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Subject: Re: [RFC v1 2/3] drivers: nvmem: Add driver for QTI qfprom-efuse
 support
To:     Doug Anderson <dianders@chromium.org>
Cc:     "Ravi Kumar Bokka (Temp)" <rbokka@codeaurora.org>,
        Rob Herring <robh+dt@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>,
        dhavalp@codeaurora.org, mturney@codeaurora.org,
        sparate@codeaurora.org, c_rbokka@codeaurora.org,
        mkurumel@codeaurora.org
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From:   Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Message-ID: <9864496c-b066-3fe8-5608-bd9af69663f4@linaro.org>
Date:   Wed, 20 May 2020 15:35:23 +0100
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On 18/05/2020 19:31, Doug Anderson wrote:
> Hi,
> 
> On Mon, May 18, 2020 at 3:45 AM Srinivas Kandagatla
> <srinivas.kandagatla@linaro.org> wrote:
>>
>> On 18/05/2020 11:39, Ravi Kumar Bokka (Temp) wrote:
>>>
>>> Based on the compatible, do i need to separate probe function for
>>> qfprom-efuse and maintain separate nvmem object to register nvmem
>>> framework. Is this what you are suggesting to implementing this in to
>>> one existing driver?
>>
>> Yes for same driver we should add new compatible string and add support
>> to this in existing qfprom driver.
>> Ideally we should allocate nvmem_config object at probe with different
>> parameters based on compatible string.
> 
> I wish I had better documentation for exactly what was in the SoC
> instead of the heavily redacted stuff Qualcomm provides.  Really the
> answer here is: how do you best describe the hardware?  OK, so I just
> spent the past hour or so trying to patch together all the bits and
> fragments that Qualcomm provided me.  Just like a scavenger hunt!
> Fun!  The best I can patch together is that there is a single QFPROM
> with these ranges:
> 
> 0x00780000 - 0x007800FF
> QFPROM HW registers, range 1/2
> 
> 0x00780120 - 0x007808FF
> QFPROM "raw" space
> 

so this is the only region is the QFPROM fuses can be programmed!

> 0x00782000 - 0x007820FF
> QFPROM HW registers, range 2/2
> 
> 0x00784120 - 0x007848FF
> QFPROM "corrected" space

Is this some kind of FEC corrected regions?


> 
> 0x00786000 - 0x00786FFF
> QFPROM memory range that I don't really understand and maybe we don't
> worry about right now?

> 
> Did I get that right?  If so, is there a prize for winning the scavenger hunt?
> 
> ---
> 
> If so then, IMO, it wouldn't be insane to actually keep it as two
> drivers and two device tree nodes, as you've done.  I'd defer to
> Srinivas and Rob Herring, though.  The existing driver would be a
> read-only driver and provide access to the "corrected" versions of all
> the registers.  Its node would have "#address-cells = <1>" and
> "#size-cells = <1>" because it's expected that other drivers might
> need to refer to data stored here.
> 
> Your new driver would be read-write and provide access to the "raw"
> values.  A read from your new driver would not necessarily equal a
> read from the old driver if the FEC (forward error correction) kicked

Is this only applicable for corrected address space?

> in.  Other drivers should never refer to the non-corrected values so
> you wouldn't have "#address-cells" and "#size-cells".  The only way to
> really read or write it would be through sysfs.
> 
> It would be super important to document what's happening, of course.
> ...and ideally name them to make it clearer too.
> 
> ---
> 
> Another alternative (if Srinivas and/or Rob H prefer it) would be to
> deprecate the old driver and/or bindings and say that there really
> should just be one node and one driver.  In that case you'd replace
> the old node with:
> 
> qfprom@780000 {
>    compatible = "qcom,sc7180-qfprom-efuse";

May be "qcom,sc7180-qfprom"


>    reg = <0 0x00780000 0 0x6fff>;
>    #address-cells = <1>;
>    #size-cells = <1>;
> 
>    clocks = <&gcc GCC_SEC_CTRL_CLK_SRC>;
>    clock-names = "sec";
> 
>    qusb2p_hstx_trim: hstx-trim-primary@25b {
>      reg = <0x25b 0x1>;
>      bits = <1 3>;
>    };
> };
> 
> You'd use the of_match_table solution to figure out the relevant
> offsets (0x120, 0x2000, 0x4120, 0x6000) for sc7180 and this new driver
> would be responsible for being able to read the corrected values and


Encompassing these offsets in driver as part of the register defines 
itself should be a good start!

It will also be nice to understand how similar this thing is with w.rt 
other Qcom SoCs?


> also for programming.  In this case I'm not sure how (assuming it's
> valuable) you'd provide read access to the uncorrected data.
I will leave this question to the author of the driver.

--srini

> 
> 
> -Doug
>