linux-kernel - Re: [PATCH v1 03/11] drivers: soc: hisi: Add support for Hisilicon Djtag driver

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Message-ID: <2bac1a34-591b-6557-15bf-40db25c3d129@huawei.com>
Date:   Tue, 8 Nov 2016 13:49:43 +0000
From:   John Garry <john.garry@...wei.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        Anurup M <anurupvasu@...il.com>, <anurup.m@...wei.com>,
        <linux-kernel@...r.kernel.org>, <mark.rutland@....com>,
        <shyju.pv@...wei.com>, <gabriele.paoloni@...wei.com>,
        <will.deacon@....com>, <linuxarm@...wei.com>,
        <xuwei5@...ilicon.com>, <zhangshaokun@...ilicon.com>,
        <sanil.kumar@...ilicon.com>, <tanxiaojun@...wei.com>,
        <shiju.jose@...wei.com>
Subject: Re: [PATCH v1 03/11] drivers: soc: hisi: Add support for Hisilicon
 Djtag driver

On 08/11/2016 11:45, Arnd Bergmann wrote:
> On Tuesday, November 8, 2016 11:23:35 AM CET John Garry wrote:
>> On 07/11/2016 20:08, Arnd Bergmann wrote:
>>> On Monday, November 7, 2016 2:15:10 PM CET John Garry wrote:
>>>>
>>>> Hi Arnd,
>>>>
>>>> The new bus type tries to model the djtag in a similar way to I2C/USB
>>>> driver arch, where we have a host bus adapter and child devices attached
>>>> to the bus. The child devices are bus driver devices and have bus
>>>> addresses. We think of the djtag as a separate bus, so we are modelling
>>>> it as such.
>>>>
>>>> The bus driver offers a simple host interface for clients to read/write
>>>> to the djtag bus: bus accesses are hidden from the client, the host
>>>> drives the bus.
>>>
>>> Ok, in that case we should probably start out by having a bus specific
>>> DT binding, and separating the description from that of the bus master
>>> device.
>>
>> OK
>>
>>>
>>> I'd suggest requiring #address-cells=<1> and #size-cells=<0> in the master
>>> node, and listing the children by reg property. If the address is not
>>> easily expressed as a single integer, use a larger #address-cells value.
>>
>> We already have something equivalent to reg in "module-id" (see patch
>> 02/11), which is the slave device bus address; here's a sample:
>> +		/* For L3 cache PMU */
>> +		pmul3c0 {
>> +			compatible = "hisilicon,hisi-pmu-l3c-v1";
>> +			scl-id = <0x02>;
>> +			num-events = <0x16>;
>> +			num-counters = <0x08>;
>> +			module-id = <0x04>;
>> +			num-banks = <0x04>;
>> +			cfgen-map = <0x02 0x04 0x01 0x08>;
>> +			counter-reg = <0x170>;
>> +			evctrl-reg = <0x04>;
>> +			event-en = <0x1000000>;
>> +			evtype-reg = <0x140>;
>> +		};
>>
>> FYI, "module-id" is our own internal hw nomenclature.
>
> Yes, that was my interpretation as well. Please use the standard
> "reg" property for this then.
>
>>> Another option that we have previously used was to actually pretend that
>>> a vendor specific bus is an i2c bus and use the i2c probing infrastructure,
>>> but that only makes sense if the software side closely resembles i2c
>>> (this was the case for Allwinner I think, but I have not looked at
>>> your driver in enough detail to know if it is true here as well).
>>>
>>
>> OK, let me check this. By chance do you remember the specific AllWinner
>> driver/hw?
>
> drivers/i2c/busses/i2c-sun6i-p2wi.c

Hi Arnd,

Thanks for the reference.

I think the i2c interface doesn't fully satisfy our requirements as we 
need more than just a slave bus address when accessing the slave device 
(which I think is what i2c uses). We also need to pass "offset" and 
"mod_mask" arguments to the djtag adapter to access specific registers 
in the slave device.

Cheers,
John

>
> 	Arnd
>
> .
>