linux-kernel - Re: [PATCH v10 06/12] peci: Add a PECI adapter driver for Aspeed AST24xx/AST25xx

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Date:   Tue, 15 Jan 2019 15:36:43 -0800
From:   Jae Hyun Yoo <jae.hyun.yoo@...ux.intel.com>
To:     Joel Stanley <joel@....id.au>
Cc:     devicetree <devicetree@...r.kernel.org>,
        Ryan Chen <ryan_chen@...eedtech.com>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Jeffery <andrew@...id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        James Feist <james.feist@...ux.intel.com>,
        Vernon Mauery <vernon.mauery@...ux.intel.com>,
        Robin Murphy <robin.murphy@....com>,
        Haiyue Wang <haiyue.wang@...ux.intel.com>
Subject: Re: [PATCH v10 06/12] peci: Add a PECI adapter driver for Aspeed
 AST24xx/AST25xx

On 1/15/2019 3:14 PM, Joel Stanley wrote:
> On Tue, 15 Jan 2019 at 09:49, Jae Hyun Yoo <jae.hyun.yoo@...ux.intel.com> wrote:
>>>> +       /**
>>>> +        * We check that the regmap works on this very first access,
>>>> +        * but as this is an MMIO-backed regmap, subsequent regmap
>>>> +        * access is not going to fail and we skip error checks from
>>>> +        * this point.
>>>
>>> Why do you use a regmap for this driver? AFAICT it has exclusive
>>> ownership over the register range it uses, which is sometimes a reason
>>> to use a regmap over a mmio region.
>>>
>>> I'm not sure if you've ever disassembled drivers/base/regmap/regmap.o,
>>> but if you do you will find that a single mmio read turns into
>>> hundreds of instructions.
>>>
>>
>> No specific reason. regmap makes some overhead as you mentioned but it
>> also provides some advantages on access simplification, endianness
>> handling and register dump at run time. I would not insist using of
>> regmap if you prefer using of raw readl and writel. Do you want replace
>> regmap with readl and writel in this driver?
> 
> I think that would be best.
> 
> You raise some good points about the regmap API, and you're not alone
> in using it for these reasons. We should look in to providing a
> suitable API without the overhead, or making regmap more efficient for
> the mmio case.
> 

Okay. I'll rewrite this driver code using readl and writel instead of 
regmap mmio. Thanks again for your review!

Regards,
Jae