[PATCH] drm/xe/xe_guc_ads: Add whitelist registers to write list

[Cavitt, Jonathan]

-----Original Message-----
From: Harrison, John C <john.c.harrison at intel.com> 
Sent: Friday, November 1, 2024 11:46 AM
To: Cavitt, Jonathan <jonathan.cavitt at intel.com>; intel-xe at lists.freedesktop.org
Cc: Gupta, saurabhg <saurabhg.gupta at intel.com>; Zuo, Alex <alex.zuo at intel.com>; Nerlige Ramappa, Umesh <umesh.nerlige.ramappa at intel.com>; Roper, Matthew D <matthew.d.roper at intel.com>; De Marchi, Lucas <lucas.demarchi at intel.com>; Dixit, Ashutosh <ashutosh.dixit at intel.com>
Subject: Re: [PATCH] drm/xe/xe_guc_ads: Add whitelist registers to write list
> 
> On 11/1/2024 11:04, Jonathan Cavitt wrote:
> > When performing a guc_mmio_regset_write, we add all the registers in the
> > reg_sr list to the save/restore list, but do not do the same for the
> > whitelist registers.  Add them in.
> >
> > Closes: https://gitlab.freedesktop.org/drm/xe/kernel/issues/2249
> > Signed-off-by: Jonathan Cavitt <jonathan.cavitt at intel.com>
> > CC: Lucas de Marchi <lucas.demarchi at intel.com>
> > CC: Matt Roper <matthew.d.roper at intel.com>
> > CC: John Harrison <john.c.harrison at intel.com>
> > CC: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> > CC: Ashutosh Dixit <ashutosh.dixit at intel.com>
> > ---
> >   drivers/gpu/drm/xe/xe_guc_ads.c | 11 ++++++++++-
> >   1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/xe/xe_guc_ads.c b/drivers/gpu/drm/xe/xe_guc_ads.c
> > index 943146e5b460..2fc6b1ccc8fc 100644
> > --- a/drivers/gpu/drm/xe/xe_guc_ads.c
> > +++ b/drivers/gpu/drm/xe/xe_guc_ads.c
> > @@ -239,9 +239,12 @@ static size_t calculate_regset_size(struct xe_gt *gt)
> >   	enum xe_hw_engine_id id;
> >   	unsigned int count = 0;
> >   
> > -	for_each_hw_engine(hwe, gt, id)
> > +	for_each_hw_engine(hwe, gt, id) {
> >   		xa_for_each(&hwe->reg_sr.xa, sr_idx, sr_entry)
> >   			count++;
> > +		xa_for_each(&hwe->reg_whitelist.xa, sr_idx, sr_entry)
> > +			count++;
> > +	}
> >   
> >   	count += ADS_REGSET_EXTRA_MAX * XE_NUM_HW_ENGINES;
> >   
> > @@ -727,6 +730,12 @@ static unsigned int guc_mmio_regset_write(struct xe_guc_ads *ads,
> >   	xa_for_each(&hwe->reg_sr.xa, idx, entry)
> >   		guc_mmio_regset_write_one(ads, regset_map, entry->reg, count++);
> >   
> > +	i = 0;
> > +	xa_for_each(&hwe->reg_whitelist.xa, idx, entry)
> > +		guc_mmio_regset_write_one(ads, regset_map,
> > +					  RING_FORCE_TO_NONPRIV(hwe->mmio_base, i++),
> > +					  count++);
> > +
> The code that actually writes to the NONPRIV registers 
> (xe_reg_sr_apply_whitelist() in xe_reg_src.c) explicitly clears all the 
> unused registers with a comment of "clear the rest in case of garbage". 

The code in xe_reg_sr_apply_whitelist calls xe_mmio_write32 to write the
registers, whereas the code in guc_mmio_regset_write uses xe_map_memcpy_to
internally.  While the former seems to be writing to the
xe_mmio_adjusted_addr(mmio, reg.addr) + mmio->regs, the latter appears to be
writing to IOSYS_MAP_INIT_OFFSET(ads_to_map(ads), guc_ads_regset_offset(ads).

I'm not particularly well-versed in these functions, but it looks to me that these
two functions write to different locations and thus would not impact each other.
Or, in other words, I don't think the garbage we're clearing in xe_reg_sr_apply_whitelist
is the same as the data we're writing in guc_mmio_regset_write.

-Jonathan Cavitt

> If we don't trust the reset state to be valid then we need to ensure all 
> of them are saved/restored across a reset. Otherwise, that garbage can 
> come back and cause problems.
> 
> John.
> 
> 
> >   	for (e = extra_regs; e < extra_regs + ARRAY_SIZE(extra_regs); e++) {
> >   		if (e->skip)
> >   			continue;
> 
>