linux-kernel - Re: [PATCH 07/18] regmap: mmio: add config option to allow relaxed MMIO accesses

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Message-ID: <87lfg97yix.fsf@collabora.com>
Date:   Wed, 14 Oct 2020 16:00:38 +0300
From:   Adrian Ratiu <adrian.ratiu@...labora.com>
To:     Mark Brown <broonie@...nel.org>,
        Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     Ezequiel Garcia <ezequiel@...labora.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Fruehberger Peter <Peter.Fruehberger@...bosch.com>,
        kuhanh.murugasen.krishnan@...el.com,
        Daniel Vetter <daniel@...ll.ch>, kernel@...labora.com,
        linux-media@...r.kernel.org, linux-rockchip@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/18] regmap: mmio: add config option to allow relaxed
 MMIO accesses

On Wed, 14 Oct 2020, Mark Brown <broonie@...nel.org> wrote:
> On Wed, Oct 14, 2020 at 02:51:14PM +0300, Adrian Ratiu wrote: 
>> On Tue, 13 Oct 2020, Mark Brown <broonie@...nel.org> wrote: 
>> > On Mon, Oct 12, 2020 at 11:59:46PM +0300, Adrian Ratiu wrote: 
> 
>> > > -	writeb(val, ctx->regs + reg); +	if 
>> > > (ctx->relaxed_mmio) + writeb_relaxed(val, ctx->regs + reg); 
>> > > +	else + writeb(val, ctx->regs + reg); 
> 
>> > There is no point in doing a conditional operation on every 
>> > I/O, it'd be better to register a different set of ops when 
>> > doing relaxed I/O. 
> 
>> Indeed I have considered adding new functions but went with 
>> this solution because it's easier for the users to only have to 
>> define a "relaxed" config then test the regmap ctx as above. 
> 
> It seems like you've taken this in a direction other than what 
> I was thinking of here - defining separate ops doesn't mean we 
> have to do anything which has any impact on the interface seen 
> by users.  The regmap config is supplied at registration time, 
> it's just as available then as it is when doing I/O.

Right. I got confused by the meaning of ops :) Sorry about that.
 
> 
>> Thinking a bit more about it, yes, it makes more sense to have 
>> dedicated ops: this way users don't have to be explicit about 
>> adding membarriers and can combine relaxed and non-relaxed more 
>> easily, so it's also a better API trade-off in addition to 
>> avoiding the conditional. Thanks! 
> 
> I'm not sure what you're proposing here - it does seem useful to 
> be able to combine relaxed and non-relaxed I/O but that seems 
> like it'd break down the abstraction for regmap since tht's not 
> really a concept other buses are going to have?  Unless we 
> provide an operation to switch by setting flags or somethin 
> possibly and integrate it with the cache perhaps.  Could you be 
> a bit more specific about what you were thinking of here please?

I was thinking about exposing a relaxed API like 
regmap_write_relaxed but now that I know what you meant by ops and 
also that it doesn't make sense for other busses / violates the 
abstraction, I realize that is a bad idea and I will continue 
improving this to avoid the conditional and send a separete 
patch. Thanks again!

>
>> Question: Do you want me to split this patch from the series and send it
>> separately just for the regmap subsystem to be easier to review / apply?
>
> Sure.