Re: [PATCH v3 1/2] drm/i915/gmbus: Increase the Bytes per Rd/Wr Op

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Apr 18, 2018 at 09:20:23AM +0300, Jani Nikula wrote:
> On Wed, 18 Apr 2018, Ramalingam C <ramalingam.c@intel.com> wrote:
> > On Tuesday 17 April 2018 11:39 PM, Ville Syrjälä wrote:
> >> On Tue, Apr 17, 2018 at 02:25:32PM +0530, Ramalingam C wrote:
> >>> >From Gen9 onwards Bspec says HW supports Max Bytes per single RD/WR op is
> >>> 511Bytes instead of previous 256Bytes used in SW.
> >>>
> >>> This change allows the max bytes per op upto 511Bytes from Gen9 onwards.
> >>>
> >>> v2:
> >>>    No Change.
> >>> v3:
> >>>    Inline function for max_xfer_size and renaming of the macro.[Jani]
> >>>
> >>> Cc: Jani Nikula <jani.nikula@intel.com>
> >>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> >>> Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
> >>> ---
> >>>   drivers/gpu/drm/i915/i915_reg.h  |  1 +
> >>>   drivers/gpu/drm/i915/intel_i2c.c | 11 +++++++++--
> >>>   2 files changed, 10 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> >>> index 475cac07d3e6..be6114a0e8ab 100644
> >>> --- a/drivers/gpu/drm/i915/i915_reg.h
> >>> +++ b/drivers/gpu/drm/i915/i915_reg.h
> >>> @@ -3013,6 +3013,7 @@ enum i915_power_well_id {
> >>>   #define   GMBUS_CYCLE_STOP	(4<<25)
> >>>   #define   GMBUS_BYTE_COUNT_SHIFT 16
> >>>   #define   GMBUS_BYTE_COUNT_MAX   256U
> >>> +#define   GEN9_GMBUS_BYTE_COUNT_MAX 511U
> >>>   #define   GMBUS_SLAVE_INDEX_SHIFT 8
> >>>   #define   GMBUS_SLAVE_ADDR_SHIFT 1
> >>>   #define   GMBUS_SLAVE_READ	(1<<0)
> >>> diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
> >>> index e6875509bcd9..4367827d7661 100644
> >>> --- a/drivers/gpu/drm/i915/intel_i2c.c
> >>> +++ b/drivers/gpu/drm/i915/intel_i2c.c
> >>> @@ -361,6 +361,13 @@ gmbus_wait_idle(struct drm_i915_private *dev_priv)
> >>>   	return ret;
> >>>   }
> >>>   
> >>> +static inline
> >>> +unsigned int gmbus_max_xfer_size(struct drm_i915_private *dev_priv)
> >>> +{
> >>> +	return (INTEL_GEN(dev_priv) >= 9) ? GEN9_GMBUS_BYTE_COUNT_MAX :
> >>> +		GMBUS_BYTE_COUNT_MAX;
> >> Hmm. You sure about this 256 limit on older HW? The spec does sort of
> >> say that 0-256 is the valid range, but the SPT+ docs still have that
> >> same text, and the register has always had 9 bits for byte count. I
> >> don't see any statements saying that they changed this in any way for
> >> SPT. It only talks about >511 bytes needing the special treatment.
> >>
> >> If we do this the I think you should just drop the defines and put the
> >> raw numbers into this function. The extra indirection just makes life
> >> harder. Also pointless parens around the GEN>9 check.
> > Even I couldn't get any place where BSpec says 256Bytes is the limit for 
> > any platform. Everywhere 9bits are used.
> > And when I cross verified with other OS usage 511Bytes is used as limit 
> > across all platforms.
> >
> > Just to be cautious for not breaking any older platforms out in linux 
> > world, I limited the extension of the limit to the known
> > and easily testable platforms at my desk (Gen9+)
> >
> > Do you suggest we should apply 511Bytes as max limit for all platforms?
> > Do we have any means to test this new limit on all supported legacy 
> > platforms?
> >
> > Except enabling the full potential of the HW in SW, I dont see any ROI 
> > here as most of the GMBUS reqs are <256Bytes.
> > Only in case of HDCP2.2 we need single read cycle for 538Bytes.
> >
> > we have couple of options here: Please share your opinion to choose one 
> > of them.
> > 1. Just dont change the upper limit for RD/WR. Keep it as it is at 
> > 256Bytes. Anyway no user demands it.
> > 2. As per HW capability, Change the upper limit for RD/WR to 511Bytes 
> > for all platforms. This is needs the functional verification on all 
> > legacy plat supported.
> > 3. Change the upper limit for RD/WR to 511Bytes for newer platforms, say 
> > Gen9+.
> 
> Please let's not change the limit for old platforms for absolutely no
> gain. And if Ville insists anyway, let's leave that as a separate
> follow-up change that can easily be reverted later.
> 
> I might consider using the 511 limit only for platforms that
> HAS_GMBUS_BURST_READ too.
> 
> The original limit seems to have been added in 9535c4757b88 ("drm/i915:
> cope with large i2c transfers") citing "the specs". Any recollection
> anyone? Chris?

"Duble buffered data register and a 9 bit counter support 0 byte to 256
Byte transfers."
has always been in the spec (and still is for SPT+).

-- 
Ville Syrjälä
Intel
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