Re: [PATCH v8 09/10] drivers: qcom: rpmh: add support for batch RPMH request

[Lina Iyer]

On Tue, May 15 2018 at 09:52 -0600, Doug Anderson wrote:
> Hi,
>
> On Mon, May 14, 2018 at 12:59 PM, Lina Iyer <ilina@xxxxxxxxxxxxxx> wrote:
>
> > > >  /**
> > > > @@ -77,12 +82,14 @@ struct rpmh_request {
> > > >   * @cache: the list of cached requests
> > > >   * @lock: synchronize access to the controller data
> > > >   * @dirty: was the cache updated since flush
> > > > + * @batch_cache: Cache sleep and wake requests sent as batch
> > > >   */
> > > >  struct rpmh_ctrlr {
> > > >         struct rsc_drv *drv;
> > > >         struct list_head cache;
> > > >         spinlock_t lock;
> > > >         bool dirty;
> > > > +       const struct rpmh_request *batch_cache[RPMH_MAX_BATCH_CACHE];
> > >
> > >
> > > I'm pretty confused about why the "batch_cache" is separate from the
> > > normal cache.  As far as I can tell the purpose of the two is the same
> > > but you have two totally separate code paths and data structures.
> > Due to a hardware limitation, requests made by bus drivers must be set
> > up in the sleep and wake TCS first before setting up the requests from
> > other drivers. Bus drivers use batch mode for any and all RPMH
> > communication. Hence their request are the only ones in the batch_cache.
>
> This is totally not obvious and not commented.  Why not rename to
> "priority" instead of "batch"?
>
> If the only requirement is that these come first, that's still no
> reason to use totally separate code paths and mechanisms.  These
> requests can still use the same data structures / functions and just
> be ordered to come first, can't they?  ...or given a boolean
> "priority" and you can do two passes through your queue: one to do the
> priority ones and one to do the secondary ones.
The bus requests have a certain order and cannot be mutated by the
RPMH driver. It has to be maintained in the TCS in the same order.
Also, the bus requests have quite a churn during the course of an
usecase. They are setup and invalidated often.
It is faster to have them separate and invalidate the whole lot of the
batch_cache instead of intertwining them with requests from other
drivers and then figuring out which all must be invalidated and rebuilt
when the next batch requests comes in. Remember, that requests may come
from any driver any time and therefore will be mangled if we use the
same data structure. So to be faster and to avoid having mangled requests
in the TCS, I have kept the data structures separate.

> > > > +       spin_unlock_irqrestore(&ctrlr->lock, flags);
> > > > +
> > > > +       return ret;
> > > > +}
> > >
> > >
> > > As part of my overall confusion about why the batch cache is different
> > > than the normal one: for the normal use case you still call
> > > rpmh_rsc_write_ctrl_data() for things you put in your cache, but you
> > > don't for the batch cache.  I still haven't totally figured out what
> > > rpmh_rsc_write_ctrl_data() does, but it seems strange that you don't
> > > do it for the batch cache but you do for the other one.
> > flush_batch does write to the controller using
> > rpmh_rsc_write_ctrl_data()
>
> My confusion is that they happen at different times.  As I understand it:
>
> * For the normal case, you immediately calls
> rpmh_rsc_write_ctrl_data() and then later do the rest of the work.
>
> * For the batch case, you call both later.
>
> Is there a good reason for this, or is it just an arbitrary
> difference?  If there's a good reason, it should be documented.
In both the cases, the requests are cached in the rpmh library and are
only sent to the controller only when the flushed. I am not sure the
work is any different. The rpmh_flush() flushes out batch requests and
then the requests from other drivers.

Thanks,
Lina