
> Signed-off-by: Charles Mirabile <cmirabil@xxxxxxxxxx>
> ---
>  drivers/auxdisplay/Kconfig            |   8 +
>  drivers/auxdisplay/Makefile           |   1 +
>  drivers/auxdisplay/sensehat-display.c | 264 ++++++++++++++++++++++++++
>  include/uapi/linux/sensehat.h         |  28 +++
>  4 files changed, 301 insertions(+)
>  create mode 100644 drivers/auxdisplay/sensehat-display.c
>  create mode 100644 include/uapi/linux/sensehat.h
> 
> diff --git a/drivers/auxdisplay/Kconfig b/drivers/auxdisplay/Kconfig
> index 64012cda4d12..9bad1aade7a0 100644
> --- a/drivers/auxdisplay/Kconfig
> +++ b/drivers/auxdisplay/Kconfig
> @@ -203,6 +203,14 @@ config ARM_CHARLCD
>  	  line and the Linux version on the second line, but that's
>  	  still useful.
>  
> +config SENSEHAT_DISPLAY
> +	tristate "Raspberry Pi Sense HAT display driver"
> +	depends on I2C
> +	select MFD_SIMPLE_MFD_I2C
> +	help
> +	 This is a driver for the Raspberry Pi Sensehat 8x8 RBG-LED matrix
> +	 you can access it as a misc device at /dev/sense-hat
> +
>  menuconfig PARPORT_PANEL
>  	tristate "Parallel port LCD/Keypad Panel support"
>  	depends on PARPORT
> diff --git a/drivers/auxdisplay/Makefile b/drivers/auxdisplay/Makefile
> index 6968ed4d3f0a..30b2b7934046 100644
> --- a/drivers/auxdisplay/Makefile
> +++ b/drivers/auxdisplay/Makefile
> @@ -14,3 +14,4 @@ obj-$(CONFIG_HT16K33)		+= ht16k33.o
>  obj-$(CONFIG_PARPORT_PANEL)	+= panel.o
>  obj-$(CONFIG_LCD2S)		+= lcd2s.o
>  obj-$(CONFIG_LINEDISP)		+= line-display.o
> +obj-$(CONFIG_SENSEHAT_DISPLAY)	+= sensehat-display.o
> diff --git a/drivers/auxdisplay/sensehat-display.c b/drivers/auxdisplay/sensehat-display.c
> new file mode 100644
> index 000000000000..08b397a544f0
> --- /dev/null
> +++ b/drivers/auxdisplay/sensehat-display.c
> @@ -0,0 +1,264 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Raspberry Pi Sense HAT 8x8 LED matrix display driver
> + * http://raspberrypi.org
> + *
> + * Copyright (C) 2015 Raspberry Pi
> + * Copyright (C) 2021 Charles Mirabile, Mwesigwa Guma, Joel Savitz
> + *
> + * Original Author: Serge Schneider
> + * Revised for upstream Linux by: Charles Mirabile, Mwesigwa Guma, Joel Savitz
> + */
> +
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include <linux/errno.h>
> +#include <linux/string.h>
> +#include <linux/mm.h>
> +#include <linux/slab.h>
> +#include <linux/uaccess.h>
> +#include <linux/delay.h>
> +#include <linux/init.h>
> +#include <linux/platform_device.h>
> +#include <linux/mod_devicetable.h>
> +#include <linux/miscdevice.h>
> +#include <linux/regmap.h>
> +#include <linux/property.h>
> +#include <linux/sensehat.h>
> +
> +#define GAMMA_SIZE 32
> +#define VMEM_SIZE 192
> +
> +struct sensehat_display {
> +	struct platform_device *pdev;
> +	struct miscdevice mdev;
> +	struct mutex rw_mtx;
> +	u8 gamma[GAMMA_SIZE];
> +	u8 vmem[VMEM_SIZE];
> +	u32 display_register;
> +};
> +
> +static bool lowlight;
> +module_param(lowlight, bool, 0);
> +MODULE_PARM_DESC(lowlight, "Reduce LED matrix brightness to one third");
> +
> +static const u8 gamma_presets[][GAMMA_SIZE] = {
> +	[SENSEHAT_GAMMA_DEFAULT] = {
> +		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01,
> +		0x02, 0x02, 0x03, 0x03, 0x04, 0x05, 0x06, 0x07,
> +		0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0E, 0x0F, 0x11,
> +		0x12, 0x14, 0x15, 0x17, 0x19, 0x1B, 0x1D, 0x1F,
> +	},
> +	[SENSEHAT_GAMMA_LOWLIGHT] = {
> +		0x00, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,
> +		0x01, 0x01, 0x01, 0x01, 0x01, 0x02, 0x02, 0x02,
> +		0x03, 0x03, 0x03, 0x04, 0x04, 0x05, 0x05, 0x06,
> +		0x06, 0x07, 0x07, 0x08, 0x08, 0x09, 0x0A, 0x0A,
> +	},
> +};
> +
> +static void sensehat_update_display(struct sensehat_display *display)
> +{
> +	int i, ret;
> +	struct regmap *regmap = dev_get_regmap(display->pdev->dev.parent, NULL);

Same comment as with joystick.

> +	u8 temp[VMEM_SIZE];
> +
> +	for (i = 0; i < VMEM_SIZE; ++i)
> +		temp[i] = display->gamma[display->vmem[i] & 0x1f];
> +
> +	ret = regmap_bulk_write(regmap, display->display_register, temp,
> +				VMEM_SIZE);
> +	if (ret < 0)
> +		dev_err(&display->pdev->dev,
> +			"Update to 8x8 LED matrix display failed");
> +}
> +
> +static loff_t sensehat_display_llseek(struct file *filp, loff_t offset,
> +				      int whence)
> +{
> +	loff_t base, pos;
> +
> +	switch (whence) {
> +	case SEEK_SET:
> +		base = 0;
> +		break;
> +	case SEEK_CUR:
> +		base = filp->f_pos;
> +		break;
> +	case SEEK_END:
> +		base = VMEM_SIZE;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +	pos = base + offset;
> +	if (pos < 0 || pos >= VMEM_SIZE)
> +		return -EINVAL;
> +	filp->f_pos = pos;
> +	return base;
> +}
> +
> +static ssize_t sensehat_display_read(struct file *filp, char __user *buf,
> +				     size_t count, loff_t *f_pos)
> +{
> +	struct sensehat_display *sensehat_display =
> +		container_of(filp->private_data, struct sensehat_display, mdev);
> +	ssize_t retval = -EFAULT;
> +
> +	if (*f_pos >= VMEM_SIZE)
> +		return 0;
> +	if (*f_pos + count > VMEM_SIZE)
> +		count = VMEM_SIZE - *f_pos;
> +	if (mutex_lock_interruptible(&sensehat_display->rw_mtx))
> +		return -ERESTARTSYS;
> +	if (copy_to_user(buf, sensehat_display->vmem + *f_pos, count))
> +		goto out;
> +	*f_pos += count;
> +	retval = count;
> +out:
> +	mutex_unlock(&sensehat_display->rw_mtx);
> +	return retval;
> +}
> +
> +static ssize_t sensehat_display_write(struct file *filp, const char __user *buf,
> +				      size_t count, loff_t *f_pos)
> +{
> +	struct sensehat_display *sensehat_display =
> +		container_of(filp->private_data, struct sensehat_display, mdev);
> +	int ret = count;
> +
> +	if (*f_pos >= VMEM_SIZE)
> +		return -EFBIG;
> +	if (*f_pos + count > VMEM_SIZE)
> +		count = VMEM_SIZE - *f_pos;
> +	if (mutex_lock_interruptible(&sensehat_display->rw_mtx))
> +		return -ERESTARTSYS;
> +	if (copy_from_user(sensehat_display->vmem + *f_pos, buf, count)) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +	sensehat_update_display(sensehat_display);
> +	*f_pos += count;
> +out:
> +	mutex_unlock(&sensehat_display->rw_mtx);
> +	return ret;
> +}
> +
> +static long sensehat_display_ioctl(struct file *filp, unsigned int cmd,
> +				   unsigned long arg)
> +{
> +	struct sensehat_display *sensehat_display =
> +		container_of(filp->private_data, struct sensehat_display, mdev);
> +	void __user *user_ptr = (void __user *)arg;
> +	int i, ret = 0;
> +
> +	if (mutex_lock_interruptible(&sensehat_display->rw_mtx))
> +		return -ERESTARTSYS;

nit: add space, here and after the switch statement.

> +	switch (cmd) {
> +	case SENSEDISP_IOGET_GAMMA:
> +		if (copy_to_user(user_ptr, sensehat_display->gamma, GAMMA_SIZE))
> +			ret = -EFAULT;
> +		goto no_update;
> +	case SENSEDISP_IOSET_GAMMA:
> +		if (copy_from_user(sensehat_display->gamma, user_ptr,
> +				   GAMMA_SIZE))
> +			ret = -EFAULT;
> +		break;
> +	case SENSEDISP_IORESET_GAMMA:
> +		if (arg >= SENSEHAT_GAMMA_PRESET_COUNT) {
> +			ret = -EINVAL;
> +			goto no_update;
> +		}
> +		memcpy(sensehat_display->gamma, gamma_presets[arg], GAMMA_SIZE);
> +		goto no_check;
> +	default:
> +		ret = -EINVAL;
> +		break;
> +	}
> +	for (i = 0; i < GAMMA_SIZE; ++i)
> +		sensehat_display->gamma[i] &= 0x1f;
> +no_check:
> +	sensehat_update_display(sensehat_display);
> +no_update:
> +	mutex_unlock(&sensehat_display->rw_mtx);
> +	return ret;
> +}
> +
> +static const struct file_operations sensehat_display_fops = {
> +	.owner = THIS_MODULE,
> +	.llseek = sensehat_display_llseek,
> +	.read = sensehat_display_read,
> +	.write = sensehat_display_write,
> +	.unlocked_ioctl = sensehat_display_ioctl,
> +};
> +
> +static int sensehat_display_probe(struct platform_device *pdev)
> +{
> +	int ret;
> +
> +	struct sensehat_display *sensehat_display =
> +		devm_kzalloc(&pdev->dev, sizeof(*sensehat_display), GFP_KERNEL);
> +
> +	sensehat_display->pdev = pdev;
> +
> +	memcpy(sensehat_display->gamma, gamma_presets[lowlight], GAMMA_SIZE);
> +
> +	memset(sensehat_display->vmem, 0, VMEM_SIZE);

You don't nee this, kzalloc already zeroes the memory.

> +
> +	mutex_init(&sensehat_display->rw_mtx);
> +
> +	ret = device_property_read_u32(&pdev->dev, "reg",
> +				       &sensehat_display->display_register);

Same comment as with joystick.

> +	if (ret) {
> +		dev_err(&pdev->dev, "Could not read register property.\n");
> +		return ret;
> +	}
> +
> +	sensehat_display->mdev = (struct miscdevice){
> +		.minor = MISC_DYNAMIC_MINOR,
> +		.name = "sense-hat",
> +		.mode = 0666,
> +		.fops = &sensehat_display_fops,
> +	};
> +
> +	ret = misc_register(&sensehat_display->mdev);
> +	if (ret < 0) {
> +		dev_err(&pdev->dev,
> +			"Could not register 8x8 LED matrix display.\n");
> +		return ret;
> +	}
> +
> +	ret = devm_add_action(&pdev->dev, (void *)misc_deregister,
> +			      &sensehat_display->mdev);
> +	if (ret < 0) {
> +		dev_err(&pdev->dev, "Could not add misc device to devm\n");
> +		return ret;
> +	}

Instead of doing this I'd add a .remove callback to the platform driver.

> +
> +	dev_info(&pdev->dev,
> +		 "8x8 LED matrix display registered with minor number %i",
> +		 sensehat_display->mdev.minor);
> +
> +	sensehat_update_display(sensehat_display);

This could potentially race with an app accessing the misc device?

> +	return 0;
> +}
> +
> +static const struct of_device_id sensehat_display_device_id[] = {
> +	{ .compatible = "raspberrypi,sensehat-display" },
> +	{},
> +};
> +MODULE_DEVICE_TABLE(of, sensehat_display_device_id);
> +
> +static struct platform_driver sensehat_display_driver = {
> +	.probe = sensehat_display_probe,
> +	.driver = {
> +		.name = "sensehat-display",
> +		.of_match_table = sensehat_display_device_id,
> +	},
> +};
> +
> +module_platform_driver(sensehat_display_driver);
> +
> +MODULE_DESCRIPTION("Raspberry Pi Sense HAT 8x8 LED matrix display driver");
> +MODULE_AUTHOR("Serge Schneider <serge@xxxxxxxxxxxxxxx>");
> +MODULE_LICENSE("GPL");
> diff --git a/include/uapi/linux/sensehat.h b/include/uapi/linux/sensehat.h
> new file mode 100644
> index 000000000000..cae07568a5eb
> --- /dev/null
> +++ b/include/uapi/linux/sensehat.h
> @@ -0,0 +1,28 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later WITH Linux-syscall-note */
> +/*
> + * Raspberry Pi Sense HAT core driver
> + * http://raspberrypi.org
> + *
> + * Copyright (C) 2015 Raspberry Pi
> + * Copyright (C) 2021 Charles Mirabile, Mwesigwa Guma, Joel Savitz
> + *
> + * Original Author: Serge Schneider
> + * Revised for upstream Linux by: Charles Mirabile, Mwesigwa Guma, Joel Savitz
> + */
> +
> +#ifndef _UAPILINUX_SENSEHAT_H_
> +#define _UAPILINUX_SENSEHAT_H_
> +
> +#define SENSEDISP_IOC_MAGIC 0xF1
> +
> +#define SENSEDISP_IOGET_GAMMA _IO(SENSEDISP_IOC_MAGIC, 0)
> +#define SENSEDISP_IOSET_GAMMA _IO(SENSEDISP_IOC_MAGIC, 1)
> +#define SENSEDISP_IORESET_GAMMA _IO(SENSEDISP_IOC_MAGIC, 2)
> +
> +enum gamma_preset {
> +	SENSEHAT_GAMMA_DEFAULT = 0,
> +	SENSEHAT_GAMMA_LOWLIGHT,
> +	SENSEHAT_GAMMA_PRESET_COUNT,
> +};
> +
> +#endif

-- 
Nicolás Sáenz