Re: [PATCH 1/2] dt-bindings: Documentation for qcom,llcc

[Channa <ckadabi@codeaurora.org>]

On 2018-02-02 03:05, Mark Rutland wrote:
> On Thu, Feb 01, 2018 at 12:39:09PM -0800, Channa wrote:
>> On 2018-02-01 02:44, Mark Rutland wrote:
>> > On Thu, Jan 25, 2018 at 03:55:12PM -0800, Channagoud Kadabi wrote:
>> > > Documentation for last level cache controller device tree bindings,
>> > > client bindings usage examples.
>> > >
>> > > Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>
>> > > ---
>> > >  .../devicetree/bindings/arm/msm/qcom,llcc.txt      | 93
>> > > ++++++++++++++++++++++
>> > >  1 file changed, 93 insertions(+)
>> > >  create mode 100644
>> > > Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>> > >
>> > > diff --git a/Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>> > > b/Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>> > > new file mode 100644
>> > > index 0000000..d433b0c
>> > > --- /dev/null
>> > > +++ b/Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>> > > @@ -0,0 +1,93 @@
>> > > +* LLCC (Last Level Cache Controller)
>> > > +
>> > > +Properties:
>> > > +- compatible:
>> > > +	Usage: required
>> > > +	Value type: <string>
>> > > +	Definition: must be "qcom,llcc-core"
>> > > +
>> > > +- reg:
>> > > +	Usage: required
>> > > +	Value Type: <prop-encoded-array>
>> > > +	Definition: must be addresses and sizes of the LLCC registers
>> > > +
>> > > +- llcc-bank-off:
>> > > +	Usage: required
>> > > +	Value Type: <u32 array>
>> > > +	Definition: Offsets of llcc banks from llcc base address starting
>> > > from
>> > > +		    LLCC bank0.
>> > > +
>> > > +- llcc-broadcast-off:
>> > > +	Usage: required
>> > > +	Value Type: <u32>
>> > > +	Definition: Offset of broadcast register from LLCC bank0 address.
>> >
>> > Please could we use "offset" rather than "off" for both of these? That
>> > way it's obvious these aren't properties for disabling some feature.
>> >
>> > How variable are these offsets in practice? Is the memory map not fixed?
>> 
>> The offsets depends on the number of LLCC HW blocks. These number of 
>> HW
>> blocks vary from
>> chipset to chipset and new registers could be added that changes the 
>> offset.
> 
> Surely if new registers are added, we need a new compatible string?
> 
> Can't we encode the number of LLCC HW blocks, instead? Presumably that
> would give enough information to cover both llcc-bank-off and
> llcc-broadcast-off.
> 
> [...]

Are you suggesting to move these offset handing out of DTS files and 
manage in the driver?

> 
>> > > +
>> > > +compatible devices:
>> > > +		qcom,sdm845-llcc
>> >
>> > Huh? The "qcom,sdm845-llcc" bindings wasn't described above, and it's
>> > not clear what this means.
>> >
>> > > +
>> > > +Example:
>> > > +
>> > > +	qcom,system-cache@1300000 {
>> > > +		compatible = "qcom,llcc-core", "syscon", "simple-mfd";
>> >
>> > This looks very wrong. Why do you need syscon and simple-mfd?
>> 
>> LLCC HW block has 3 functionalities:
>> System cache core, ECC & AMON drivers for debugging.
>> All three drivers use the same register space for configuration, 
>> status etc.
>> In order to avoid remapping the same address region across multiple 
>> drivers,
>> I have implemented this driver as a syncon and simple-mfd.
> 
> Please don't do that; that's completely dependent on Linux
> implementation details.

Why do you think simple-mfd is not good here? The LLCC HW clock is 
outside of CPUSS and has
multiple functional blocks.

> 
> Have one top level driver for the whole LLCC block, which maps the
> registers, and provides an API for accessing them. When that probes, it
> can cause the other drivers to be probed (e.g. with a platform device),
> and those can access the LLCC registers via that API.
> 
>> > > +		reg = <0x1300000 0x50000>;
>> > > +		reg-names = "llcc_base";
>> > > +
>> > > +		llcc: qcom,sdm845-llcc {
>> > > +			compatible = "qcom,sdm845-llcc";
>> >
>> > Why is this a sub-node?
>> qcom,sdm845-llcc: This core driver as mentioned in the list above.
>> >
>> > Why isn't the top-level node just "qcom,sdm845-llcc" ?
>> >
>> > > +			#cache-cells = <1>;
>> > > +			max-slices = <32>;
>> > > +		};
>> > > +
>> > > +		qcom,llcc-ecc {
>> > > +			compatible = "qcom,llcc-ecc";
>> > > +		};
>> 
>> qcom,llcc-ecc: Driver #2 for ECC
>> 
>> > > +
>> > > +		qcom,llcc-amon {
>> > > +			compatible = "qcom,llcc-amon";
>> > > +			qcom,fg-cnt = <0x7>;
>> > > +		};
>> > > +
>> 
>> qcom,llcc-amon: Driver #3 for AMON
> 
> Please describe the HW, not the drivers.
> 
> As above, I don't believe you need multiple nodes here. Linux can
> instantiate the drivers as necessary.
> 
> [...]
> 
>> > > +- cache-slices:
>> > > +	Usage: required
>> > > +	Value type: <prop-encoded-array>
>> > > +	Definition: The tuple has phandle to llcc device as the first
>> > > argument and the
>> > > +		    second argument is the usecase id of the client.
>> >
>> > What is a "usecase id" ?
>> 
>> Usecase id for use case that wants to use system cache for eg: 
>> video-encode
>> and video-decode
> 
> Sure, but how is the value used? Is it the index of a slice? Or
> something more abstract?

This is used as an index to the SCT (System cache Table) configuration 
data that controls
the behavior of each cache slice.

> 
> Thanks,
> Mark.
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