Re: [PATCH v9 3/3] soc: qcom: rpmh: Invoke rpmh_flush() for dirty caches

[Maulik Shah <mkshah@codeaurora.org>]

On 2/29/2020 5:15 AM, Doug Anderson wrote:
> Hi,
>
> On Fri, Feb 28, 2020 at 3:38 AM Maulik Shah <mkshah@codeaurora.org> wrote:
>> Add changes to invoke rpmh flush() from within cache_lock when the data
>> in cache is dirty.
>>
>> This is done only if OSI is not supported in PSCI. If OSI is supported
>> rpmh_flush can get invoked when the last cpu going to power collapse
>> deepest low power mode.
>>
>> Also remove "depends on COMPILE_TEST" for Kconfig option QCOM_RPMH so the
>> driver is only compiled for arm64 which supports psci_has_osi_support()
>> API.
>>
>> Signed-off-by: Maulik Shah <mkshah@codeaurora.org>
>> Reviewed-by: Srinivas Rao L <lsrao@codeaurora.org>
>> ---
>>   drivers/soc/qcom/Kconfig |  2 +-
>>   drivers/soc/qcom/rpmh.c  | 33 ++++++++++++++++++++++-----------
>>   2 files changed, 23 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/soc/qcom/Kconfig b/drivers/soc/qcom/Kconfig
>> index d0a73e7..2e581bc 100644
>> --- a/drivers/soc/qcom/Kconfig
>> +++ b/drivers/soc/qcom/Kconfig
>> @@ -105,7 +105,7 @@ config QCOM_RMTFS_MEM
>>
>>   config QCOM_RPMH
>>          bool "Qualcomm RPM-Hardened (RPMH) Communication"
>> -       depends on ARCH_QCOM && ARM64 || COMPILE_TEST
>> +       depends on ARCH_QCOM && ARM64
>>          help
>>            Support for communication with the hardened-RPM blocks in
>>            Qualcomm Technologies Inc (QTI) SoCs. RPMH communication uses an
>> diff --git a/drivers/soc/qcom/rpmh.c b/drivers/soc/qcom/rpmh.c
>> index f28afe4..6a5a60c 100644
>> --- a/drivers/soc/qcom/rpmh.c
>> +++ b/drivers/soc/qcom/rpmh.c
>> @@ -12,6 +12,7 @@
>>   #include <linux/module.h>
>>   #include <linux/of.h>
>>   #include <linux/platform_device.h>
>> +#include <linux/psci.h>
>>   #include <linux/slab.h>
>>   #include <linux/spinlock.h>
>>   #include <linux/types.h>
>> @@ -158,6 +159,13 @@ static struct cache_req *cache_rpm_request(struct rpmh_ctrlr *ctrlr,
>>          }
>>
>>   unlock:
>> +       if (ctrlr->dirty && !psci_has_osi_support()) {
>> +               if (rpmh_flush(ctrlr)) {
>> +                       spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>> +                       return ERR_PTR(-EINVAL);
>> +               }
>> +       }
> It's been a long time since I looked in depth at RPMH, but upon a
> first glance this seems like it's gonna be terrible for performance.
> You're going to send every entry again and again, aren't you?  In
> other words in pseudo-code:
>
> 1. rpmh_write(addr=0x10, data=0x99);
> ==> writes on the bus (0x10, 0x99)
>
> 2. rpmh_write(addr=0x11, data=0xaa);
> ==> writes on the bus (0x10, 0x99)
> ==> writes on the bus (0x11, 0xaa)
>
> 3. rpmh_write(addr=0x10, data=0xbb);
> ==> writes on the bus (0x10, 0xbb)
> ==> writes on the bus (0x11, 0xaa)
>
> 4. rpmh_write(addr=0x12, data=0xcc);
> ==> writes on the bus (0x10, 0xbb)
> ==> writes on the bus (0x11, 0xaa)
> ==> writes on the bus (0x12, 0xcc)
>
> That seems bad.

Hi Doug,

No this is NOT how data is sent to RPMh/AOSS.
The rpmh_flush() fills up DRV-2 (HLOS) TCSes, makes it ready and The HW 
takes care of
sending data of Sleep TCSes for each of the EL/ DRV(s) when Last cpu is 
going to deepest
low power mode and of WAKE TCSes while first cpu is waking up.

> Why can't you just send the new request itself and
> forget adding it to the cache?  In other words don't even call
> cache_rpm_request() in the non-OSI case and then in __rpmh_write()
> just send right away...

This won’t work out. Let me explain why…

We have 3 SLEEP and 3 WAKE TCSes from below config..
                         qcom,tcs-config = <ACTIVE_TCS  2>,
                                           <SLEEP_TCS   3>,
                                           <WAKE_TCS    3>,
Each TCS has total 16 commands so total 48 commands(16*3) for each SLEEP 
and WAKE TCSes,
that can be filled up.

Now Lets take a example in pseudo-code on what could happen if we don’t 
cache and
immediately fill up TCSes commands. The triggering part doesn’t happen 
as explained above
it fills up TCSes and makes them ready..

Time-t0 (from client_x invoking rpmh_write_batch() for SLEEP SET, a 
batch of 3 commands)

rpmh_write_batch(
addr=0x10, data=0x99,  -> fills up CMD0 in SLEEP TCS_0
addr=0x11, data=0xaa,  -> fills up CMD1 in SLEEP TCS_0
addr=0x10, data=0xbb); -> fills up CMD2 in SLEEP TCS_0

Time-t1 (from client_y invoking rpmh_write(), a single command)

rpmh_write(
addr=0x12, data=0xcc,  -> fills up CMD3 in SLEEP TCS_0
);

Time-t2 (from client_x invokes rpmh_invalidate() which invalidates all 
previous *batch requests* only)

At this point, it should have CMD3 only in TCS while CMD 0,1,2 needs to 
be freed up, since we expect
a new batch request now.

Since driver didn’t cache anything in the first place, it doesn’t know 
details about previous batch request
like how many commands it had, what were the commands of those batches 
when filling up in TCSes, and so on…
(basically all the data required to free up only CMD 0,1,2, and don’t 
disturb CMD3)

Whats more?

The new batch request could be of let say 5 commands after invalidation, 
instead of 3 commands in previous batch.
So it will not fit in CMD-0,1,2 and we might want to allocate from 
CMD-4,5,6,7,8 now.

This will leave a hole in TCS CMDs (each TCS has 16 total commands) 
unless we re-arrange everything.
Also we may want to fill up batch request first and then single 
requests, by not caching anything, driver don’t
know which one is batch and which one is single request.

There are other cases like below which also gets impacted if driver 
don't cache anything...

for example, when we don’t have dedicated ACTIVE TCS ( if we have below 
config with ACTIVE TCS count 0)
     qcom,tcs-config = <ACTIVE_TCS  0>,
                           <SLEEP_TCS   3>,
                           <WAKE_TCS    3>,

Now to send active data, driver may re-use/ re-purpose few of the sleep 
or wake TCS, to be used as ACTIVE TCS and once work is done,
it will be re-allocated in SLEEP/ WAKE TCS pool accordingly. If driver 
don’t cache, all the SLEEP and WAKE data is lost when one
of TCS is repurposed to use as ACTIVE TCS.

Hope above explanation clears why caching is important and gives clear 
view of caching v/s not caching.

Thanks,
Maulik

> I tried to test this and my printouts didn't show anything actually
> happening in rpmh_flush().  Maybe I just don't have the write patches
> to exercise this properly...

it may be due to missing interconnect patches series
https://patchwork.kernel.org/project/linux-arm-msm/list/?series=247175

>> +
>>          spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>>
>>          return req;
>> @@ -285,26 +293,35 @@ int rpmh_write(const struct device *dev, enum rpmh_state state,
>>   }
>>   EXPORT_SYMBOL(rpmh_write);
>>
>> -static void cache_batch(struct rpmh_ctrlr *ctrlr, struct batch_cache_req *req)
>> +static int cache_batch(struct rpmh_ctrlr *ctrlr, struct batch_cache_req *req)
>>   {
>>          unsigned long flags;
>>
>>          spin_lock_irqsave(&ctrlr->cache_lock, flags);
>> +
>>          list_add_tail(&req->list, &ctrlr->batch_cache);
>>          ctrlr->dirty = true;
>> +
>> +       if (!psci_has_osi_support()) {
>> +               if (rpmh_flush(ctrlr)) {
>> +                       spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>> +                       return -EINVAL;
>> +               }
>> +       }
>> +
>>          spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>> +
>> +       return 0;
>>   }
>>
>>   static int flush_batch(struct rpmh_ctrlr *ctrlr)
>>   {
>>          struct batch_cache_req *req;
>>          const struct rpmh_request *rpm_msg;
>> -       unsigned long flags;
>>          int ret = 0;
>>          int i;
>>
>>          /* Send Sleep/Wake requests to the controller, expect no response */
>> -       spin_lock_irqsave(&ctrlr->cache_lock, flags);
>>          list_for_each_entry(req, &ctrlr->batch_cache, list) {
>>                  for (i = 0; i < req->count; i++) {
>>                          rpm_msg = req->rpm_msgs + i;
>> @@ -314,7 +331,6 @@ static int flush_batch(struct rpmh_ctrlr *ctrlr)
>>                                  break;
>>                  }
>>          }
>> -       spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>>
>>          return ret;
>>   }
>> @@ -386,10 +402,8 @@ int rpmh_write_batch(const struct device *dev, enum rpmh_state state,
>>                  cmd += n[i];
>>          }
>>
>> -       if (state != RPMH_ACTIVE_ONLY_STATE) {
>> -               cache_batch(ctrlr, req);
>> -               return 0;
>> -       }
>> +       if (state != RPMH_ACTIVE_ONLY_STATE)
>> +               return cache_batch(ctrlr, req);
> I'm curious: why not just do:
>
> if (state != RPMH_ACTIVE_ONLY_STATE && psci_has_osi_support()) {
>    cache_batch(ctrlr, req);
>    return 0;
> }
>
> ...AKA don't even cache it up if we're not in OSI mode.  IIUC this
> would be a huge deal because with your code you're doing the whole
> RPMH transfer under "spin_lock_irqsave", right?  And presumably RPMH
> transfers are somewhat slow, otherwise why did anyone come up with
> this whole caching / last-man-down scheme to start with?
>
> OK, it turned out to be at least slightly more complex because it
> appears that we're supposed to use rpmh_rsc_write_ctrl_data() for
> sleep/wake stuff and that they never do completions, but it really
> wasn't too hard.  I prototyped it at <http://crrev.com/c/2080916>.
> Feel free to hijack that change if it looks like a starting point and
> if it looks like I'm not too confused.
I looked at this change and thought of it earlier but it won’t work out 
for the reasons in above example.
I have thought of few optimizations in rpmh_flush() to reduce its time, 
if we *really* see any performance impact…

below is high level idea…
When  rpmh_write_batch() is invoked for SLEEP_SETs, currently 
rpmh_flush() will update both SLEEP and WAKE TCS contents,
However we may change it to update only SLEEP TCS, and when 
rpmh_write_batch() is invoked for WAKE SETs, update only WAKE TCS contents.
This way it may reduce time by roughly ~50%.
>
>>          for (i = 0; i < count; i++) {
>>                  struct completion *compl = &compls[i];
>> @@ -455,9 +469,6 @@ static int send_single(struct rpmh_ctrlr *ctrlr, enum rpmh_state state,
>>    * Return: -EBUSY if the controller is busy, probably waiting on a response
>>    * to a RPMH request sent earlier.
>>    *
>> - * This function is always called from the sleep code from the last CPU
>> - * that is powering down the entire system. Since no other RPMH API would be
>> - * executing at this time, it is safe to run lockless.
> Interesting that you removed this comment but not the copy of the
> comment inside this function.  Was that on purpose?
Its a miss. my bad. will remove in next revision.

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