Hi Ashutosh,
On 30-09-2023 03:11, Dixit, Ashutosh wrote:
On Fri, 29 Sep 2023 09:48:36 -0700, Dixit, Ashutosh wrote:
Hi Badal,
On Thu, 28 Sep 2023 23:37:35 -0700, Nilawar, Badal wrote:
On 28-09-2023 10:25, Dixit, Ashutosh wrote:
On Wed, 27 Sep 2023 01:39:46 -0700, Nilawar, Badal wrote:
On 27-09-2023 10:23, Dixit, Ashutosh wrote:
On Mon, 25 Sep 2023 01:18:38 -0700, Badal Nilawar wrote:
+static umode_t
+xe_hwmon_is_visible(const void *drvdata, enum hwmon_sensor_types type,
+ u32 attr, int channel)
+{
+ struct xe_hwmon *hwmon = (struct xe_hwmon *)drvdata;
+ int ret;
+
+ xe_device_mem_access_get(gt_to_xe(hwmon->gt));
Maybe we do xe_device_mem_access_get/put in xe_hwmon_process_reg where it
is needed? E.g. xe_hwmon_is_visible doesn't need to do this because it
doesn't read/write registers.
Agreed, but visible function is called only once while registering hwmon
interface, which happen during driver probe. During driver probe device
will be in resumed state. So no harm in keeping
xe_device_mem_access_get/put in visible function.
To me it doesn't make any sense to keep xe_device_mem_access_get/put
anywhere except in xe_hwmon_process_reg where the HW access actually
happens. We can eliminate xe_device_mem_access_get/put's all over the place
if we do it. Isn't it?
Agreed, thought process here suggest that take rpm wakeref at lowest
possible level. I already tried this in rfc series and in some extent in
rev2. There is problem with this approach. See my comments below.
The only restriction I have heard of (though not sure why) is that
xe_device_mem_access_get/put should not be called under lock>. Though I am
not sure it is for spinlock or also mutex. So as we were saying the locking
will also need to move to xe_hwmon_process_reg.
Yes from rev2 comments its dangerous to take mutex before
xe_device_mem_access_get/put. With code for "PL1 disable/restore during
resume" I saw deadlock. Scenario was power1_max write -> mutex lock -> rpm
resume -> disable pl1 -> mutex lock (dead lock here).
But this is already the wrong order as mentioned below. If we follow the
below order do we still see deadlock?
So:
xe_hwmon_process_reg()
{
xe_device_mem_access_get
mutex_lock
...
mutex_unlock
xe_device_mem_access_put
}
So once again if this is not possible for some reason let's figure out why.
There are two problems with this approach.
Problem 1: If you see implementation of xe_hwmon_power_max_write, reg
access is happening 3 times, so there will be 3 rpm suspend/resume
cycles. I was observing the same with rfc implementation. So in subsequent
series xe_device_mem_access_put/get is moved to top level functions
i.e. hwmon hooks.
This is not exactly correct because there is also a 1 second autosuspend
delay which will prevent such rpm suspend/resume cycles:
xe_pm_runtime_init:
pm_runtime_set_autosuspend_delay(dev, 1000);
rpm auto suspend delay can be 0 as well, IGT does set it to 0. In that
case there will be rpm cycle for every register access. So it better to
keep xe_device_mem_access_get/put at attribute level i.e. in hwmon hooks.
Problem 2: If locking moved inside xe_hwmon_process_reg then between two
subsequent reg accesses it will open small window during which race can
happen.
As Anshuman suggested in other thread for read are sequential and protected
by sysfs layer. So lets apply locking only for RW attributes.
But what is the locking trying to protect? As far as I understand it is
just the registers which have to be atomically modified/read. So it seems
sufficient to just protect the register accesses with the lock.
So I am still not convinced.
In i915 initially rmw accesses were protected with lock but later with
addition of PL1 disable (during resume) logic in some extent locking got
extended to attribute level.
Another scenario where locking is needed is for rw attributes where
write and read can happen from different threads.
For readonly attributes as per this
(https://elixir.bootlin.com/linux/latest/source/fs/seq_file.c) locking
is not needed.
I think lets apply locking at attribute level.
Let's figure out the locking first depending on what needs to be protected
(just registers or other data too). And then we can see where to put the
xe_device_mem_access_get/put's (following the rule that
xe_device_mem_access_get/put's should not be called under lock).
Regards,
Badal
Thanks.
--
Ashutosh
