Re: [PATCH RFC v2 3/3] drm/dp: Add a drm_aux-dev module for reading/writing dpcd registers.

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Sep 15, 2015 at 04:55:04PM -0700, Rafael Antognolli wrote:
> This module is heavily based on i2c-dev. Once loaded, it provides one
> dev node per DP AUX channel, named drm_aux-N.
> 
> It's possible to know which connector owns this aux channel by looking
> at the respective sysfs /sys/class/drm_aux-dev/drm_aux-N/connector, if
> the connector device pointer was correctly set in the aux helper struct.
> 
> Two main operations are provided on the registers: read and write. The
> address of the register to be read or written is given using lseek.
> Reading or writing does not update the offset of the file.
> 
> Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
> ---
>  drivers/gpu/drm/Kconfig       |   4 +
>  drivers/gpu/drm/Makefile      |   1 +
>  drivers/gpu/drm/drm_aux-dev.c | 326 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 331 insertions(+)
>  create mode 100644 drivers/gpu/drm/drm_aux-dev.c
> 
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 1a0a8df..eae847c 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -25,6 +25,10 @@ config DRM_MIPI_DSI
>  	bool
>  	depends on DRM
>  
> +config DRM_AUX_CHARDEV
> +	tristate "DRM DP AUX Interface"
> +	depends on DRM
> +
>  config DRM_KMS_HELPER
>  	tristate
>  	depends on DRM
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index 45e7719..a1a94306 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -32,6 +32,7 @@ CFLAGS_drm_trace_points.o := -I$(src)
>  
>  obj-$(CONFIG_DRM)	+= drm.o
>  obj-$(CONFIG_DRM_MIPI_DSI) += drm_mipi_dsi.o
> +obj-$(CONFIG_DRM_AUX_CHARDEV) += drm_aux-dev.o
>  obj-$(CONFIG_DRM_TTM)	+= ttm/
>  obj-$(CONFIG_DRM_TDFX)	+= tdfx/
>  obj-$(CONFIG_DRM_R128)	+= r128/
> diff --git a/drivers/gpu/drm/drm_aux-dev.c b/drivers/gpu/drm/drm_aux-dev.c
> new file mode 100644
> index 0000000..fcc334a
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_aux-dev.c
> @@ -0,0 +1,326 @@
> +/*
> + * Copyright © 2015 Intel Corporation
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a
> + * copy of this software and associated documentation files (the "Software"),
> + * to deal in the Software without restriction, including without limitation
> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
> + * and/or sell copies of the Software, and to permit persons to whom the
> + * Software is furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice (including the next
> + * paragraph) shall be included in all copies or substantial portions of the
> + * Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
> + * IN THE SOFTWARE.
> + *
> + * Authors:
> + *    Rafael Antognolli <rafael.antognolli@intel.com>
> + *
> + */
> +
> +#include <linux/device.h>
> +#include <linux/fs.h>
> +#include <linux/slab.h>
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <asm/uaccess.h>
> +#include <drm/drm_dp_helper.h>
> +#include <drm/drm_crtc.h>
> +
> +struct drm_aux_dev {
> +	struct list_head list;
> +	unsigned index;
> +	struct drm_dp_aux *aux;
> +	struct device *dev;
> +};
> +
> +#define DRM_AUX_MINORS	256
> +static int drm_aux_dev_count = 0;
> +static LIST_HEAD(drm_aux_dev_list);
> +static DEFINE_SPINLOCK(drm_aux_dev_list_lock);
> +
> +static struct drm_aux_dev *drm_aux_dev_get_by_minor(unsigned index)
> +{
> +	struct drm_aux_dev *aux_dev;
> +
> +	spin_lock(&drm_aux_dev_list_lock);
> +	list_for_each_entry(aux_dev, &drm_aux_dev_list, list) {
> +		if (aux_dev->index == index)
> +			goto found;
> +	}
> +
> +	aux_dev = NULL;
> +found:
> +	spin_unlock(&drm_aux_dev_list_lock);
> +	return aux_dev;
> +}
> +
> +static struct drm_aux_dev *drm_aux_dev_get_by_aux(struct drm_dp_aux *aux)
> +{
> +	struct drm_aux_dev *aux_dev;
> +
> +	spin_lock(&drm_aux_dev_list_lock);
> +	list_for_each_entry(aux_dev, &drm_aux_dev_list, list) {
> +		if (aux_dev->aux == aux)
> +			goto found;
> +	}
> +
> +	aux_dev = NULL;
> +found:
> +	spin_unlock(&drm_aux_dev_list_lock);
> +	return aux_dev;
> +}
> +
> +static struct drm_aux_dev *get_free_drm_aux_dev(struct drm_dp_aux *aux)
> +{
> +	struct drm_aux_dev *aux_dev;
> +	int index;
> +
> +	spin_lock(&drm_aux_dev_list_lock);
> +	index = drm_aux_dev_count;
> +	spin_unlock(&drm_aux_dev_list_lock);
> +	if (index >= DRM_AUX_MINORS) {
> +		printk(KERN_ERR "i2c-dev: Out of device minors (%d)\n",
> +		       index);
> +		return ERR_PTR(-ENODEV);
> +	}
> +
> +	aux_dev = kzalloc(sizeof(*aux_dev), GFP_KERNEL);
> +	if (!aux_dev)
> +		return ERR_PTR(-ENOMEM);
> +	aux_dev->aux = aux;
> +	aux_dev->index = index;
> +
> +	spin_lock(&drm_aux_dev_list_lock);
> +	drm_aux_dev_count++;
> +	list_add_tail(&aux_dev->list, &drm_aux_dev_list);
> +	spin_unlock(&drm_aux_dev_list_lock);
> +	return aux_dev;
> +}
> +
> +static void return_drm_aux_dev(struct drm_aux_dev *aux_dev)
> +{
> +	spin_lock(&drm_aux_dev_list_lock);
> +	list_del(&aux_dev->list);
> +	spin_unlock(&drm_aux_dev_list_lock);
> +	kfree(aux_dev);
> +}
> +
> +static ssize_t name_show(struct device *dev,
> +			 struct device_attribute *attr, char *buf)
> +{
> +	struct drm_aux_dev *aux_dev = drm_aux_dev_get_by_minor(MINOR(dev->devt));
> +
> +	if (!aux_dev)
> +		return -ENODEV;
> +	return sprintf(buf, "%s\n", aux_dev->aux->name);
> +}
> +static DEVICE_ATTR_RO(name);
> +
> +static ssize_t connector_show(struct device *dev,
> +			      struct device_attribute *attr, char *buf)
> +{
> +	struct drm_aux_dev *aux_dev = drm_aux_dev_get_by_minor(MINOR(dev->devt));
> +	struct drm_dp_aux *aux;
> +	struct device *conn_dev;
> +	struct drm_connector *connector = NULL;
> +
> +	if (!aux_dev)
> +		return -ENODEV;
> +	aux = aux_dev->aux;
> +	conn_dev = aux->connector;
> +	if (!conn_dev)
> +		return sprintf(buf, "unknown\n");
> +
> +	connector = dev_get_drvdata(aux->connector);
> +
> +	return sprintf(buf, "%s\n", connector->name);
> +}
> +static DEVICE_ATTR_RO(connector);
> +
> +static struct attribute *drm_aux_attrs[] = {
> +	&dev_attr_name.attr,
> +	&dev_attr_connector.attr,
> +	NULL,
> +};
> +ATTRIBUTE_GROUPS(drm_aux);
> +
> +static int auxdev_open(struct inode *inode, struct file *file)
> +{
> +	unsigned int minor = iminor(inode);
> +	struct drm_aux_dev *aux_dev;
> +
> +	aux_dev = drm_aux_dev_get_by_minor(minor);
> +	if (!aux_dev)
> +		return -ENODEV;
> +
> +	file->private_data = aux_dev;
> +	return 0;
> +}
> +
> +static ssize_t auxdev_read(struct file *file, char __user *buf, size_t count,
> +			   loff_t *offset)
> +{
> +	char *localbuf;
> +	ssize_t res;
> +	struct drm_aux_dev *aux_dev = file->private_data;
> +
> +	localbuf = memdup_user(buf, count);
> +	if (IS_ERR(localbuf))
> +		return PTR_ERR(buf);
> +
> +	res = drm_dp_dpcd_read(aux_dev->aux, *offset, localbuf, count);
> +	if (res < 0)
> +		goto finish;

read/write will need to do the access in 16 byte chunks. That's the max
amount of data that can be transferred with a single AUX operation. I
suppose that would also avoid the need for allocating large buffers
since you can just have a 16 byte buffer on the stack and keep reusing
it while you iterate through the total count. I'm not sure what are
the expected semantics of read/write if you do it in chunks and hit
a copy_{to,from}_user() failure somewhere down the line. But I suppose
access_ok() upfront for the whole user buffer could avoid that issue.

Also the size of the DPCD address space is 1<<20 bytes, so you'll need to
limit the access to stay within that. 'size = lseek(fd, 0, SEEK_END)'
can be used to get the file size, so you should allow the offset to
reach 1<<20, but not beyond. read/write should process as much much of
the data that they can given the current file offset, and return how
many bytes made it in/out (can also be 0 if offset was already at 1<<20).

> +
> +	if (copy_to_user(buf, localbuf, res))
> +		res = -EFAULT;
> +
> +finish:
> +	kfree(localbuf);
> +	return res;
> +}
> +
> +static ssize_t auxdev_write(struct file *file, const char __user *buf,
> +			    size_t count, loff_t *offset)
> +{
> +	char *localbuf;
> +	ssize_t res;
> +	struct drm_aux_dev *aux_dev = file->private_data;
> +
> +	localbuf = memdup_user(buf, count);
> +	if (IS_ERR(localbuf))
> +		return PTR_ERR(buf);
> +
> +	res = drm_dp_dpcd_write(aux_dev->aux, *offset, localbuf, count);
> +	kfree(localbuf);
> +
> +	return res;
> +}
> +
> +static const struct file_operations auxdev_fops = {
> +	.owner		= THIS_MODULE,
> +	.llseek		= generic_file_llseek,
> +	.read		= auxdev_read,
> +	.write		= auxdev_write,
> +	.open		= auxdev_open,
> +};
> +
> +static struct class *drm_aux_dev_class;
> +static int drm_dev_major = -1;
> +
> +#define to_auxdev(d) container_of(d, struct drm_aux_dev, aux)
> +
> +static int auxdev_attach_aux(struct drm_dp_aux *aux, void *data)
> +{
> +	int *major = data;
> +	struct drm_aux_dev *aux_dev;
> +	int res;
> +
> +	aux_dev = get_free_drm_aux_dev(aux);
> +	if (IS_ERR(aux_dev))
> +		return PTR_ERR(aux_dev);
> +
> +	aux_dev->dev = device_create(drm_aux_dev_class, aux->dev,
> +				     MKDEV(*major, aux_dev->index), NULL,
> +				     "drm_aux-%d", aux_dev->index);
> +	if (IS_ERR(aux_dev->dev)) {
> +		res = PTR_ERR(aux_dev->dev);
> +		goto error;
> +	}
> +
> +	pr_debug("drm_aux-dev: aux [%s] registered as minor %d\n",
> +		 aux->name, aux_dev->index);
> +	return 0;
> +error:
> +	return_drm_aux_dev(aux_dev);
> +	return res;
> +
> +}
> +
> +static int auxdev_detach_aux(struct drm_dp_aux *aux, void *data)
> +{
> +	int *major = data;
> +	int minor;
> +	struct drm_aux_dev *aux_dev;
> +
> +	aux_dev = drm_aux_dev_get_by_aux(aux);
> +	if (!aux_dev) /* attach must have failed */
> +		return 0;
> +
> +	minor = aux_dev->index;
> +	return_drm_aux_dev(aux_dev);
> +	device_destroy(drm_aux_dev_class, MKDEV(*major, minor));
> +
> +	pr_debug("drm_aux-dev: aux [%s] unregistered\n", aux->name);
> +	return 0;
> +}
> +
> +static int auxdev_inform_cb(int action, struct drm_dp_aux *aux)
> +{
> +	switch (action) {
> +	case DRM_DP_ADD_AUX:
> +		return auxdev_attach_aux(aux, &drm_dev_major);
> +	case DRM_DP_DEL_AUX:
> +		return auxdev_detach_aux(aux, &drm_dev_major);
> +	}
> +
> +	return 0;
> +}
> +
> +static int __init drm_aux_dev_init(void)
> +{
> +	int res;
> +
> +	printk(KERN_INFO "drm dp aux /dev entries driver\n");
> +
> +	res = register_chrdev(0, "aux", &auxdev_fops);
> +	if (res < 0)
> +		goto out;
> +	drm_dev_major = res;
> +
> +	drm_aux_dev_class = class_create(THIS_MODULE, "drm_aux-dev");
> +	if (IS_ERR(drm_aux_dev_class)) {
> +		res = PTR_ERR(drm_aux_dev_class);
> +		goto out_unreg_chrdev;
> +	}
> +	drm_aux_dev_class->dev_groups = drm_aux_groups;
> +
> +	/* Keep track of DP aux that will be added or removed later */
> +	drm_dp_aux_set_aux_dev(&auxdev_inform_cb);
> +
> +	/* Bind to already existing DP aux */
> +	res = drm_dp_aux_for_each(&drm_dev_major, auxdev_attach_aux);
> +
> +	return 0;
> +
> +out_unreg_chrdev:
> +	unregister_chrdev(drm_dev_major, "aux");
> +out:
> +	printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);
> +	return res;
> +}
> +
> +static void __exit drm_aux_dev_exit(void)
> +{
> +	printk(KERN_INFO "drm dp aux /dev entries driver - unloading\n");
> +	drm_dp_aux_set_aux_dev(NULL);
> +	drm_dp_aux_for_each(&drm_dev_major, auxdev_detach_aux);
> +	class_destroy(drm_aux_dev_class);
> +	unregister_chrdev(drm_dev_major, "aux");
> +}
> +
> +MODULE_AUTHOR("Rafael Antognolli <rafael.antognolli@intel.com>");
> +MODULE_DESCRIPTION("DRM DP AUX /dev entries driver");
> +MODULE_LICENSE("GPL");
> +
> +module_init(drm_aux_dev_init);
> +module_exit(drm_aux_dev_exit);
> -- 
> 2.4.3
> 
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-- 
Ville Syrjälä
Intel OTC
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