Message-ID: <97a28ade85f31f709c7848da5ebab0bdc802afd2.camel@gmail.com>
Date:   Mon, 29 Nov 2021 20:19:01 +0100
From:   David Virag <virag.david003@...il.com>
To:     Sam Protsenko <semen.protsenko@...aro.org>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jaewon Kim <jaewon02.kim@...sung.com>,
        Chanho Park <chanho61.park@...sung.com>,
        Youngmin Nam <youngmin.nam@...sung.com>,
        devicetree@...r.kernel.org, linux-spi@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-i2c@...r.kernel.org
Subject: Re: [PATCH 0/8] soc: samsung: Add USIv2 driver

On Mon, 2021-11-29 at 15:56 +0200, Sam Protsenko wrote:
> On Sun, 28 Nov 2021 at 05:15, David Virag <virag.david003@...il.com>
> wrote:
> > 
> > On Sun, 2021-11-28 at 00:32 +0200, Sam Protsenko wrote:
> > > USIv2 IP-core provides selectable serial protocol (UART, SPI or
> > > High-Speed I2C); only one can be chosen at a time. This series
> > > implements USIv2 driver, which allows one to select particular USI
> > > function in device tree, and also performs USI block
> > > initialization.
> > > 
> > > With that driver implemented, it's not needed to do USI
> > > initialization
> > > in protocol drivers anymore, so that code is removed from the
> > > serial
> > > driver.
> > > 
> > 
> > I think the downstream way of doing this (USI node reg being on the
> > SW_CONF register itself rather than an offset from uart/i2c/spi, the
> > USI driver only controlling the SW_CONF, and the uart/i2c/spi drivers
> > controlling their USI_CON and USI_OPTION regs) is cleaner, better,
> > and
> > easier to adapt to USIv1 too.
> > 
> 
> One reason why I think it's better to provide SW_CONF register via
> syscon node, is that it helps us to avoid possible register access
> conflicts in future, and also conflicts when requesting corresponding
> resources. In other words, the System Register block can be used by
> many consumers (drivers) in future; those consumers might try to
> modify the same registers simultaneously, which might lead to race
> conditions (as RMW operation is not atomic), so some kind of
> serialization should be done (like locking in regmap), which is
> provided by syscon. Also, that wouldn't even come to that: you just
> can't request the same I/O area twice in Linux. So if SW_CONF is
> passed via "reg" property to USI driver, and then we try to map the
> whole System Register (or its portion that includes SW_CONF), that
> request would fail.

I've got to admit, that's something I didn't think about much, partly
because the lack of TRM on my hand, as I'm working with just vendor
kernel sources and consumer phones. What other things are in the sysreg
in your case? Looking at my vendor device tree, the USI SW_CONF
registers are at 0x10032000-0x10032008 in my case and the DT lacks
anything else close by (in the 0x1003xxxx region).

> 
> Although passing one SW_CONF register via "reg" might look easier to
> implement, it might also bring us all sort of problems later on. And I
> think a good design should account for such pitfalls.
> 
> As for the USI registers: I really don't think that duplicating the
> code for USI block reset across uart/i2c/spi drivers would help us to
> accomplish anything. Why those drivers should be even aware of USI
> reset? At least in USIv2 block, the USI registers and uart/i2c/spi
> registers are not mixed: they are located at different and always
> fixed addresses. We can benefit from that fact, and provide Device
> Tree structure which reflects the hardware one, separating USI control
> from actual protocol nodes.
> 
> > For example: I'm sure this is the case on USIv2 devices too, but on
> > Exynos7885, different devices have USI modes configured differently.
> > For example a Samsung Galaxy A8 (2018) has all the USI blocks
> > configured as SPI while a Samsung Galaxy M20 has the first USI
> > configured as dual HSI2C, the second as HSI2C on the first 2 pins and
> > the third as HSI2C on the last 2 pins. With this way of doing
> > everything on USIv2 we'd need 3 disabled USIv2 nodes in the SoC DTSI
> > for one USI block, each for every protocol the USI block can do, all
> > having a single child for their protocol and each referencing the
> > same
> > sysreg (not even sure if that's even supported). Then the board DTS
> > could enable the USI node it needs.
> > 
> 
> If I'm following you correctly, then it's not like that. I guess
> Krzysztof already replied to that, so I'll probably just repeat his
> words. In that case you'll have something like this in your SoC dtsi
> (for your USIv1 case of course, because dual HSI2C is not present in
> USIv2):
> 
> <<<<<<<<<<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>>>>>>>>
> usi1 {
>     spi1 {
>     };
> 
>     hsi2c1_1 {
>     };
> 
>     hsi2c1_2 {
>     };
> };
> 
> usi2 {
>     spi2 {
>     };
> 
>     hsi2c2_1 {
>     };
> };
> 
> 
> usi3 {
>     spi3 {
>     };
> 
>     hsi2c2_2 {
>     };
> };
> <<<<<<<<<<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>>>>>>>>
> 
> and then in your board dts you just have to enable corresponding usi's
> with proper modes, and enable chosen protocol nodes, like this:
> 
> <<<<<<<<<<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>>>>>>>>
> &usi1 {
>     status = "okay"
>     samsung,mode = <USI_V1_DUAL_I2C>;
> };
> 
> &hsi2c1_1 {
>     status = "okay"
> };
> 
> &hsi2c1_2 {
>     status = "okay"
> };
> <<<<<<<<<<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>>>>>>>>

What got me confused is the following: Upon checking vendor drivers I
was under the impression that we have all 3 protocols at seperate
addresses, and the USI SW_CONF register kind of works like a
multiplexer for the USI pins to switch between protocols. Now I see
that I was wrong, and the addresses are in fact the same. Now on a
hardware level it might still work just as a multiplexer but it
swithches the entire address space for a whole different protocol
block. Dumb little misunderstanding on my part, never mind! They are on
the same address even on USIv1. Not sure how I haven't noticed that
before, I guess since I never started experimenting with USI before,
just looked at the code as a reference I assumed a lot of things.

> 
> > With the downstream way we could have just one USI node and we could
> > add the 3 protocols it can do disabled as seperate or child nodes.
> > This
> > way the board DTS only needs to set the appropriate mode setting and
> > enable the protocol it needs. I'd say much better than having 3 USI
> > nodes for the same USI block.
> > 
> 
> Not sure if with downstream USI driver you can actually have protocols
> as sub-nodes in USI node though. It doesn't do anything like
> of_platform_populate().

It can't as far as I'm aware, I was just thinking that did seem like a
good idea to keep.

> 
> Also, with this USIv2 driver you can do the same thing you described:
> you can have just one USI node with 3 protocols as sub-nodes (or you
> can even have protocol nodes outside of USI node, but I'd not
> recommend that).
> 
> Actually I can see that it's my fault for not describing that case in
> bindings example. I'll make sure to do that in v2. You also got me
> thinking about default mode: sometimes SW_CONF reset value chooses
> some protocol. In that case maybe it'd useful to have something like
> USI_V2_DEFAULT, to tell driver to not touch SW_CONF at all.

Not sure if that's useful, I'm thinking we specify some protocol for
the USIs in board dts anyways, and if we don't, then we probably don't
use that USI block anyways, so at a minimum all protocols should be
probably disabled in that case, and probably the USI block as a whole
too. (SoC dtsi has them disabled, board dts doesn't touch them, so they
remain disabled). May I know how do you think a defult mode would be
useful?

> And also I
> can add USI_V2_NONE while at it, so that driver can write 0x0 to
> SW_CONF: that way no protocol will be selected. Maybe that can be
> beneficial for PM reasons, if some board doesn't use some USI blocks
> at all. Do you think it's feasible to add those two values to
> dt-bindings header? And is it possible to do so in USIv1?

I think I saw some downstream driver do something similiar, that sounds
like a good idea. In USIv1 I can see the HSI2C driver writing 0 to the
SW_CONF register at pm suspend. Not sure why that's in the HSI2C driver
rather than the USI but I'm guessing it should do the same thing as for
you. I have no TRM though, so not sure. We'll probably just have to
assume that's how it works here, maybe someone that has access to an
USIv1 SoC TRM could confirm? Probably won't get any response from
anyone who has it though.
 
> 
> > Also this way is pretty USIv2 centric. Adding USIv1 support to this
> > driver is difficult this way because of the the lack of USI_CON and
> > USI_OPTION registers as a whole (so having nowhere to actually set
> > the
> > reg of the USI node to, as the only thing USIv1 has is the SW_CONF
> > register). In my opinion being able to use the same driver and same
> > device tree layout for USIv1 and USIv2 is a definite plus
> > 
> 
> Well, it's USIv2 driver after all. I never expected it can be extended
> for USIv1 support. If you think it can be reused for USIv1, it's fine
> by me. But we need to consider next things:
>   - rename the driver to just "usi.c" (and also its configuration
> symbol)
>   - provide different compatible for USIv1 (and maybe corresponding
> driver data)
>   - rework bindings (header and doc); make sure existing bindings are
> intact (we shouldn't change already introduced interfaces)
>   - in case of USIv1 compatible; don't try to tinker with USIv2
> registers
>   - samsung,clkreq-on won't be available in case of USIv1 compatible
> 
> Because I don't have USIv1 SoC TRM (and neither do I possess some
> USIv1 board which I can use for test), I don't think it's my place to
> add USIv1 support. But I think it's possible to do so, using my input
> above.
> 
> I can see how it might be frustrating having to do some extra work
> (comparing to just using the code existing in downstream). But I guess
> that's the difference: vendor is mostly concerned about competitive
> advantage and getting to market fast, while upstream is more concerned
> about quality, considering all use cases, and having proper design.

It's not really the extra work, I just didn't see the benefits of this
way, and my misunderstanding caused me to not see how this would work.
I never really wanted to use the downstream driver as is, but in my
head I was thinking that "layout" should work.

> Anyway, we can work together to make it right, and to have both
> IP-cores support. In the worst case, if those are too different, we
> can have two separate drivers for those.
> 
> > The only real drawback of that way is having to add code for USIv2
> > inside the UART, HSI2C, and SPI drivers but in my opinion the
> > benefits
> > overweigh the drawbacks greatly. We could even make the
> > uart/spi/hsi2c
> > drivers call a helper function in the USI driver to set their
> > USI_CON
> > and USI_OPTION registers up so that code would be shared and not
> > duplicated. Wether this patch gets applied like this is not my
> > choice
> > though, I'll let the people responsible decide
> > :-)
> > 
> 
> I'd argue that there are a lot of real drawbacks of using downstream
> driver as is. That's why I completely re-designed and re-implemented
> it. Downstream driver can't be built and function as a module, it
> doesn't respect System Register sharing between consumers, it leads
> to
> USI reset code duplication scattered across protocol drivers (that
> arguably shouldn't even be aware of that), it doesn't reflect HW
> structure clearly, it's not holding clocks needed for registers
> access
> (btw, sysreg clock can be provided in syscon node, exactly for that
> reason). As Krzysztof said, it also can't handle correct probe order
> and deferred probes. Downstream driver might work fine for some
> particular use-cases the vendor has, but in upstream it's better to
> cover more cases we can expect, as upstream kernel is used on more
> platforms, with more user space variants, etc.

I do agree now, as I said a bit of a misunderstanding made me believe
this was wrong. (as if the addresses were different and the downstream
drivers worked the same way that would mean each USIv2 would have 3
sets of USI_CON and USI_OPTION registers for each protocol which would
definitely have to be handled somewhat differently.

> 
> I don't really think protocol drivers should be aware of USI
> registers
> at all, but if we they do -- we can provide some API from USIv2
> driver
> later, with EXPORT_SYMBOL(), referencing corresponding USI instance
> by
> phandle or using some other mechanism for inter-driver communication.
> 
> Of course, it's not my place to decide on patch acceptance too. But I
> was under the impression that maintainers would be ok with this
> course
> of actions. Also, upstream kernel seems to already follow the same
> design for some similar drivers. See for example
> drivers/soc/qcom/qcom_gsbi.c.
> 
> > Anyways, soon enough I can write an USIv1 driver after I submit all
> > the
> > 7885 stuff I'm working on currently. If you want to, you can add
> > USIv2
> > support to that driver, or if an USIv2 driver is already in
> > upstream at
> > that point, if it is written in the downstream way I can add v1
> > support
> > to that, or if it's like this I'll have to make a whole seperate
> > driver
> > with a whole seperate DT structure.
> > 
> 
> If it's like you said (USIv1 only touches the SW_CONF register), I
> guess USIv2 driver can be extended for USIv1 case. I already provided
> my thoughts on such rework above. It's probably better to consult
> with
> Krzysztof first. I guess the only way to figure out if it's feasible
> or it's better to have separate exynos-usi-v1.c for USIv1, is to try
> and add USIv1 support into USIv2 driver and see how pretty or ugly it
> is :) Whatever the way you decide to go with, please add me to Cc
> list
> when sending USIv1 patches.

Sure, I'll try doing it on top of the final version of your driver
then! Sorry for the misunderstanding there!

> 
> > Best regards,
> > David

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