[PATCH] rtc: sun6i: Add NVMEM provider

[PATCH] rtc: sun6i: Add NVMEM provider

[Samuel Holland]

The sun6i RTC provides 32 bytes of general-purpose data registers.
They can be used to save data in the always-on RTC power domain.
The registers are writable via 32-bit MMIO accesses only.

Expose the region as a NVMEM provider so it can be used by userspace and
other drivers.

Signed-off-by: Samuel Holland <samuel@sholland.org>
---
 drivers/rtc/rtc-sun6i.c | 42 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/drivers/rtc/rtc-sun6i.c b/drivers/rtc/rtc-sun6i.c
index e75020ab8024..f4a5e7465148 100644
--- a/drivers/rtc/rtc-sun6i.c
+++ b/drivers/rtc/rtc-sun6i.c
@@ -69,6 +69,10 @@
 #define SUN6I_LOSC_OUT_GATING			0x0060
 #define SUN6I_LOSC_OUT_GATING_EN_OFFSET		0
 
+/* General-purpose data */
+#define SUN6I_GP_DATA				0x0100
+#define SUN6I_GP_DATA_SIZE			0x20
+
 /*
  * Get date values
  */
@@ -641,6 +645,39 @@ static const struct rtc_class_ops sun6i_rtc_ops = {
 	.alarm_irq_enable	= sun6i_rtc_alarm_irq_enable
 };
 
+static int sun6i_rtc_nvmem_read(void *priv, unsigned int offset, void *_val, size_t bytes)
+{
+	struct sun6i_rtc_dev *chip = priv;
+	u32 *val = _val;
+	int i;
+
+	for (i = 0; i < bytes / 4; ++i)
+		val[i] = readl(chip->base + SUN6I_GP_DATA + offset + 4 * i);
+
+	return 0;
+}
+
+static int sun6i_rtc_nvmem_write(void *priv, unsigned int offset, void *_val, size_t bytes)
+{
+	struct sun6i_rtc_dev *chip = priv;
+	u32 *val = _val;
+	int i;
+
+	for (i = 0; i < bytes / 4; ++i)
+		writel(val[i], chip->base + SUN6I_GP_DATA + offset + 4 * i);
+
+	return 0;
+}
+
+static struct nvmem_config sun6i_rtc_nvmem_cfg = {
+	.type		= NVMEM_TYPE_BATTERY_BACKED,
+	.reg_read	= sun6i_rtc_nvmem_read,
+	.reg_write	= sun6i_rtc_nvmem_write,
+	.size		= SUN6I_GP_DATA_SIZE,
+	.word_size	= 4,
+	.stride		= 4,
+};
+
 /* Enable IRQ wake on suspend, to wake up from RTC. */
 static int sun6i_rtc_suspend(struct device *dev)
 {
@@ -728,6 +765,11 @@ static int sun6i_rtc_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	sun6i_rtc_nvmem_cfg.priv = chip;
+	ret = devm_rtc_nvmem_register(chip->rtc, &sun6i_rtc_nvmem_cfg);
+	if (ret)
+		return ret;
+
 	dev_info(&pdev->dev, "RTC enabled\n");
 
 	return 0;
-- 
2.26.3

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[Maxime Ripard]

[-- Attachment #1: Type: text/plain, Size: 692 bytes --]

Hi,

On Sun, Apr 18, 2021 at 08:45:49PM -0500, Samuel Holland wrote:
> The sun6i RTC provides 32 bytes of general-purpose data registers.
> They can be used to save data in the always-on RTC power domain.
> The registers are writable via 32-bit MMIO accesses only.
> 
> Expose the region as a NVMEM provider so it can be used by userspace and
> other drivers.
> 
> Signed-off-by: Samuel Holland <samuel@sholland.org>

As far as I understood, you want to use those registers to implement
super-standby? If so, while it makes sense for the kernel to be able to
be able to write to those registers, I guess it would be a bit unwise to
allow the userspace to access it?

Maxime

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[Alexandre Belloni]

On 30/04/2021 11:02:06+0200, Maxime Ripard wrote:
> Hi,
> 
> On Sun, Apr 18, 2021 at 08:45:49PM -0500, Samuel Holland wrote:
> > The sun6i RTC provides 32 bytes of general-purpose data registers.
> > They can be used to save data in the always-on RTC power domain.
> > The registers are writable via 32-bit MMIO accesses only.
> > 
> > Expose the region as a NVMEM provider so it can be used by userspace and
> > other drivers.
> > 
> > Signed-off-by: Samuel Holland <samuel@sholland.org>
> 
> As far as I understood, you want to use those registers to implement
> super-standby? If so, while it makes sense for the kernel to be able to
> be able to write to those registers, I guess it would be a bit unwise to
> allow the userspace to access it?

I would think nvmem is still the proper subsystem. I guess maybe we
should have a version of __nvmem_device_get that would ensure exclusive
access to a cell, thus preventing userspace accessing it as long a the
kernel is using it.

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[Samuel Holland]

On 4/30/21 4:02 AM, Maxime Ripard wrote:
> Hi,
> 
> On Sun, Apr 18, 2021 at 08:45:49PM -0500, Samuel Holland wrote:
>> The sun6i RTC provides 32 bytes of general-purpose data registers.
>> They can be used to save data in the always-on RTC power domain.
>> The registers are writable via 32-bit MMIO accesses only.
>>
>> Expose the region as a NVMEM provider so it can be used by userspace and
>> other drivers.
>>
>> Signed-off-by: Samuel Holland <samuel@sholland.org>
> 
> As far as I understood, you want to use those registers to implement
> super-standby? If so, while it makes sense for the kernel to be able to
> be able to write to those registers, I guess it would be a bit unwise to
> allow the userspace to access it?

I want the user to be able to pass information to the bootloader (to
select a boot device, e.g. reboot to FEL). I also want the user to be
able to read data stored to these registers by system firmware (e.g.
crust writes exception information there). It's not really related to
standby.

I would want to stack a nvmem-reboot-mode on top to give friendlier
names to some of the numbers, but I don't see a problem with root having
direct access to the registers. It's no different from /dev/nvram
providing access to the PC CMOS RAM.

Regards,
Samuel

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[Alexandre Belloni]

On 09/05/2021 22:39:30-0500, Samuel Holland wrote:
> On 4/30/21 4:02 AM, Maxime Ripard wrote:
> > Hi,
> > 
> > On Sun, Apr 18, 2021 at 08:45:49PM -0500, Samuel Holland wrote:
> >> The sun6i RTC provides 32 bytes of general-purpose data registers.
> >> They can be used to save data in the always-on RTC power domain.
> >> The registers are writable via 32-bit MMIO accesses only.
> >>
> >> Expose the region as a NVMEM provider so it can be used by userspace and
> >> other drivers.
> >>
> >> Signed-off-by: Samuel Holland <samuel@sholland.org>
> > 
> > As far as I understood, you want to use those registers to implement
> > super-standby? If so, while it makes sense for the kernel to be able to
> > be able to write to those registers, I guess it would be a bit unwise to
> > allow the userspace to access it?
> 
> I want the user to be able to pass information to the bootloader (to
> select a boot device, e.g. reboot to FEL). I also want the user to be
> able to read data stored to these registers by system firmware (e.g.
> crust writes exception information there). It's not really related to
> standby.
> 
> I would want to stack a nvmem-reboot-mode on top to give friendlier
> names to some of the numbers, but I don't see a problem with root having
> direct access to the registers. It's no different from /dev/nvram
> providing access to the PC CMOS RAM.
> 

(which is deprecated in favor of nvmem)


-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[Maxime Ripard]

[-- Attachment #1: Type: text/plain, Size: 1972 bytes --]

On Sun, May 09, 2021 at 10:39:30PM -0500, Samuel Holland wrote:
> On 4/30/21 4:02 AM, Maxime Ripard wrote:
> > Hi,
> > 
> > On Sun, Apr 18, 2021 at 08:45:49PM -0500, Samuel Holland wrote:
> >> The sun6i RTC provides 32 bytes of general-purpose data registers.
> >> They can be used to save data in the always-on RTC power domain.
> >> The registers are writable via 32-bit MMIO accesses only.
> >>
> >> Expose the region as a NVMEM provider so it can be used by userspace and
> >> other drivers.
> >>
> >> Signed-off-by: Samuel Holland <samuel@sholland.org>
> > 
> > As far as I understood, you want to use those registers to implement
> > super-standby? If so, while it makes sense for the kernel to be able to
> > be able to write to those registers, I guess it would be a bit unwise to
> > allow the userspace to access it?
> 
> I want the user to be able to pass information to the bootloader (to
> select a boot device, e.g. reboot to FEL). I also want the user to be
> able to read data stored to these registers by system firmware (e.g.
> crust writes exception information there). It's not really related to
> standby.

What information do you want to provide? This looks like punching
through the abstraction layer provided by the kernel. This is also an
issue since it ties an ABI to the use of crust: if there's another user
for those RTC registers at some point, the userspace would have no way
to tell whether or not crust is being used and might get complete
garbage (compared to what crust usually provides) instead.

> I would want to stack a nvmem-reboot-mode on top to give friendlier
> names to some of the numbers, but I don't see a problem with root having
> direct access to the registers. It's no different from /dev/nvram
> providing access to the PC CMOS RAM.

And those solutions have issues too. efivarfs for example can totally
brick the system it runs on if the user has an unfortunate rm -rf.

Maxime

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[Samuel Holland]

On 5/25/21 3:24 AM, Maxime Ripard wrote:
> On Sun, May 09, 2021 at 10:39:30PM -0500, Samuel Holland wrote:
>> On 4/30/21 4:02 AM, Maxime Ripard wrote:
>>> Hi,
>>>
>>> On Sun, Apr 18, 2021 at 08:45:49PM -0500, Samuel Holland wrote:
>>>> The sun6i RTC provides 32 bytes of general-purpose data registers.
>>>> They can be used to save data in the always-on RTC power domain.
>>>> The registers are writable via 32-bit MMIO accesses only.
>>>>
>>>> Expose the region as a NVMEM provider so it can be used by userspace and
>>>> other drivers.
>>>>
>>>> Signed-off-by: Samuel Holland <samuel@sholland.org>
>>>
>>> As far as I understood, you want to use those registers to implement
>>> super-standby? If so, while it makes sense for the kernel to be able to
>>> be able to write to those registers, I guess it would be a bit unwise to
>>> allow the userspace to access it?
>>
>> I want the user to be able to pass information to the bootloader (to
>> select a boot device, e.g. reboot to FEL). I also want the user to be
>> able to read data stored to these registers by system firmware (e.g.
>> crust writes exception information there). It's not really related to
>> standby.
> 
> What information do you want to provide? This looks like punching
> through the abstraction layer provided by the kernel.

I don't see how registering an NVMEM provider is punching through any
abstraction layer. NVMEM _is_ the relevant abstraction layer provided by
the kernel. What else would you suggest I use?

> This is also an issue since it ties an ABI to the use of crust: if
> there's another user for those RTC registers at some point, the
> userspace would have no way to tell whether or not crust is being
> used and might get complete garbage (compared to what crust usually
> provides) instead.

If the user chooses to interact with the NVMEM via sysfs, then the user
has to deal with the consequences. This is equivalent to the user's
responsibility when using, for example, /dev/mem or libusb.

For the purpose of providing the abstraction, the actual data stored
there is irrelevant. And if you want to partition up the registers to
reserve for different users, that is what NVMEM cells accomplish.

>> I would want to stack a nvmem-reboot-mode on top to give friendlier
>> names to some of the numbers, but I don't see a problem with root having
>> direct access to the registers. It's no different from /dev/nvram
>> providing access to the PC CMOS RAM.
> 
> And those solutions have issues too. efivarfs for example can totally
> brick the system it runs on if the user has an unfortunate rm -rf.

The sun6i RTC, like PC CMOS, is battery-backed RAM. efivarfs writes to
flash. The magnitude of any such "danger" is vastly different between
the two. (And I think we can do better than vendors who release those
brickable EFI implementations.)

Regards,
Samuel

[PATCH] rtc: sun6i: Add NVMEM provider

[Samuel Holland]

The sun6i RTC provides 32 bytes of general-purpose data registers.
They can be used to save data in the always-on RTC power domain.
The registers are writable via 32-bit MMIO accesses only.

Expose them with a NVMEM provider so they can be used by other drivers.

Signed-off-by: Samuel Holland <samuel@sholland.org>
---

 drivers/rtc/rtc-sun6i.c | 42 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/drivers/rtc/rtc-sun6i.c b/drivers/rtc/rtc-sun6i.c
index 5b3e4da63406..755aeb82a285 100644
--- a/drivers/rtc/rtc-sun6i.c
+++ b/drivers/rtc/rtc-sun6i.c
@@ -71,6 +71,10 @@
 #define SUN6I_LOSC_OUT_GATING			0x0060
 #define SUN6I_LOSC_OUT_GATING_EN_OFFSET		0
 
+/* General-purpose data */
+#define SUN6I_GP_DATA				0x0100
+#define SUN6I_GP_DATA_SIZE			0x20
+
 /*
  * Get date values
  */
@@ -662,6 +666,39 @@ static const struct rtc_class_ops sun6i_rtc_ops = {
 	.alarm_irq_enable	= sun6i_rtc_alarm_irq_enable
 };
 
+static int sun6i_rtc_nvmem_read(void *priv, unsigned int offset, void *_val, size_t bytes)
+{
+	struct sun6i_rtc_dev *chip = priv;
+	u32 *val = _val;
+	int i;
+
+	for (i = 0; i < bytes / 4; ++i)
+		val[i] = readl(chip->base + SUN6I_GP_DATA + offset + 4 * i);
+
+	return 0;
+}
+
+static int sun6i_rtc_nvmem_write(void *priv, unsigned int offset, void *_val, size_t bytes)
+{
+	struct sun6i_rtc_dev *chip = priv;
+	u32 *val = _val;
+	int i;
+
+	for (i = 0; i < bytes / 4; ++i)
+		writel(val[i], chip->base + SUN6I_GP_DATA + offset + 4 * i);
+
+	return 0;
+}
+
+static struct nvmem_config sun6i_rtc_nvmem_cfg = {
+	.type		= NVMEM_TYPE_BATTERY_BACKED,
+	.reg_read	= sun6i_rtc_nvmem_read,
+	.reg_write	= sun6i_rtc_nvmem_write,
+	.size		= SUN6I_GP_DATA_SIZE,
+	.word_size	= 4,
+	.stride		= 4,
+};
+
 #ifdef CONFIG_PM_SLEEP
 /* Enable IRQ wake on suspend, to wake up from RTC. */
 static int sun6i_rtc_suspend(struct device *dev)
@@ -795,6 +832,11 @@ static int sun6i_rtc_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	sun6i_rtc_nvmem_cfg.priv = chip;
+	ret = devm_rtc_nvmem_register(chip->rtc, &sun6i_rtc_nvmem_cfg);
+	if (ret)
+		return ret;
+
 	dev_info(&pdev->dev, "RTC enabled\n");
 
 	return 0;
-- 
2.35.1

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[Jernej Škrabec]

Dne četrtek, 14. april 2022 ob 01:17:30 CEST je Samuel Holland napisal(a):
> The sun6i RTC provides 32 bytes of general-purpose data registers.
> They can be used to save data in the always-on RTC power domain.
> The registers are writable via 32-bit MMIO accesses only.
> 
> Expose them with a NVMEM provider so they can be used by other drivers.
> 
> Signed-off-by: Samuel Holland <samuel@sholland.org>

Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>

Best regards,
Jernej

> ---
> 
>  drivers/rtc/rtc-sun6i.c | 42 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/drivers/rtc/rtc-sun6i.c b/drivers/rtc/rtc-sun6i.c
> index 5b3e4da63406..755aeb82a285 100644
> --- a/drivers/rtc/rtc-sun6i.c
> +++ b/drivers/rtc/rtc-sun6i.c
> @@ -71,6 +71,10 @@
>  #define SUN6I_LOSC_OUT_GATING			0x0060
>  #define SUN6I_LOSC_OUT_GATING_EN_OFFSET		0
> 
> +/* General-purpose data */
> +#define SUN6I_GP_DATA				0x0100
> +#define SUN6I_GP_DATA_SIZE			0x20
> +
>  /*
>   * Get date values
>   */
> @@ -662,6 +666,39 @@ static const struct rtc_class_ops sun6i_rtc_ops = {
>  	.alarm_irq_enable	= sun6i_rtc_alarm_irq_enable
>  };
> 
> +static int sun6i_rtc_nvmem_read(void *priv, unsigned int offset, void
> *_val, size_t bytes) +{
> +	struct sun6i_rtc_dev *chip = priv;
> +	u32 *val = _val;
> +	int i;
> +
> +	for (i = 0; i < bytes / 4; ++i)
> +		val[i] = readl(chip->base + SUN6I_GP_DATA + offset + 4 * 
i);
> +
> +	return 0;
> +}
> +
> +static int sun6i_rtc_nvmem_write(void *priv, unsigned int offset, void
> *_val, size_t bytes) +{
> +	struct sun6i_rtc_dev *chip = priv;
> +	u32 *val = _val;
> +	int i;
> +
> +	for (i = 0; i < bytes / 4; ++i)
> +		writel(val[i], chip->base + SUN6I_GP_DATA + offset + 4 * 
i);
> +
> +	return 0;
> +}
> +
> +static struct nvmem_config sun6i_rtc_nvmem_cfg = {
> +	.type		= NVMEM_TYPE_BATTERY_BACKED,
> +	.reg_read	= sun6i_rtc_nvmem_read,
> +	.reg_write	= sun6i_rtc_nvmem_write,
> +	.size		= SUN6I_GP_DATA_SIZE,
> +	.word_size	= 4,
> +	.stride		= 4,
> +};
> +
>  #ifdef CONFIG_PM_SLEEP
>  /* Enable IRQ wake on suspend, to wake up from RTC. */
>  static int sun6i_rtc_suspend(struct device *dev)
> @@ -795,6 +832,11 @@ static int sun6i_rtc_probe(struct platform_device
> *pdev) if (ret)
>  		return ret;
> 
> +	sun6i_rtc_nvmem_cfg.priv = chip;
> +	ret = devm_rtc_nvmem_register(chip->rtc, &sun6i_rtc_nvmem_cfg);
> +	if (ret)
> +		return ret;
> +
>  	dev_info(&pdev->dev, "RTC enabled\n");
> 
>  	return 0;

Re: [PATCH] rtc: sun6i: Add NVMEM provider

[alexandre.belloni]

From: Alexandre Belloni <alexandre.belloni@bootlin.com>

On Wed, 13 Apr 2022 18:17:30 -0500, Samuel Holland wrote:
> The sun6i RTC provides 32 bytes of general-purpose data registers.
> They can be used to save data in the always-on RTC power domain.
> The registers are writable via 32-bit MMIO accesses only.
> 
> Expose them with a NVMEM provider so they can be used by other drivers.
> 
> 
> [...]

Applied, thanks!

[1/1] rtc: sun6i: Add NVMEM provider
      commit: 581d6d8f483696ff164f52a71beb43a87b718592

Best regards,
-- 
Alexandre Belloni <alexandre.belloni@bootlin.com>