Re: [PATCH v4 9/9] powerpc/pseries/vas: Write 'nr_total_credits' for QoS credits change

[Haren Myneni <haren@linux.ibm.com>]

On Wed, 2022-02-23 at 17:33 +1000, Nicholas Piggin wrote:
> Excerpts from Haren Myneni's message of February 20, 2022 6:03 am:
> > pseries supports two types of credits - Default (uses normal
> > priority
> > FIFO) and Qality of service (QoS uses high priority FIFO). The user
> > decides the number of QoS credits and sets this value with HMC
> > interface. With the core add/removal, this value can be changed in
> > HMC
> > which invokes drmgr to communicate to the kernel.
> > 
> > This patch adds an interface so that drmgr command can write the
> > new
> > target QoS credits in sysfs. But the kernel gets the new QoS
> > capabilities from the hypervisor whenever nr_total_credits is
> > updated
> > to make sure sync with the values in the hypervisor.
> > 
> > Signed-off-by: Haren Myneni <haren@linux.ibm.com>
> > ---
> >  arch/powerpc/platforms/pseries/vas-sysfs.c | 33
> > +++++++++++++++++++++-
> >  arch/powerpc/platforms/pseries/vas.c       |  2 +-
> >  arch/powerpc/platforms/pseries/vas.h       |  1 +
> >  3 files changed, 34 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/platforms/pseries/vas-sysfs.c
> > b/arch/powerpc/platforms/pseries/vas-sysfs.c
> > index e24d3edb3021..20745cd75f27 100644
> > --- a/arch/powerpc/platforms/pseries/vas-sysfs.c
> > +++ b/arch/powerpc/platforms/pseries/vas-sysfs.c
> > @@ -25,6 +25,33 @@ struct vas_caps_entry {
> >  
> >  #define to_caps_entry(entry) container_of(entry, struct
> > vas_caps_entry, kobj)
> >  
> > +/*
> > + * This function is used to get the notification from the drmgr
> > when
> > + * QoS credits are changed. Though receiving the target total QoS
> > + * credits here, get the official QoS capabilities from the
> > hypervisor.
> > + */
> > +static ssize_t nr_total_credits_store(struct vas_cop_feat_caps
> > *caps,
> > +				       const char *buf, size_t count)
> > +{
> > +	int err;
> > +	u16 creds;
> > +
> > +	/*
> > +	 * Nothing to do for default credit type.
> > +	 */
> > +	if (caps->win_type == VAS_GZIP_DEF_FEAT_TYPE)
> > +		return -EOPNOTSUPP;
> > +
> > +	err = kstrtou16(buf, 0, &creds);
> > +	if (!err)
> > +		err = vas_reconfig_capabilties(caps->win_type);
> 
> So what's happening here? The creds value is ignored? Can it just
> be a write-only file which is named appropriately to indicate it
> can be written-to to trigger an update?

Yes, new credits value is ignored. When the user changes QoS credits
with the HMC interface, it should reflect in QoS capability in the
hypervisor. So ignoring the credit value here and get the capability
value from the hypervisor.

This file should be read/write - the user space should be able to read
the current configured value for both credit types - default and QoS

Can I say nr_total_credits_update?

Thanks
Haren

> 
> Thanks,
> Nick