netdev - Re: [RFT][PATCH V2 09/10] net: dsa: microchip: Factor out regmap config generation into common header

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Date:   Thu, 10 Jan 2019 15:37:28 +0100
From:   Marek Vasut <marex@...x.de>
To:     Tristram.Ha@...rochip.com
Cc:     f.fainelli@...il.com, andrew@...n.ch, Woojung.Huh@...rochip.com,
        netdev@...r.kernel.org, UNGLinuxDriver@...rochip.com
Subject: Re: [RFT][PATCH V2 09/10] net: dsa: microchip: Factor out regmap
 config generation into common header

On 1/10/19 3:10 AM, Tristram.Ha@...rochip.com wrote:
>>> I just looked at your regmap code and you use 3 regmap pointers for
>> specific 8-bit, 16-bit, and 32-bit accesses.  The switch access is always 8-bit.  It
>> has automatic register increment so that you can access arbitrary length of
>> registers.  The use of 16-bit and 32-bit accesses makes access efficient if it
>> makes sense.
>>
>> Right, that's what happens here.
>>
>>> Most older switches define registers in 8-bit.  Exceptions are the default
>> VID and indirect access.
>>>
>>> A specific switch mostly defines registers in 16-bit because it shares the
>> core design with an Ethernet controller.
>>>
>>> KSZ9477 is the newer designed switch and it gets some designs from older
>> switches and that is why it has a mix of 8-bit, 16-bit, and 32-bit register
>> definitions.
>>
>> Right, that's quite horrible.
>>
>>> In my code I just use regmap_bulk_read and regmap_bulk_write and still
>> use the old spi access functions for specific 8-bit, 16-bit, and 32-bit accesses.
>>
>> Let's not mix regmap and non-regmap accesses, that'd be a mess. Let's
>> stick to one, regmap.
>>
>>> We can combine the logic of ksz_spi_read8 and others into ksz_read8 and
>> so they can be used for both SPI and I2C accesses.
>>
>> You can just use regmap_*() accessors and regmap will deal with i2c/spi
>> abstraction for you, that's the idea.
>>
> 
> What I meant is I use bulk_read as a base and modify it to access 16-bit and 32-bit instead of using regmap[1] and regmap[2].  We can keep regmap[2] for 32-bit access just for the regmap_update_bits function.
> 
> I intend to keep the 3 regmap pointers as a specific switch actually requires those specific accesses as it does not have automatic register increment.

I don't think the bulk read is a good idea due to register alignment.
You see, with bulk read, the user might try to read two bytes from the
middle of 4 byte register and I'm not sure how the HW would like that.
If we have 32bit regmap for 32bit registers, all behaves as expected.

-- 
Best regards,
Marek Vasut