From mboxrd@z Thu Jan  1 00:00:00 1970
From: lists@kaiser.cx (Martin Kaiser)
Date: Tue, 9 May 2017 15:47:55 +0200
Subject: devm_ioremap() in a sysfs show method
In-Reply-To: <20170424095321.GA30492@botnar.kaiser.cx>
References: <20170424095321.GA30492@botnar.kaiser.cx>
Message-ID: <20170509134755.GA18996@botnar.kaiser.cx>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

I've just had another look at this.

Thus wrote Martin Kaiser (lists at kaiser.cx):

> static ssize_t
> my_read_reg(struct device *pdev, struct device_attribute *attr, char *buf)
> {
>     u32 __iomem *reg;

>     reg = devm_ioremap(pdev, 0x581018C4, 4);
>     return scnprintf(buf, PAGE_SIZE, "0x%02x\n", ioread8(reg));
> }

> Is it ok to use devm_ioremap() on the device that I get here? If so, may
> I assume that devm_ioremap() re-uses an existing mapping and it's ok to
> call it for each sysfs access?

Using devm_ioremap() seems a bad idea. I get a different virtual address
in reg for each call to the read method.

Also, using dev_info() or similar debug prints on the device would crash
my kernel.

[   19.802171] [<c01aa494>] (dev_driver_string+0x0/0x4c) from [<c01aa60c>] (__dev_printk+0x44/0x7c)
[   19.811011] [<c01aa5c8>] (__dev_printk+0x0/0x7c) from [<c01aa6cc>] (dev_info+0x44/0x50)
[   19.819117]  r6:d388f1a0 r5:d3afd000 r4:d48f68d4
[   19.823840] [<c01aa688>] (dev_info+0x0/0x50) from [<c0013c6c>] (my_read_reg+0x3c/0x64)

It seems the device that's passed to the read method has some
limitations. In my setup, I created a kobject with NULL parent and
called sysfs_create_group() to attach my nodes to the kobject.

I'd appreciate any hints that help me understand what's going on behind
the scenes.

Thanks,
Martin