linux-kernel - Re: [RFC PATCH v2 3/4] drivers/auxdisplay: senshat Raspberry Pi Sense HAT display driver

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Date:   Thu, 16 Sep 2021 13:00:14 +0200
From:   Matthias Brugger <mbrugger@...e.com>
To:     nsaenzju@...hat.com, Charles Mirabile <cmirabil@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Serge Schneider <serge@...pberrypi.org>,
        Stefan Wahren <stefan.wahren@...e.com>,
        linux-rpi-kernel@...ts.infradead.org, fedora-rpi@...glegroups.com,
        Mwesigwa Guma <mguma@...hat.com>,
        Joel Savitz <jsavitz@...hat.com>
Subject: Re: [RFC PATCH v2 3/4] drivers/auxdisplay: senshat Raspberry Pi Sense
 HAT display driver

On 30/08/2021 15:28, nsaenzju@...hat.com wrote:
> Hi Charles,
> 
> On Fri, 2021-08-20 at 14:08 -0400, Charles Mirabile wrote:
>> This patch implements control of the 8x8 RGB LED matrix display.
> 
> It'd be nice to get a more information on the i2c interface, and what each byte
> is supposed to represent.
> 
>> Signed-off-by: Charles Mirabile <cmirabil@...hat.com>
>> Signed-off-by: Mwesigwa Guma <mguma@...hat.com>
>> Signed-off-by: Joel Savitz <jsavitz@...hat.com>
>> ---
> 
> [...]
> 
>> +static long sensehat_display_ioctl(struct file *filp, unsigned int cmd,
>> +			     unsigned long arg)
>> +{
>> +	struct sensehat *sensehat = container_of(filp->private_data, struct sensehat, display.mdev);
>> +	struct sensehat_display *sensehat_display = &sensehat->display;
>> +	void __user *user_ptr = (void __user *)arg;
>> +	u8 temp[GAMMA_SIZE];
>> +	int ret;
>> +
>> +	if (mutex_lock_interruptible(&sensehat_display->rw_mtx))
>> +		return -ERESTARTSYS;
>> +	switch (cmd) {
>> +	case SENSEDISP_IOGET_GAMMA:
>> +		if (copy_to_user(user_ptr, sensehat_display->gamma, GAMMA_SIZE)) {
>> +			ret = -EFAULT;
>> +			goto out_unlock;
>> +		}
>> +		ret = 0;
>> +		goto out_unlock;
>> +	case SENSEDISP_IOSET_GAMMA:
>> +		if (copy_from_user(temp, user_ptr, GAMMA_SIZE)) {
>> +			ret = -EFAULT;
>> +			goto out_unlock;
>> +		}
>> +		ret = 0;
>> +		goto out_update;
>> +	case SENSEDISP_IORESET_GAMMA:
>> +		if (arg < GAMMA_DEFAULT || arg >= GAMMA_PRESET_COUNT) {
>> +			ret = -EINVAL;
>> +			goto out_unlock;
>> +		}
>> +		memcpy(temp, gamma_presets[arg], GAMMA_SIZE);
>> +		ret = 0;
>> +		goto out_update;
>> +	default:
>> +		ret = -EINVAL;
>> +		goto out_unlock;
>> +	}
>> +out_update:
>> +	memcpy(sensehat_display->gamma, temp, GAMMA_SIZE);
>> +	sensehat_update_display(sensehat);
>> +out_unlock:
>> +	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,
>> +};
> 
> I doubt this approach will make it upstream. This should use an already
> existing kernel interface, or if not good enough, extend it. I'm sure this is
> not the first RGB led matrix to show up anyway. Maybe drivers/leds has
> infrastructure to deal with this. Or else a fb device?

I'm working on upstreaming that again and my approach is to use framebuffer for 
this. What is the reason you decided against framebuffer?

Regards,
Matthias

> 
> I presume you want to keep the IOCTL in order to be able to run RPi specific
> aplications/libraries. It'll be up to them to change once we settle on a proper
> way of handling this, not the other way around. Note that the RPi engineers
> have always preffered using official kernel interfaces when available, so I
> don't think this'll be problematic.
> 
> Regards,
> Nicolas
>