Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock
On Mon, Mar 08, 2021 at 03:03:58PM +0100, Sebastian Reichel wrote: > Hi, > > On Sat, Mar 06, 2021 at 11:56:45AM -0800, Rob Herring wrote: > > On Tue, Feb 23, 2021 at 02:26:57AM +0100, Sebastian Reichel wrote: > > > On Mon, Feb 22, 2021 at 10:26:26PM +0100, Alexandre Belloni wrote: > > > > On 22/02/2021 22:20:47+0100, Alexandre Belloni wrote: > > > > > On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote: > > > > > > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The > > > > > > modules SQW clock output defaults to 32768 Hz. This behaviour is > > > > > > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed, > > > > > > the clock is disabled and all i.MX6 functionality depending on > > > > > > the 32 KHz clock has undefined behaviour. On systems using hardware > > > > > > watchdog it seems to likely trigger a lot earlier than configured. > > > > > > > > > > > > The proper solution would be to describe this dependency in DT, > > > > > > but that will result in a deadlock. The kernel will see, that > > > > > > i.MX6 system clock needs the RTC clock and do probe deferral. > > > > > > But the i.MX6 I2C module never becomes usable without the i.MX6 > > > > > > CKIL clock and thus the RTC's clock will not be probed. So from > > > > > > the kernel's perspective this is a chicken-and-egg problem. > > > > > > > > > > > > > > > > Reading the previous paragraph, I was going to suggest describing the > > > > > dependency and wondering whether this would cause a circular > > > > > dependency. > > > > > I guess this will keep being an issue for clocks on an I2C or SPI > > > > > bus... > > > > > > Yes, it is a circular dependency on this particular system on > > > module. It only works because the RTC enables the clock by > > > default. The i.MX6 CKIL is expected to be always enabled. > > > > I think you should describe the circular clocking and then provide a way > > to break the dependency. > > This is very much not trivial. The clock is required during early > initialization of the i.MX. At this point we are far from probing > I2C drivers and without the I2C driver the clock is not registered. > The current i.MX code expects the system clocks to be fixed clocks, > since they must be enabled before any code is executed (incl. > bootloader) and must never be disabled. From a HW design point of > view it does not make sense to have a SW controllable clock for it, > since it just adds extra cost. I believe for QMX6 it is only SW > controllable, because that avoids the need for an extra crystal. > > So how is the clock framework supposed to know, that it can ignore > the clock during registration? I see the following options: > > 1. My solution is the simplest one. Keep i.MX clock code the same >(it assumes a fixed-clock being used for CKIL) and avoid >registering RTC clock. This basically means the RTC is considered >to be a fixed-clock on this system, which is what the HW designers >seemed to have in mind (vendor kernel for the QMX6 is old enough >(4.9.x) to not to have CLK feature in the RTC driver. Vendor >U-Boot also does not touch the RTC. Booting mainline kernel once >bricks QMX6 boards until RTC battery is removed, so one could >actually argue addition of the CLK feature in 1373e77b4f10 (4.13) >is a regression). Currently Qualcomm device uses "protected-clocks" >for FW controlled clocks where Linux would crash the system by >trying to access them. IMHO the RTC is similar, since disabling >or modifying its frequency on QMX6 results in undefined behaviour >and possibly system crash. > > 2. Make i.MX clock code use the RTC as CKIL clock provider, but >ignore it somehow. I see three sub-options: > > 2.1. Add a property 'boot-enabled' to the RTC node, so that the > clock framework is aware of clock being enabled. This can > be used to satisfy clock dependencies somehow. > > 2.2. The RTC device is not probed without I2C bus, but the driver > could also register a fake clock purely based on DT > information by adding some early init hook and take over > the clock once the I2C part is being probed. I think this > is a bad idea regarding maintainability of the driver. > Also for systems not using the RTC clock, the early clock > registration is basically wrong: If the kernel disables > the RTC it will stay disabled across boots if the RTC has > a backup battery. Basically we cannot imply anything from > the RTC compatible value alone. > > 2.3 The i.MX core code could request CKIL with some flag, that > it's fine to have an unresolvable clock and just expect it > to be boot-enabled. The rationale would be, that CKIL must > be always-enabled. I think 2.1 or 2.3 is fine. It boils down to detecting a cycle and then either you have a property or implicitly know to ignore a dependency. > > It's a somewhat common issue. > > It is? This only works, because one can
Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock
Hi, On Sat, Mar 06, 2021 at 11:56:45AM -0800, Rob Herring wrote: > On Tue, Feb 23, 2021 at 02:26:57AM +0100, Sebastian Reichel wrote: > > On Mon, Feb 22, 2021 at 10:26:26PM +0100, Alexandre Belloni wrote: > > > On 22/02/2021 22:20:47+0100, Alexandre Belloni wrote: > > > > On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote: > > > > > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The > > > > > modules SQW clock output defaults to 32768 Hz. This behaviour is > > > > > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed, > > > > > the clock is disabled and all i.MX6 functionality depending on > > > > > the 32 KHz clock has undefined behaviour. On systems using hardware > > > > > watchdog it seems to likely trigger a lot earlier than configured. > > > > > > > > > > The proper solution would be to describe this dependency in DT, > > > > > but that will result in a deadlock. The kernel will see, that > > > > > i.MX6 system clock needs the RTC clock and do probe deferral. > > > > > But the i.MX6 I2C module never becomes usable without the i.MX6 > > > > > CKIL clock and thus the RTC's clock will not be probed. So from > > > > > the kernel's perspective this is a chicken-and-egg problem. > > > > > > > > > > > > > Reading the previous paragraph, I was going to suggest describing the > > > > dependency and wondering whether this would cause a circular dependency. > > > > I guess this will keep being an issue for clocks on an I2C or SPI bus... > > > > Yes, it is a circular dependency on this particular system on > > module. It only works because the RTC enables the clock by > > default. The i.MX6 CKIL is expected to be always enabled. > > I think you should describe the circular clocking and then provide a way > to break the dependency. This is very much not trivial. The clock is required during early initialization of the i.MX. At this point we are far from probing I2C drivers and without the I2C driver the clock is not registered. The current i.MX code expects the system clocks to be fixed clocks, since they must be enabled before any code is executed (incl. bootloader) and must never be disabled. From a HW design point of view it does not make sense to have a SW controllable clock for it, since it just adds extra cost. I believe for QMX6 it is only SW controllable, because that avoids the need for an extra crystal. So how is the clock framework supposed to know, that it can ignore the clock during registration? I see the following options: 1. My solution is the simplest one. Keep i.MX clock code the same (it assumes a fixed-clock being used for CKIL) and avoid registering RTC clock. This basically means the RTC is considered to be a fixed-clock on this system, which is what the HW designers seemed to have in mind (vendor kernel for the QMX6 is old enough (4.9.x) to not to have CLK feature in the RTC driver. Vendor U-Boot also does not touch the RTC. Booting mainline kernel once bricks QMX6 boards until RTC battery is removed, so one could actually argue addition of the CLK feature in 1373e77b4f10 (4.13) is a regression). Currently Qualcomm device uses "protected-clocks" for FW controlled clocks where Linux would crash the system by trying to access them. IMHO the RTC is similar, since disabling or modifying its frequency on QMX6 results in undefined behaviour and possibly system crash. 2. Make i.MX clock code use the RTC as CKIL clock provider, but ignore it somehow. I see three sub-options: 2.1. Add a property 'boot-enabled' to the RTC node, so that the clock framework is aware of clock being enabled. This can be used to satisfy clock dependencies somehow. 2.2. The RTC device is not probed without I2C bus, but the driver could also register a fake clock purely based on DT information by adding some early init hook and take over the clock once the I2C part is being probed. I think this is a bad idea regarding maintainability of the driver. Also for systems not using the RTC clock, the early clock registration is basically wrong: If the kernel disables the RTC it will stay disabled across boots if the RTC has a backup battery. Basically we cannot imply anything from the RTC compatible value alone. 2.3 The i.MX core code could request CKIL with some flag, that it's fine to have an unresolvable clock and just expect it to be boot-enabled. The rationale would be, that CKIL must be always-enabled. > It's a somewhat common issue. It is? This only works, because one can treat the RTC's clock output like a fixed clock by not messing around with it. -- Sebastian signature.asc Description: PGP signature
Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock
On Tue, Feb 23, 2021 at 02:26:57AM +0100, Sebastian Reichel wrote: > Hi, > > On Mon, Feb 22, 2021 at 10:26:26PM +0100, Alexandre Belloni wrote: > > On 22/02/2021 22:20:47+0100, Alexandre Belloni wrote: > > > On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote: > > > > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The > > > > modules SQW clock output defaults to 32768 Hz. This behaviour is > > > > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed, > > > > the clock is disabled and all i.MX6 functionality depending on > > > > the 32 KHz clock has undefined behaviour. On systems using hardware > > > > watchdog it seems to likely trigger a lot earlier than configured. > > > > > > > > The proper solution would be to describe this dependency in DT, > > > > but that will result in a deadlock. The kernel will see, that > > > > i.MX6 system clock needs the RTC clock and do probe deferral. > > > > But the i.MX6 I2C module never becomes usable without the i.MX6 > > > > CKIL clock and thus the RTC's clock will not be probed. So from > > > > the kernel's perspective this is a chicken-and-egg problem. > > > > > > > > > > Reading the previous paragraph, I was going to suggest describing the > > > dependency and wondering whether this would cause a circular dependency. > > > I guess this will keep being an issue for clocks on an I2C or SPI bus... > > Yes, it is a circular dependency on this particular system on > module. It only works because the RTC enables the clock by > default. The i.MX6 CKIL is expected to be always enabled. I think you should describe the circular clocking and then provide a way to break the dependency. It's a somewhat common issue. Rob
Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock
Hi, On Mon, Feb 22, 2021 at 10:26:26PM +0100, Alexandre Belloni wrote: > On 22/02/2021 22:20:47+0100, Alexandre Belloni wrote: > > On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote: > > > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The > > > modules SQW clock output defaults to 32768 Hz. This behaviour is > > > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed, > > > the clock is disabled and all i.MX6 functionality depending on > > > the 32 KHz clock has undefined behaviour. On systems using hardware > > > watchdog it seems to likely trigger a lot earlier than configured. > > > > > > The proper solution would be to describe this dependency in DT, > > > but that will result in a deadlock. The kernel will see, that > > > i.MX6 system clock needs the RTC clock and do probe deferral. > > > But the i.MX6 I2C module never becomes usable without the i.MX6 > > > CKIL clock and thus the RTC's clock will not be probed. So from > > > the kernel's perspective this is a chicken-and-egg problem. > > > > > > > Reading the previous paragraph, I was going to suggest describing the > > dependency and wondering whether this would cause a circular dependency. > > I guess this will keep being an issue for clocks on an I2C or SPI bus... Yes, it is a circular dependency on this particular system on module. It only works because the RTC enables the clock by default. The i.MX6 CKIL is expected to be always enabled. > > > Technically everything is fine by not touching anything, since > > > the RTC clock correctly enables the clock on reset (i.e. on > > > battery backup power loss) and also the bootloader enables it > > > in case a kernel without this support has been booted. > > > > > > The 'protected-clocks' property is already in use for some clocks > > > that may not be touched because of firmware limitations and is > > > described in Documentation/devicetree/bindings/clock/clock-bindings.txt. > > > > > > Signed-off-by: Sebastian Reichel > > Acked-by: Alexandre Belloni > > Or maybe you expected me to apply the patch, how are the following > patches dependent on this one? The last patch, which introduces a new board has a runtime dependency on this patch. Without this feature the board goes into a reboot loop because its bootloader enables the i.MX6 watchdog and without the CKIL its timing is messed up. But it's a pure runtime dependency for a new board, so it should be fine to merge this via your tree. It basically means the board is only working once your tree and arm tree have been merged, which seems ok from my POV. -- Sebastian signature.asc Description: PGP signature
Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock
On 22/02/2021 22:20:47+0100, Alexandre Belloni wrote: > On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote: > > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The > > modules SQW clock output defaults to 32768 Hz. This behaviour is > > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed, > > the clock is disabled and all i.MX6 functionality depending on > > the 32 KHz clock has undefined behaviour. On systems using hardware > > watchdog it seems to likely trigger a lot earlier than configured. > > > > The proper solution would be to describe this dependency in DT, > > but that will result in a deadlock. The kernel will see, that > > i.MX6 system clock needs the RTC clock and do probe deferral. > > But the i.MX6 I2C module never becomes usable without the i.MX6 > > CKIL clock and thus the RTC's clock will not be probed. So from > > the kernel's perspective this is a chicken-and-egg problem. > > > > Reading the previous paragraph, I was going to suggest describing the > dependency and wondering whether this would cause a circular dependency. > I guess this will keep being an issue for clocks on an I2C or SPI bus... > > > Technically everything is fine by not touching anything, since > > the RTC clock correctly enables the clock on reset (i.e. on > > battery backup power loss) and also the bootloader enables it > > in case a kernel without this support has been booted. > > > > The 'protected-clocks' property is already in use for some clocks > > that may not be touched because of firmware limitations and is > > described in Documentation/devicetree/bindings/clock/clock-bindings.txt. > > > > Signed-off-by: Sebastian Reichel > Acked-by: Alexandre Belloni Or maybe you expected me to apply the patch, how are the following patches dependent on this one? -- Alexandre Belloni, co-owner and COO, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock
On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote: > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The > modules SQW clock output defaults to 32768 Hz. This behaviour is > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed, > the clock is disabled and all i.MX6 functionality depending on > the 32 KHz clock has undefined behaviour. On systems using hardware > watchdog it seems to likely trigger a lot earlier than configured. > > The proper solution would be to describe this dependency in DT, > but that will result in a deadlock. The kernel will see, that > i.MX6 system clock needs the RTC clock and do probe deferral. > But the i.MX6 I2C module never becomes usable without the i.MX6 > CKIL clock and thus the RTC's clock will not be probed. So from > the kernel's perspective this is a chicken-and-egg problem. > Reading the previous paragraph, I was going to suggest describing the dependency and wondering whether this would cause a circular dependency. I guess this will keep being an issue for clocks on an I2C or SPI bus... > Technically everything is fine by not touching anything, since > the RTC clock correctly enables the clock on reset (i.e. on > battery backup power loss) and also the bootloader enables it > in case a kernel without this support has been booted. > > The 'protected-clocks' property is already in use for some clocks > that may not be touched because of firmware limitations and is > described in Documentation/devicetree/bindings/clock/clock-bindings.txt. > > Signed-off-by: Sebastian Reichel Acked-by: Alexandre Belloni > --- > Documentation/devicetree/bindings/rtc/rtc-m41t80.txt | 1 + > drivers/rtc/rtc-m41t80.c | 3 +++ > 2 files changed, 4 insertions(+) > > diff --git a/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt > b/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt > index c746cb221210..ea4bbf5c4282 100644 > --- a/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt > +++ b/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt > @@ -19,6 +19,7 @@ Optional properties: > - interrupts: rtc alarm interrupt. > - clock-output-names: From common clock binding to override the default > output >clock name > +- protected-clocks: Bool, if set operating system should not handle clock. > - wakeup-source: Enables wake up of host system on alarm > > Example: > diff --git a/drivers/rtc/rtc-m41t80.c b/drivers/rtc/rtc-m41t80.c > index 160dcf68e64e..3296583853a8 100644 > --- a/drivers/rtc/rtc-m41t80.c > +++ b/drivers/rtc/rtc-m41t80.c > @@ -546,6 +546,9 @@ static struct clk *m41t80_sqw_register_clk(struct > m41t80_data *m41t80) > struct clk_init_data init; > int ret; > > + if (of_property_read_bool(node, "protected-clocks")) > + return 0; > + > /* First disable the clock */ > ret = i2c_smbus_read_byte_data(client, M41T80_REG_ALARM_MON); > if (ret < 0) > -- > 2.30.0 > -- Alexandre Belloni, co-owner and COO, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
